Archive for the ‘Climate Change’ Category

The Role of a Battery in Meeting Winter Electricity Demand

November 26, 2021

Click Image for a larger version. The 23rd November 2021 was a glorious day in West London.

Friends, these last few days of November have finally been cold enough for all the parts of my plan for a low-emissions home to come together.

  • The triple glazing and external wall insulation are reducing the heating required to keep the house comfortable.
  • The air source heat pump is reducing the amount of energy required to produce that heat.
  • The battery is charging itself at night on cheap electricity and running the house during the day, only running out of charge in the evening.
  • And on sunny days, the solar panels are topping up the battery during the day!

And so far, nothing major has gone wrong!

In this article I will outline the role of the battery in all of this by looking at how the battery operated during two recent quite chilly days, one sunny and one dull.

Battery: Capacity

The battery is a Tesla Powerwall 2, with 13.5 kWh of storage. This is large for a domestic battery but still only 25% the capacity of a typical mid-size EV battery.

We use about 10 kWh/day of electricity for day-to-day living, and in summer the battery and solar PV allow us to operate off-grid for almost 6 months.

But now we are heating our home electrically, and expect to have to provide around 50 kWh/day of heating – something achieved using around 15 kWh/day of electricity using an air source heat pump.

So winter demand is expected to be around 25 kWh/day of electricity – roughly twice as much electricity as the battery can store.

So in winter the main role of the battery is to allow us to buy cheap rate electricity, and then use it later in the day, minimising the cost of the extra electricity we are using for heating.

Battery: Losses

Intrinsically, battery cells can only store DC electricity, but the Powerwall needs to store and discharge AC electricity.

So the Powerwall, includes an AC to DC converter on its charging input and a DC to AC converter (called an inverter) on its output. Overall, it promises to deliver back 90% of the electric energy stored in it.  Tesla call this ‘90% round trip efficiency’.

Additionally the battery requires power to maintain itself: it needs to keep its internal controllers going, and to operate a heating and cooling system to maintain the battery at a suitable temperature during charging and discharging in order to ensure the longevity of the battery. Tesla guarantee that the battery will retain 80% capacity after 10 years.

These two effects – round trip efficiency and self-consumption – make it difficult to estimate the so-called ‘state of charge’ of the battery (SOC) i.e. how ‘full’ it is. This is because energy stored in the battery seems to slowly disappear and it is not clear quite how to account for that.

So I have crudely approximated both effects by a simple average power loss in the battery, typically between 100 W (2.4 kWh/day) and 150 W (3.6 kWh/day). I have then adjusted this rate to make sure the Powerwall is ’empty’ at approximately the observed time. The Powerwall also reports what ‘it’ thinks it’s internal state of charge is.

Household Demand

The graph below shows that household demand on Tuesday 23rd November 2021 and Wednesday 24th November 2021 was broadly similar.

Click Image for a larger version. Household demand on the 23rd and 24th November. Most of the roughly 1 kW of demand is from the heat pump (~0.6kW).

Normal non-heating household demand is typically ~10 kWh/day but on these days, the house used ~24 kWh of electricity with ~15 kWh being used to operate the heat pump to provide hot water and space heating:

The battery itself seemed to consume about 3 kWh with probably ~1.3 kWh of that being round-trip losses, and 1.7 kWh (~70 W) being self-consumed to maintain its temperature and operating system.

State of Charge

The graph below shows both my estimate of the state of charge of the battery (i.e. how full it is ) through each day. Also shown as red dots is the self-reported state of charge of the battery.

Click Image for a larger version. The estimated state of charge (SOC) of the battery (kWh) during the 23rd and 24th November. The green line is my estimate and the red dots show the battery’s self-reported estimate. Also shown is the solar power (kW) during the day which was substantial on the 23rd – enough to meet household demand and partially re-charge the battery – but not very substantial on the 24th. The green dotted line shows how the battery would discharge if there were no ‘solar boost’

The overall uncertainty in my estimate of the state of charge is quite large – perhaps about 0.5 kWh – as shown by the fact that on the 23rd the battery apparently ‘overfills’ and the 24th it ‘underfills’. However my estimates do coincide reasonably well with the Powerwall’s own estimates.

  • On the 23rd, the battery was initially not quite empty and then charged at approximately 3.6 kW using off-peak electricity available from 0:30 to 04:30. It discharged during the day at around 1 kW, and then after being boosted by 7.4 kWh of solar PV, ran out of charge between 23:00 and 24:00. That evening I was obliged to purchase only ~ 0.5 kWh of full-price electricity.
  • On the 24th, the battery was initially empty and then charged at approximately 3.6 kW using off-peak electricity available from 0:30 to 04:30. Discharging at ~1 kW as on the 23rd, but with just 0.94 kWh of ‘solar boost’, it ran out of charge between 18:00 and 19:00. That evening I was obliged to purchase ~5.4 kWh of full-price electricity!

Because the battery had been fully charged and emptied on the 24th, I could evaluate its round trip efficiency using data reported by the battery itself. It reported that it had received 13.4 kWh of charge and discharged 12.1 kWh – just over 90% of the charge, which is in line with specification. But I do not think this figure includes ‘self-consumption’ which I believe appears as a ‘phantom’ domestic load.

Click Image for a larger version. Grid use during the 23rd was almost exclusively during the cheap rate when the battery was charged while using cheap-rate electricity operate the domestic load, including a dishwasher cycle. On the 24th the battery ran out in the early evening and around 5.5 kWh of full price grid electricity was used.

This behaviour makes sense in general terms. If the battery is full (13.5 kWh) at 4:30 a.m., and demand is around 1 kW, then we would expect the battery to run out 13.5 h later i.e. around 6:00 p.m.. Each kWh of solar generation delays that time by an hour.

Costs & Carbon Emissions.

In the Table below I have summarised as best I can the costs and carbon emissions arising from the house on these two days and compared them with two alternative situations:

  • The first imagines that we had the same solar PV, and heated the house with a heat pump but with no battery.
  • The second imagines that we had the same solar PV, but heated the house with a gas boiler.

Click the image for a larger version of the table. Entries associated with burning gas are coloured in blue. Calaulations are bsed on Electricity prices of 5p/kWh (off-peak) and 16.3 p/kWh (peak) and gas prices of 3.83 p/kWh.

The analysis and evaluation of the alternative scenarios is tedious beyond measure, so allow me to simply summarise.

In terms of money:

  • The battery saves lots of money every day, with or without the ‘solar boost’.
  • It makes the use of an ASHP not only low-carbon, but also very cheap.

In terms of carbon dioxide emissions:

  • The battery reduces ‘my’ carbon dioxide emissions by optimally capturing solar energy.
  • On dull winter days the carbon dioxide emissions would be similar with or without a battery.
  • Using an ASHP – with or without a battery – drastically reduces carbon dioxide emissions compared to using a gas boiler.


As I have commented before, the battery is primarily a financial investment.

In summer:

  • The battery it allows me to fully utilise the solar PV, taking the house essentially off-grid for around 6 months.
  • This saves me hundreds of pounds a year.

In winter:

  • The battery it allows me to fully utilise the small amount of solar PV available, and to time-shift the use of off-peak electricity.
  • This also saves me hundreds of pounds a year.

The apparent savings in carbon dioxide emissions associated with using a battery are illusory and in fact the battery is really an additional electrical item using power and causing further emissions.

The solar PV provides low-carbon electricity, but without a battery, any mismatch between production and our domestic use of electricity is expensive.

  • Overproduction of electricity is exported to the grid and I get 3p/kWh.
  • Underproduction of electricity requires me to import from the grid and I must pay 16.3p/kWh.

So without a battery, the low carbon dioxide electricity is still used and still helps the planet, displacing generation from gas-fired power stations. But I don’t get much benefit for it!

What if… we had taken Climate Change seriously in the 1990’s?

November 21, 2021

Friends, reading comments about COP26, I noticed that many people viewed it as ‘an apocalyptic failure’, while others viewed it as ‘meaningful progress’.

In the face of such different interpretations, I began to wonder what unequivocal ‘success’ at COP26 would have looked like?

And reflecting further, I began to wonder what the world would be like now if humanity had begun taking climate change seriously in the 1990’s.

I picked the 1990’s because the basic reality of the phenomenon became mainstream in 1980’s. And so considering (say) 1995 as a starting point would have given  a clear 10 to 15 years for debate about what actions humanity might best take. Remember the landmark Kyoto Protocol was signed in 1992.

And my conclusion surprised me.

I came to the conclusion, that although I wished profoundly that we had acted differently back then, in fact I don’t think that we would be substantially further along the road to a renewable-energy economy than we are now.


Well it’s all hypothetical and I could be wrong, but I am probably not completely wrong.

Anyway: that’s what this article is about: how much lower might CO2 concentrations be now if we had acted earlier?

Carbon dioxide

The measure of success or failure in our attempts to avoid the most dangerous impacts of anthropogenic global warming is the atmospheric concentration of carbon dioxide (CO2).

In 1995 atmospheric CO2 concentrations were ~360 ppm, and they have risen in the intervening 26 years to roughly 415 ppm.

Click the image for a larger version. The ‘Keeling Curve’ showing the rise in atmospheric concentration of CO2 since 1959 as measured at the Mauna Loa observatory: link. Also shown are three alternative trends that might have happened. The green dotted line shows what would have happened if annual emissions had remained the same as in 1995. The orange dotted line shows what would have happened if annual emissions had fallen since 1995. The red dotted line shows what would have happened if annual emissions increased even faster than they actually did..

If in the years since 1995 we had managed to stop growth in emissions i.e. just emit the same amount of CO2 every year – then the Keeling curve would have continued in roughly a straight line – shown as a green dotted line above.

In that case, the atmospheric CO2 concentrations now would be around 400 ppm. This is 15 ppm lower than the situation now – which would represent a saving of 7 years emissions.

But could we reasonably have hoped for better? Surely if we had started back then we would have hoped for a reduction in annual emissions by now?

I am not sure. The world population has increased from ~5.6 billion to ~7.6 billion over the period since 1995, and it is not obvious to me that we could reasonably have hoped for a significant reduction in annual emissions.

And this is especially the case since many of the technologies which are today commercially available at scale did not exist back in 1995.

Let me give you some examples.

Wind Turbines

I could be wrong, but I think the idea that 9.5 MW wind turbines could built and operated would have been inconceivable to most planners back in the 1990’s. The idea of having vast windfarms at sea would also have seemed deeply unrealistic. No one would have made a national plan that depended on an untried technology.

So early turbines were built and incorporated onto electricity grids, and the cost was subsidised to enable the industries to grow.

And as experience with the technology grew post 2000, we might conceivably have deployed wind power faster than we actually did. But faster deployment would have led to the installation of more lower power turbines which we would now be scrapping.

The massive and ongoing reduction in cost arises from installing monsters 10 times as large as turbines installed a decade ago.

It is probably true that we are behind the best possible wind deployment curve – but we are probably only a small number of years behind that curve. We are certainly not 25 years behind the most optimistic possible curve.


I could be wrong again, but I think the idea that solar PV generation could be built and operated on the present scale and at the present cost would have been inconceivable to most planners back in the 1990’s.

As with wind, initial solar generation in the UK was heavily subsidised and that allowed an industry to develop, both in the UK, but most significantly in China where most panels are made.

In 1995 I am pretty sure that the idea that rooftop solar panels in the UK (!) might generate more than 1 GW on a sunny day would have been inconceivable.

The idea that we would have Solar PV farms generating 9 GW on sunny days is still astounding.

The idea that the cost of solar PV would fall as low and as fast as it has could not have been reasonably predicted by anyone.

And the impact of this in other countries will be more profound than in the UK.

But it is hard to imagine that the industry could have grown and prices fallen substantially faster than they already have.

Electric Vehicles

Mainstream manufacturers of internal combustion engine vehicles (ICEVs) had the technologies to make profitable and useable electric vehicles (EVs) for decades e.g. the Chevrolet EV1, Volt, and Bolt. But for a variety of reasons they resisted the technological shift.

Despite mandates to produce EVs, the legacy automakers were so committed to ICEV technology – which was where most of their profits and commercial advantages lay – that they failed to commit to the change.

So it is possible that if mandates had been maintained, the shift to electric vehicles could have begun much earlier.

But the fact that ICEVs are now becoming obsolete before our eyes is due – in my opinion – to one person: Elon Musk.

Tesla was started in 2003 and Musk joined in 2004 (link) with the aim to “expedite the move to sustainable transport and energy, obtained through electric vehicles and solar power”. The Tesla Roadster (of which only 2450 were sold) was the first production car to use lithium ion batteries. The scale and pace of the change instigated by Tesla exceeds anything that the legacy carmakers have ever achieved. And the shift to EV manufacturing in China is staggering.

And so although an earlier mandate to shift to EV’s might possibly have moved things slightly faster, my guess is that the giga-factory concept which has been invented by Tesla, will ultimately drive the switch to EVs faster than the legacy automakers – or government planners – could ever have conceived.


Batteries will play a key role in a sustainable energy civilisation. And their performance in terms of longevity, capacity and safety, has improved astoundingly in recent years.

It is not obvious to me that it would have been possible to scale up battery production substantially faster than what has actually happened.

Indeed progress has been so rapid that it is perfectly possible that had we started earlier, we might well have started using battery technologies that were barely fit for purpose.


Several aspects of the renewable energy revolution – such as cheap solar panels, EVs, or battery manufacture – rely on sophisticated and world-leading manufacturing in China.

It is not obvious that the manufacturing capacity the world requires could have been made available much faster than it already has.

The Internet & Computers

I find it hard to believe, but the internet barely existed in 1990: today it is ubiquitous

Network speeds are fast enough that video conferencing is now routine, and the speed of the network enables many technological innovations which allow us to cope with the unpredictability of energy generation using renewables.

It is not obvious that advances in these fields could possibly have developed any faster

Nuclear Power

One area where we might conceivably have achieved much more than we have is in the area of nuclear power.

I remember at the time being dismayed when the Blair government’s refused to engage with the issue, and basically let the UK civil nuclear industry die.

I personally would have been happy if the UK had built a few more nuclear power stations over the past 25 years. And it would have been quite conceivable to have built out nuclear power much more widely on an international scale.

I don’t know how much we might reasonably have hoped that this would lower CO2 emissions. Perhaps by 10% per year globally? Perhaps by a more significant fraction for the UK.

But, if we had adopted a very pro-nuclear solution to our energy problems, then there may have been other unintended consequences.

The capital requirements of a large nuclear power station (~£20 billion) are so high – even for governments – that they could easily have deprived wind and solar energy projects of the capital they needed for their initial growth.

As we look at the field now, we can see that perhaps a small number of truly gigantic nuclear power stations are possibly not the best implementation of nuclear power. And although nuclear power does have low CO2 emissions, it is expensive – something like 10 times more expensive than wind energy.

From today’s perspective, it is likely that smaller, modular reactors (e.g. this, or this, or this) will be cheaper, safer, and contribute to the grid more flexibly.

If we had already committed to nuclear power in a big way, it is quite possible that not only would the growth of renewables have been squeezed, but the price of electricity might be substantially higher.


Friends, I would like you to be sure about what I am suggesting, and what I am not suggesting in this article.

  • I am not saying that this is the best of all possible worlds.
  • I am not saying that actually our delay in acting has been ‘for the best’.
  • I am not saying we should be blasé about our progress in addressing Climate Change.

But I am reflecting that:

  • This is not the worst of all possible worlds.
  • Our past delays mean that we have not acted on climate change at the fastest rate possible.
  • But the last 25 years have seen the growth of new industries and technologies at truly astounding rates.

The change in public consciousness has also been profound.

And I am reflecting that although it would obviously have been better to address the challenges of Climate Change sooner, by chance some aspects of recent progress has been so rapid, that we may not be a full 30 years behind where we might have been.

Any delay is regrettable, but even if we had acted sooner, we would already have already committed ourselves to at least a century of climatic challenge.

My conclusion is that it is really is not too late to act on this now – but it is getting later every day…

A weekend away…

November 15, 2021

Click image for a larger version. The view from Winchelsea village is splendid, and in my opinion, all the better for the views of Camber Sands wind farm and Dungeness nuclear power station.

Friends, I was just chatting with my heat pump (HP – he/him) the other day, when HP made an odd request:

HP: “Michael, would you and Stephanie mind going away for a few days?

I was honestly a bit alarmed. This is our home and although we have taken HP in and hope to provide him with a loving home, we really don’t want him to take over! But then HP explained.

HP: “I’ve got this issue with my Weather Compensation. And when you and Steph are inside the house, switching things on and off, cooking, and just generally heating the place with your fleshy metabolisms, I just can’t see if it’s working or not!”

It still seemed a bit previous, but after discussing it together as a family, Steph and I agreed to go away for a long weekend in Winchelsea and to leave HP to look after the house.

When we came back…

On returning, we were worried we might find the house trashed – you know what youngsters are like.

But in fact the house was pristine and HP had kept the temperature stable all weekend – all by himself!

And to cap it all, HP had left us with a whole pile of beautiful data. I was so proud that I told him I was going to share it on my blog.

Weather Compensation

Looking at 72 hours of data without us messing things up, it was clear that HP really did have good weather compensation.

As the external temperature went down, HP responded by increasing the temperature of the water flowing in the radiators.

The effect of HP’s efforts was to keep the internal temperature within ± 0.3 °C of 20.5 °C over most of this period.

And it is also worth noticing how low the flow temperatures were, peaking at just under 34 °C when the external temperature fell to 7°C.

Click image for a larger version. Graph showing the external temperature (°C) and the flow temperature (°C) of water in the radiators over the 72 hours starting at midnight on 10/11 November 2021. The grey dots show the data every 2 minutes and the green curve shows the data averaged over 1 hour. Notice how when the external temperature falls, the flow temperature rises. This is called ‘Weather Compensation’.

One can see the ‘Weather Compensation’ effect more clearly if one plots the hourly-averaged flow temperature versus the external temperature.

Click image for a larger version. Graph showing the average flow temperature (°C) of water in the radiators versus the external temperature (°C) over the 72 hours starting at midnight on 10/11 November 2021. The blue dashed line shows the nominal ‘Weather compensation’ curve that HP was trying to achieve.

The blue dashed line on the graph above shows the nominal ‘Weather Compensation Curve’ that Vaillant had suggested that HP should aim for – a curve which Vaillant helpfully label with the meaningless number ‘0.7’.

It suggests that even when the outside temperature reaches 0 °C, we should be able to stay cosy with a flow temperature in the radiators of around 40 °C.

In different houses, different compensation curves can be chosen to match the heat pump output to the heat losses from the house.

And over the period of 3 days, HP used just 19.8 kWh of electricity (275 W) but delivered a whopping 83.7 kWh of heat (1.16 kW) – an average coefficient of performance of 4.2.


HP really enjoyed his time at home alone – he seemed so proud that he had mastered the Weather Compensation which can be quite tricky even for clever heat pumps like HP.

It really is great having HP in our family, and Stephanie and I enjoyed our time away too.

I really wish everyone could have a heat pump as considerate as HP.

P.S. In case you haven’t been following, HP is a 5 kW Vaillant Arotherm plus heat pump.

Powerwall: Assessment of degradation of storage capacity after 8 months

November 9, 2021

Friends, it is now 8 months (235 days) since we installed the Tesla Powerwall domestic battery (link).

And one of the questions I am most commonly asked concerns its likely lifetime.

All batteries degrade over time, even Tesla’s, but the practical question is “How much degradation occurs and over what period?

By chance last night (8th/9th November) I had the opportunity to assess this degradation and, in case you are short of time and need to do something more important than read this blog, my estimate is that the battery degradation so far is immeasurably small.

For those of you still with me, allow me explain how I made the measurement and what the results suggest regarding the battery lifetime.

Powerwall Control

The Powerwall battery is apparently controlled by an ‘App’ on my phone.

The ‘Tesla App’ has pleasing – almost compulsively enticing – graphics showing power flowing to and fro from the grid; the battery; our house; and the solar panels. It’s a really engaging interface. It also allows detailed data to be downloaded for analysis.

Click image for a larger version. Screenshots from the Tesla ‘App’ showing the charging and discharging of the battery, its state of charge, and the overall operation of the battery system.

However, I say the battery is ‘apparently’ controlled by the App because in reality the battery is controlled and monitored 24/7 over the internet by Tesla. And Tesla give me only limited control via the App.

Currently the battery is set to ‘Time-Based Control‘ which charges the battery from solar PV when available or cheap rate electricity if required.

And Tesla make the choice about how much charge to take overnight based on it’s estimate for how much solar power will be available the following day.

I don’t have the algorithm it uses, but it seems to do a fair job.

The reason I consent to this egregious interference with my liberty is that in return for ceding control, Tesla promised that the battery would retain 80% of its specified 13.5 kWh capacity (i.e. 10.8 kWh) in 10 years time i.e. after a nominal 3650 partial charge cycles.

I think this is a guarantee worth having and so I submit to the Tesla-Brain.

In fact since I bought the Powerwall I believe this guarantee has been degraded to 70% after 10 years – which suggests it really is quite a tough specification.

Expected Battery Degradation

Various reports on the web, and Tesla’s 2020 environmental impact report, indicate that Tesla car batteries seem to retain around 90% of their range after 200,000 miles (320,000 km).

Click image to see a larger version. Excerpt from Tesla’s 2020 Environmental Impact Report showing roughly 10% reduction in EV range after 200,000 miles.

It’s hard to know how that colossal range would translate into the 3650 partial charge and discharge cycles of a domestic battery over 10 years.

In part it depends strongly on the range of the charge-discharge cycles. Charging and discharging over the middle of a battery’s range – between say 10% and 90% – is relatively benign. But rapidly charging and discharging from 0% to 100% degrades battery capacity. This is why Tesla want to have control over the battery.

My thought when I bought the battery, was that domestic service would be generally less stressful than service in a motor car. Why?

  • The Powerwall has it’s own re-circulating fluid temperature control and does not need to operate in the climate extremes of a car battery.
  • A Tesla car battery is about 4 times larger than a Powerwall’s 13.5 kWh, but the maximum EV discharging rates – which can affect battery life – are up to 25 times higher than the Powerwall’s transient maximum of 7 kW.

Other reports (link) suggest that Powerwall degradation might be considerably faster than for EV’s.

However, the general pattern of battery degradation (reflected in the figure above) is that the greatest rate of degradation is at the start of the service life of the battery.

If my Powerwall were to degrade linearly to 80% capacity over 10 years (120 months) then after 8 months I might expect to see 0.18 kWh decrease in capacity. Small, but possibly detectable.

What did I measure?

By chance last night the battery ran out just before midnight, so I knew it had zero ‘state of charge’.

Additionally the Tesla-Brain decided to fully charge the battery over the four hours of cheap rate (5p/kWh) electricity starting at 00:30. So I was able to observe a full charge from empty.

The graphs below (using data downloaded via ‘the App’) show what happened. Note the data only have a time-resolution of 5 minutes.

Click image for a larger version. Graph showing the charging of the battery from 00:30 at approximately 3.6 kW and the discharging of the battery after 04:30 at approximately 300 W to meet domestic demand.

Using the charging rate and the time I can work out the ‘state of charge’ of the battery and compare this with the specified capacity.

Pleasingly, the maximum state of charge appeared to correspond closely with the initially specified capacity.

Click image for a larger version. Graph showing the calculate ‘state of charge’ of the battery during charging from 00:30 to 04:30. Within the (considerable) uncertainties of this measurement, the maximum state of charge is closely in line with its specified original capacity.

Interestingly, while the battery was charging, the Tesla control circuitry also used cheap-rate electricity to run the dishwasher and top up the domestic hot water using the heat pump.

Click image for a larger version. Graph showing the household demand from midnight to 06:00. The battery was charging in the background during these high power events.


First of all, some caveats:

  • All these measurements are self-reported by the Powerwall, and so should rightly be subject to sceptical interpretation.
  • The data have limited resolution both in power and time.

However, when I have been able to check the reported values against independent measurements – e.g. for estimates of the energy reaped from the solar panels each day – I have found them in close agreement at the level of 0.1 kWh.

So taking these measurements at face value, I find no detectable degradation in battery capacity after 8 months or 235 days.

  • If the battery capacity were degrading linearly over time to 80% of initial capacity after 8 years I would have expected to see 0.18 kWh decline in capacity.
  • If the battery capacity were degrading faster than linearly – as it plausibly might – then I would have expected to see perhaps 0.3 kWh or 0.4 kWh degradation.

Obviously I will re-visit this issue at some point in the future, but the fact that there is no detectable degradation so far suggests that the retained capacity after 10 years may indeed exceed 80%.

Which would be nice.

A Year of Solar Energy

November 8, 2021

Friends, it’s just coming up to the anniversary of the installation of solar photo-voltaic (PV) panels at Podesta Towers in Teddington. So I thought it might be interesting to see what a year of generation has brought.

First I’ll describe the installation; then explain what I expected (or at least hoped for); and then outline what has actually happened together with a discussion of the role of our domestic battery.

And finally I’ll remind you – and myself – of how solar panels fit into to my efforts to reduce carbon dioxide.

But in case you’re short of time, here are the salient points.

  • 12 solar panels generated just over 3500 kWh of electricity which is close to last year’s total domestic consumption ~3700 kWh.
  • This will have avoided emission of ~0.8 tonnes of carbon dioxide this year.
  • On the sunniest days the system generated ~25 kWh/day and in mid-winter average generation was ~ 2kWh/day which compares with around 10 kWh/day of non-heating electrical use.
  • When used with a battery, we were substantially off-grid for around 4 months.

1. The Installation

I described the installation in some detail here, but briefly it consists of 12 panels from Q-Cells, each 1.0 m x 1.7 m in size with a nominal generating power of around 340 W. I think this year’s versions are already more powerful!

Click for a larger image. Google Maps view of my home showing the shape and orientation of the available roofs. And photographs before and after the installation.

The installation cost £4,230 pounds which did not include the cost of scaffolding which was already up on the house at that point.

I chose Q-cells panels because I liked their completely black appearance; they seemed adequately specified; they were readily available; and they were cheap – around £130/panel if I recall correctly.

I later found out that they have pleasingly low embodied carbon dioxide, less than 1.6 tonnes for this installation, and so the embodied carbon dioxide payback time will be around 2 years. Q-cells also score highly for the not producing toxic waste.

2. What did I hope for?

The quotation from local installer GreenCap Energy included an estimate of the expected output – 3780 kWh/year.

But rather than trust the installer I downloaded data from an EU project that cleverly allows one to estimate how a particular solar installation with an arbitrary location and orientation would have performed hour-by-hour over the entire period from 2005 to 2016. This data suggested I might reasonably expect 3847 ± 173 kWh/year.

Click for a larger image. Estimates from an EU re-analysis project of what my solar panels WOULD have generated over the years 2005 to 2016. Year-to-Year variability appears to be about 5%.

I then averaged these 11 years of hour-by-hour data to yield my estimate for the expected monthly performance. They are shown as yellow dots on the graph below.

Click for a larger image. Expected generation in kWh per day. The yellow dots are the monthly averages of the estimated generation from 2005 to 2016. The green dotted line is a crude guess based on a ‘sine-squared’ function. The red dotted line shows our typical average daily electricity consumption ~10.5 kWh/day.

One feature of the simulated data is that the peak generation is expected to occur from April to July – a range which is not centrally arranged around the longest day (June 21st).

3. What happened?

Solar generation was broadly in line – but a little lower – than expectations. But the day-to-day and week-to-week variability was much greater than I had appreciated.

This variability makes it hard to plot readable graphs because they look chaotic! So let me introduce the data one stage at a time.

Looking at the monthly averages (below) we see that most months were close to expectations, but April was especially sunny, and August was a bit disappointing. So far, November has been a brighter than normal. Please note, the December data is from December 2020, because the data from December 2021 is not yet available.

Click for a larger image. Comparison of monthly averages of actual solar generation (green dots) with expected generation in kWh per day. (yellow dots). The black error bars show the standard deviation of that months daily data.

Now let’s additionally plot the daily data.

Click for a larger image. Similar to the graph above but now additionally showing the actual daily solar generation (kWh/day)

A few features of the daily data are really quite remarkable.

  • Firstly, the day-to-day variability is large. This means that the solar generation on a given day is almost no indication of the likely generation on the next day.
  • Secondly, the peak generation of around 25 kWh/day can occur in either April or July despite the substantial differences in day length and solar path.
  • Thirdly, even in mid-summer there can be runs of several utterly miserable days with very little solar generation.

If we average the daily data over a week, (see below) then we still see variability – deviations from the nominally-expected generation – which deviate from the expected generation consistently over periods of up to 3 weeks

Click for a larger image. Similar to the graph above but now additionally showing the ±3 day average of the generation as pink or purple lines.

Another way to look at the data which emphasises the trend more strongly than the variability, is to show cumulative generation through the year.

Click for a larger image. Cumulative generation (kWh) shown as a blue line against nominal expected generation.

Actual generation (just over 3500 kWh) is about 8% lower than I expected, but I am not especially surprised.

It could be that 2021 was a ‘a bit dull’ over the summer months when most generation takes place, or because I had incorrectly allowed for losses at the inverter – which converts DC electricity into AC mains electricity.


The PV panels were installed in November last year and to our surprise they immediately made a measurable difference – reducing our use of electricity from the grid by around 2 units per day.

Click image for a larger version. Daily electricity usage (from a smart meter) before and after solar panel installation.

However, it was not till our battery was installed in March 2021 that the transformational power of solar became apparent. Within a couple of days, the household went ‘off-grid’ and remained off-grid for around 80 days.

Click image for a larger version. Daily electricity usage (from a smart meter) since solar panel installation. Install the panels led to a small reduction in grid use. Consumption rose over Christmas. Consumption began to fall as we entered spring, and then fell to zero once we installed the battery. As we enter Autumn and Winter grid consumption is rising because since July we are using electricity to power a heat pump which heats the house and provides hot water. The bold green line shows the daily consumption averaged over ± 1 week.

As we enter autumn and winter, the solar cells still contribute significantly – 48% of our electricity in October was solar. But generation will be just around 2 kWh/day through November, December and January.

Since July, the ASHP installation has been using ~1.5 kWh of electricity a day to provide ~4 kWh of domestic hot water.

Now (November) the ASHP is providing space heating within the house, and in the coldest weather (~0°C) I expect  this will require an additional 15 kWh/day of electricity to provide ~50 kWh/day of heating. Most of this electrical energy will be downloaded at cheap rates overnight.


The PV and battery system has been installed to reduce carbon dioxide emissions from the house. They are the part of a suite of measures to reduce heating demand, eliminate gas use, electrify heating and increase the use of renewable energy.

Together they have dramatically reduced the running costs of the house.

The graph below shows expected household carbon dioxide emissions (not including consumption or travel) over the period up to 2040.

Click image for a larger version. Anticipated household carbon dioxide emissions (not including consumption or travel) over the period up to 2040. The red line shows what would have happened if I had made no changes. The green line shows the expected outcome.

In the short term, all the actions I have taken have made things worse!

The embodied carbon dioxide in the solar panels, insulation, batteries, and heat pump amounts to ~11.5 tonnes, and this ‘debt’ will not be re-paid until the end of 2023.

I would love to add more solar panels, but I am resolved to hold off until my existing carbon dioxide debt is re-paid.

Overall, I hope you can see that the solar panels are central to the plan to reduce anticipated emissions by 60 tonnes by 2040


Re-visiting the “washing-up dilemma”

November 6, 2021

Friends, I can remember a time when the main question about washing up was: “Who is going to do it?”

But dishwashers changed all that. At first there were questions about whether dishwashers or regular washing-up was technically superior.

However nowadays, dishwashers do a very creditable job of cleaning crockery and cutlery – and in what was a devastating blow for the British Tea Towel industry – they also do a fine job of drying.

[Aside: If you would like to see how dishwashers work – check out these Technology Connections videos:

But this then raised the thorny question of whether hand-washing or using a dishwasher was cheaper.

And now of course, the BIG question is which is greener i.e. has the lowest associated carbon dioxide emissions. This article is an attempt to answer this question.

Energy and Carbon Emissions

In her bookEnergy and Carbon Emissions: the way we live today“, Nicola Terry does a fine job of answering such questions.

The book is a gold mine of information on all aspects of our current ‘carbon problem’.

And she looks at this problem in Chapter 7: Should I get a more efficient X?

She concludes that:

“Washing dishes by hand probably generates less carbon [dioxide] emissions if you heat water by gas – even less if you have a solar panel for hot water

And in characteristic and admirable detail, she explains the assumptions underlying her conclusion.

But as His Bobness might have commented… things have changed.

2011 versus 2021

The energy landscape has changed significantly since 2011 when Nicola Terry published her book.

  • Carbon dioxide emissions from electricity are now 235 gCO2/kWh rather than 545 gCO2/kWh in 2011.
  • Carbon dioxide emissions from gas are still roughly 200 gCO2/kWh assuming ~90% boiler efficiency: the same as 2011.
  • In our house,
    • Hot water comes from a heat pump which produces hot water around 3 times more efficiently than a gas boiler.
    • In summer, practically all our electricity comes from solar panels with no proximate emissions: 0 gCO2/kWh
    • In winter, we set the dishwasher to run on cheap-rate electricity which typically has 10% less CO2 emissions than daytime electricity.
  • In other houses,
    • Hot water might come from solar thermal panels.

So given this complexity, can we definitively compare the environmental impact of using a dishwasher and washing up by hand?

Well I have made an attempt – and below I will run through some example calculations and then I’ll collate a more extensive set of calculations at the end.


Taking our recent Bosch model as typical, a dishwashing cycle seems to typically consume around 12 litres of water and around 1 kWh of electricity.

Comparing the consumption for different modes of operation, it is clear that nearly all this energy is used to heat the water. At 70 °C – way more than your hands could stand – fats within food residues soften considerably making them easier to remove in the water spray.

In the winter,

  • Carbon dioxide emissions associated with running a cycle in 2021 on average would have been 235 gCO2 using electricity direct from the grid.
  • At peak demand (4 pm to 7 pm), the emissions might be higher – but if run in the early hours of the morning – emissions might be ~10% lower.

In the summer,

  • Carbon dioxide emissions associated with running the dishwasher are around zero because the electricity is derived from the solar panels.

So assuming a year is 50% summer and 50% winter, carbon dioxide emissions associated with running a single cycle in 2021 on average would be 0.5 x 235 = 118 gCO2 per cycle.

So running the dishwasher 3 times per week would result in emission of around 354 gCO2 per week.

Dishwashers do offer ‘eco’ modes using cooler water which could reduce this.

Washing-up by hand

In general, hand washing probably consumes a bit more water than a dishwasher, but let us assume in the first instance that the same 12 litres of water will suffice, and that the dishes are rinsed in cold water.

Heating 12 litres of water by 40 °C from (say) 10 °C to 50 °C – roughly the hottest water one might reasonably use – requires:

12 litres x 10 °C  x 4200 J/°C/litre = 2,016,000 joules

Or in more familiar units, 0.56 kWh. If this water were heated with a heat pump with a COP of 2.5, this would require 0.224 kWh of electrical energy.

In the winter,

  • Carbon dioxide emissions associated with 0.224 kWh of electricity drawn from the grid would have been 0.224 x 235 gCO2/kWh ~ 53 gCO2

In the summer,

  • Carbon dioxide emissions associated with 0.224 kWh of electricity from the solar PV panels would have been zero.

So assuming a year is 50% summer and 50% winter, then washing up by hand 3 times a week results in weekly emissions of 3 x 27 = 81 g CO2/week

If the water were heated by gas, the equivalent emissions would have very similar in winter, but would not be reduced in summer, resulting in emissions of about 150 gCO2/week.

And if the water were heated by direct electrical heating without a heat pump, three washes per week would result in emissions of about 158 gCO2/week.


I have carried out these calculations for a number of situations using this spreadsheet (Dishwasher Calculations). I compared:

  • Hand washing with water from a heat pump powered by solar energy and a battery for half of the year.
  • Hand washing with water from a heat pump powered by grid electricity.
  • Hand washing with water from a gas boiler.
  • Hand washing with water heated by an immersion heater powered by solar energy and a battery for half of the year.
  • Hand washing with water heated by an immersion heater using grid electricity.
  • A dishwasher powered by solar energy and a battery for half of the year
  • A dishwasher powered by grid electricity.

The figure below summarises my calculations assuming that a family washes up 3 times per week either by hand or with a dishwasher.

Click on the image for a larger version. Summary of calculations showing estimated annual CO2 emissions for a variety of ways to do washing up. See text for details.

It is clear that wash-for-wash, washing-up by hand results in lower CO2 emissions. This is especially striking when using a heat pump to generate the hot water.

The lower emissions arise primarily because the water used for washing-up by hand is cooler than the water used in a dishwasher.

However different households behave in different ways, and wash-for-wash comparisons may be unrealistic.

For example, in a small household, a dishwasher can store soiled dishes out of sight for a day or two, and so operating a dishwasher (say) 3 times a week is practical while not leaving out unsightly piles of unwashed dishes. However, if a similar household were relying on hand-washing, they might wash up once each day. Let’s call this Option#2.

Or for larger households, running a dishwasher once a day is more typically part of a family routine, whereas hand-washing for a larger family would be more likely – in my experience – to use rather more water – perhaps as much as twice as much. Let’s call this Option#3.

The results for Options 2 & 3 are shown below. Note that the vertical scales are different on each of the graphs shown.

Click on the image for a larger version. Summary of calculations for Option#2 showing estimated annual CO2 emissions for a variety of ways to do washing up. See text for details. Note that the vertical is different from the other figures.

Click on the image for a larger version. Summary of calculations for Option#3 showing estimated annual CO2 emissions for a variety of ways to do washing up. See text for details. Note that the vertical is different from the other figures.

The different options have a range of CO2 emissions that vary from ‘a few kilograms per year’ to ‘a few tens of kilograms per year’.

But looking across all the options, hand-washing generally results in reduced emissions, just as Nicola Terry had concluded back in 2011. But now the minimum emissions come with water heated by a heat pump.

Click on the image for a larger version. Summary of calculations for Options #1, #2, and #3 showing estimated annual CO2 emissions for a variety of ways to do washing up. See text for details.


My first conclusion is this: How can anyone be expected to wade through such a calculation to determine how to do the washing up!

My second conclusion is not that “Dishwashers are bad”. Indeed my wife and I own a dishwasher and use it regularly.

Personally, I am convinced of the overwhelming need for society as a whole to reduce carbon dioxide emissions, and I also feel this as an intense personal responsibility too.

Nonetheless, no decision can be entirely one-dimensional. As with many similar decisions, there are other criteria that can also be significant – criteria that have nothing to do with carbon dioxide emissions.

Off the top of my head, it could be that some people just don’t like washing up! Or it could be that some people are busy and using a dishwasher is helpful. Or perhaps that someone’s partner might just want a dishwasher for some reason, and so a dishwasher could be part of a declaration of love – an important human dimension.

But… although any decision about how to live one’s life is multi-dimensional, one of those dimensions is very likely to be the associated carbon dioxide emissions, almost no matter what the actual decision concerns.

In this case, my calculations tell me that:

  • For washing up, the overwhelming contribution to the carbon emissions is caused by heating water – so using less hot water will reduce emissions no matter how the washing up is done.
  • If using a dishwasher, it is a good idea to try to ensure that it is more-or-less full, and to think about using ‘eco’ modes.
  • And when washing up is done by hand, it is a good idea to be mindful of the amount of hot water used – and possibly use cold water to rinse.

Finally, in future years, it is possible many of these emissions may well change again.

It is planned that the carbon intensity of UK electricity should fall to 100 gCO2/kWh by 2030 – and this will reduce the emissions of any appliances which use that electricity. Additionally, it could be that dishwashers will be built which can use hot water heated by a heat pump instead of electrically heating their own water.

Finally this calculation does not include the embodied carbon dioxide emissions from either a dishwasher, a heat pump, a battery or solar panels. My guess – for what it is worth – is that assuming reasonably long lifetimes for these items, the embodied carbon dioxide emissions will not significantly alter these conclusions.


Guy Callendar: Reflections on his biography.

November 4, 2021

The Callendar Effect – The Life and Work of Guy Stewart Callendar (1898–1964) The scientist who established the carbon dioxide theory of Climate Change in 1937.

Friends, as you may or may not know, Guy Callendar was the first person to discover that humanity’s emissions of carbon dioxide were measurably warming the Earth.

Astoundingly, he discovered this as an ‘amateur’ researcher in 1937, working part-time from his study in his modest home in Worthing.

His foundational paper on the discovery was published in 1938 and I wrote a précis of the paper earlier this year.

But since he lived his life as an establishment ‘outsider’, it was difficult to find out any information about his life on the web.

But last month I discovered that there is in fact a 2007 biography : The Callendar Effect by James Rodger Fleming. It arrived last week and I found reading the slim volume very moving.

So I thought I would just note down a few reflections. Please note, just about everything below is simply re-stating a few elements from James Rodger Fleming’s work.

Personal Life.

Guy Callendar was born in 1898, the third child of eminent polymath physicist and engineer, Hugh Callendar.

His father was a professor at Imperial College and Guy and his siblings had a privileged upbringing in what appears to have been an engineer’s dream household.

For example, in 1902 his father bought a motorbike, added an 8-speed gearbox and an ‘armchair’ to create a two-passenger tricycle allowing him and his wife to tour “the fearful hills of Porlock” and to treat the children to joy rides around the neighbourhood.

At age 5 his elder brother Leslie, for reasons undisclosed, “stuck a pin” in Guy’s left eye, blinding him in that eye.

He started at St Paul’s public school in 1913. His elder sister died of pneumonia in 1914, and with the start of the first world war, in 1915 he left school early.

Unfit for war service because of his blindness, he went to work for his father at Imperial College, using X-ray technology to search for defects in engine castings.

In 1919 he entered City & Guilds College – then part of Imperial – earning a certificate in Mechanics and Mathematics in 1922 – before starting work for his father on establishing the properties of steam at high temperatures and pressures. He continued this work intermittently throughout his life, and carried on the work after the death of his father in 1930.

In 1930 he married and a year later he and his wife Phyllis were “blessed” with twin daughters. He appears to have been a loving and devoted father and husband, enjoying family life, cycling, and tennis.

During and after the Second World War he worked at a secret establishment at Langhurst near Horsham on many topics but most significantly on the implementation of FIDO, a system for dispersing fog around runways.

He retired in 1958, and died of a heart attack in 1964

Throughout his life he seems to have been ‘modest and quiet’, not seeming to have been interested in advancement within the organisations within which he worked. I got the sense that he was perhaps something of a misfit, feeling most comfortable at home with his family.

Working for his father from an early age he would have been exposed to advanced ideas and technologies, but his credentials were built on practical knowledge rather than higher degrees and academic stature.


The chapter of James Rodger Fleming’s book on Callendar’s work on the climatic effect of carbon dioxide begins with a quote from Callendar himself.

“How easy it is to criticise, and how difficult to produce constructive theories of climate change

This seemed such a sad quote, because it was true in his own lifetime, when his work was initially dismissed by the meteorological establishment. Indeed during his lifetime his work was only recognised as significant within a very small circle.

And even 60 years after his death, with the climate change he predicted being seen as the greatest threat to humanity, and with his work having been validated a million times over, it is still commonplace for even otherwise well-educated people to criticise the theory without taking the effort to study it.

19th Century Insights & 20th Century Confusion

That the Earth’s surface is warmed by the atmosphere was not a new idea. In 1824 Jean Baptiste Joseph Fourier had written:

“The temperature of the Earth can be augmented by the interposition of the atmosphere, because heat in the state of light [i.e. visible light] finds less resistance in penetrating air than in re-passing into the air when converted into non-luminous heat [infrared light].

John Tyndall was, according to Callendar, the first to put forward the CO2 theory of Ice Ages. In the middle years of the 19th Century Tyndall demonstrated experimentally that:

“Perfectly invisible and colourless gases and vapours were able to absorb and emit radiant heat.

Around 1860 he wrote:

“The solar heat possesses… the power of crossing an atmosphere; but when the heat is absorbed by the planet, it is so changed in quality that the rays emanating from the planet cannot get with the same freedom back into space. Thus the atmosphere admits of the entrance of solar heat, but checks its exit; and the result is a tendency to accumulate heat at the surface of the planet.

Tyndall also concluded that:

“… changes in the amount of any radiatively active constituents of the atmosphere – water vapour, carbon dioxide, ozone or hydrocarbons – could have produced “all the mutations of climate which the geologists reveal… they constitute true causes, the extent alone of their operation remaining doubtful.

In 1896 Svante Arrhenius…

“…following Tyndall’s suggestion, demonstrated that variations in atmospheric CO2 concentration could have a very great effect on the overall heat budget and surface temperature of the planet and might trigger feedback phenomena that could account for glacial advances and retreats.

He later speculated…

“…on a ‘virtuous circle’ in which the burning of fossil fuels could help prevent a rapid return to the conditions of an ice age and could perhaps initiate a new carboniferous age of enormous plant growth.”

These 19th Century insights set the groundwork for Callendar’s work. But the first four decades of the 20th Century were filled with work which appeared to deny the possibility of the effect. One quote will suffice from the US Department of Agriculture in 1941:

“No possible increase in atmospheric carbon dioxide could materially affect either the amount of insolation reaching the surface or the amount of terrestrial radiation lost to space”

The Callendar Effect

Beginning around 1934, Callendar worked on his investigation of whether anthropogenic CO2 emissions were actively warming the Earth.  He begin this epic work in his study at home in Worthing.

Given his meagre resources I can only imagine his endeavours must have been all-consuming.

I find myself asking why it was Guy Callendar in particular that somehow pulled together all the parts of this complex problem and realised that the humanity was actually warming the Earth.

I think one part of the reason was that his eclectic experiences since an early age, and his lack of a well-defined academic role, probably allowed him to see the problem unconstrained by conservative meteorological views.

But his amateur status was probably (at least part) the reason why the meteorological establishment regarded his ideas with ongoing negativity.

In 1961, as he prepared for a summary publication of his ideas, he wrote a note listing “Reasons for the unpopularity of CO2 theory in some meteorological quarters“.

  1. The idea of a single (easily explained) factor causing world-wide climatic change seems impossible to those familiar with the complexity of the forces on which any and every climate depends.
  2. The idea that humanity’s actions could influence so vast a complex [system] is very repugnant to some.
  3. The metrological authorities of the past have pronounced against it, mainly on the basis of faulty observations of water vapour absorption, but also because they have not studied the problem to anything like the extent to required to pronounce upon it. 
  4. Last but not least. They did not think of it themselves!

But Callendar’s evident zeal for this project probably arose because of a profound understanding of the fundamental role of infrared light in establishing the temperature of the Earth, coupled with an understanding of the dynamics of CO2 in the atmosphere.

Additionally, he had worked extensively with the most accurate thermometers on Earth – developed by his father – and he was aware of the weaknesses and strengths of practical meteorological measurements.

In short, he had a uniquely eclectic combination of the necessary experience.

Together his insights led him to understand that increased atmospheric CO2 would inevitably warm the Earth independent of any degree of complexity in the details of atmospheric processes, and that that warming could be detected in spite of the difficulty.

His insights resulted a truly remarkable fact.

  • In 1937 using pencil and paper in his study, Callendar calculated that doubling the atmospheric concentration of CO2 would cause around ~1.5 °C of global warming.
  • In 2021 using resources beyond anything which Callendar might have imagined, and considering thousands of effects Callendar neglected, the IPCC 6th Assessment Report estimates that doubling the atmospheric concentration of CO2 would cause ~3°C of global warming with a likely range of 2.5°C to 4°C.

The similarity of these results is a testament to Callendar’s insight that the details don’t matter. This is analogous to the problem of putting an extra blanket on ones bed. Even though the heat transfer through woollen materials is complex, warming to some degree is inevitable.

A singular life

Ultimately, Callendar’s singular contribution to science has to remain something of a mystery. He produced an insightful work of genius in the most unlikely of circumstances.

But the slimness of his biography is indicative of a man who in many ways did not leave much of a trace, and his work was certainly not appreciated by many in his own lifetime.

But the slimness of the book also makes the little we know of his life even more poignant.


Heat Pump Operation: What it costs. And what it costs me.

October 26, 2021

Friends, yesterday I wrote about the way the new Vaillant 5 kW Arotherm plus Heat Pump performed during 24 hours of a mildly-cold autumn day.

Click image for a larger version. The graph shows electrical power consumed by the heat pump and the thermal power delivered by the pump during the 24 hours of 24th October 2021.

This was really important data for me. I have spent the best part of £60,000 insulating the house; installing solar PV and a battery; and plumbing in the air source heat pump (ASHP).

But this was the first real-world indication that the system worked more-or-less as I had anticipated.

Understanding how the heat pump works is complex. And surprisingly, explaining what it costs to operate is more complicated than I expected, and slightly personally embarrassing.

Let me explain


Recently I switched to Octopus Energy and they charge me a daily standing charge plus a rate dependent on how much gas or electricity I use.

Standing Charges 

  • Gas: 23.85p per day.
  • Electricity: 25p per day.


  • 3.83p per kWh of gas.
  • 5.0p per kWh of electricity between 00:30 and 04:30.
  • 16.26p per kWh of electricity between 04:30 and 00:30.

All these rates are likely to change – increase – in the future.

Domestic Hot Water

As the graph shows, on 24th October, the heat pump used 1.6 kWh of electricity to create 4.9 kWh of heat in the hot water cylinder.

If I had used gas to do this using a boiler which was 90% efficient I would have needed to consume 4.9/0.9 = 5.4 kWh of gas which would have cost 5.4 kWh x 3.83p/kWh = 20.7p.

Using the heat pump I consumed 1.6 kWh of electricity at 5 p/kWh (because it was heated between 00:30 and 04:30) which cost 1.6 kWh x 5.0p/kWh = 8p.

So for heating domestic hot water, the heat pump is 62% cheaper than using gas.

If we had considered heating hot water during the summer, then the electricity would have been drawn from the battery which would had been charged by solar electricity, and so would be free.

Space Heating

As the graph shows, on 24th October, I used 2.3 kWh of electricity to create 10.0 kWh of heat in the house.

If I had used gas to do this using a boiler which was 90% efficient I would have needed to consume 10/0.9 = 11.1 kWh of gas which would have cost 11.1 kWh x 3.83p/kWh = 42.5 p.

Using the heat pump I consumed 2.3 kWh of electricity at 16.26 p/kWh which cost 2.3 kWh x 16.26p/kWh = 37.4p.

So for space-heating in this mild climate, the heat pump is 12% cheaper than using gas.

Actually its a bit better than this. Because we have a battery, we fill up with cheap rate electricity at 5 p/kWh so the actual cost of using the heat pump during the day is closer to 2.3 kWh x 5p/kWh = 11.5p i.e. 73% cheaper than using gas.

And actually, its even a little better still. Even at the end of October typically 50% of our electricity is derived from the solar PV system and stored in the battery. So the actual cost per unit is closer to 2.5p/kWh, so the actual cost of using the heat pump during the day at this time of year is closer to 2.3 kWh x 2.5p/kWh = 6p i.e. 85% cheaper than using gas.

The solar contribution will go down as we head into winter, but even in midwinter we still get ~ 2 kWh/day of generation, all of which is now captured in the battery. See the graphs below for more details.

Click image for a larger version. This complicated graph shows solar generation since the solar panels were installed in November 2020. The thin green lines with circles are daily generation. The thick pink line is 7 day running average. The big green circles are monthly averages, and the yellow circles are monthly averages for this location from 2005 to 2016.

Click image for a larger version. This graph shows household electricity consumption (averaged over ±1 week) during 2021. After the battery was installed, we used very little grid electricity for about 3 months. The blue line shows actual household consumption As we head into winter, our use of grid electricity is increasing. The dotted green lines show the range into which I expect grid electricity to fall depending on how cold the winter is, and how well the heat pump works.

Preliminary summary

For heating hot water, using a heat pump costs less than half the cost of using gas when using cheap-rate electricity.

For space-heating, using a heat pump costs 12% less than using gas even when using full-price electricity. And accounting for the solar generation and use of cheap-rate electricity stored in the battery, space-heating will cost a small fraction of what it would have cost using gas.

When I take account of all the variables, including the standing charges, the heat pump, the solar PV and the battery, I estimate that my annual bills from electricity and gas will be reduced from roughly £1600 to about £500 including £178/year of standing charges.

So financially, I am benefiting from my £60,000 of investment to the tune of around £1,100 per year.

But there are some complications…


I feel obliged to disclose that when I installed the heat pump, in addition to a £5000 government grant, the kind staff at Enhabit pointed out that I could I apply for funding under the Renewable Heat Initiative (RHI).

This scheme funds people who switch to using renewable technologies to heat their homes, compensating for the fact that electrical heating is generally more expensive than gas.

It didn’t seem to matter that – as I showed above – in my circumstances the heat pump would be cheaper than using gas.

So in addition to saving around £1,100 per year, the RHI scheme will additionally pay me £129.83/quarter or £519.32 per year for the next seven (7) years. Yes, I really did write that.

So broadly speaking, I don’t expect to pay anything at all for electricity or heating for the next 7 years.


I also feel obliged to disclose that when I installed the heat pump, the kind staff at Enhabit also pointed out that I could I apply for funding to monitor the heat pump performance using the so-called Metering and Monitoring Service Package (MMSP).

Under this arrangement, I needed to pay £1,156 to have a system installed which monitored the performance of the heat pump, and reported the data back live to OFGEM every 2 minutes.

This helps OFGEM establish the real world performance of heat pumps rather than relying on manufacturer’s specifications.

In return, I get:

  • Access to the data – that’s the data I used for the previous article.
  • A one off payment of £805
  • Annual payments of £115 (paid quarterly) for the next 7 years.

Overall this amounts to £1,610. This easily covers my outlay; provides important data to OFGEM; allows me to write endless blog articles; and reduces even further the cost of heating my home for the next seven (7) years.

Final Summary 

Friends, even without any Government deals, the heat pump would already be saving me money over a gas boiler.

And the use of solar PV and a battery makes the advantage even larger.

But taking advantage of the Government’s subsidies I find myself in receipt of a financial windfall – and it seems unlikely I will need to pay for electricity or heating for the next seven years.

Frankly, I am a little embarrassed.

However, even without these deals, switching to a heat pump would already be saving me money.

Keep warm!



Heat Pump: First Space-Heating Results

October 25, 2021

Friends, the unseasonably warm autumn has meant that I have had to wait until 23rd October for our 5 kW Vaillant Arotherm plus system to switch itself on.

I only have a couple of day’s data, but the results are interesting and promising. First I will show the data for each day and then discuss what it means at the end.

23rd October 2021

Click image for a larger version. The graph shows electrical power consumed by the heat pump and the thermal power delivered by the pump during the 24 hours of 23rd October 2021.

From midnight until 6 a.m., there is no space heating, but the heat pump operates for 1 hour to heat the domestic hot water (DHW) tank. Performance for hot water heating is described in these articles (1, 2)

From 6 a.m. until midnight the system is responding to the thermostat set at 19.5 °C and heated water is circulated around the radiators.

The heat pump operates 12 times up until 6 p.m. after which no additional heating was required.

As shown on the graph, 12.1 kWh of heat was delivered using only 2.9 kWh of electrical power. which over 12 hours amounts to an average heating power of around 1 kW using only 250 W of electrical power.

The ratio of thermal to electrical power is known as the coefficient of performance (COP) and this is summarised in the graph below

Click image for a larger version. The graph shows Coefficient of Performance (COP) of the heat pump during the 24 hours of 23rd October 2021.

The COP seems to increase slowly through the day peaking for a few minutes in each cycle at values as high as 6, but the space heating average is 4.2.

Click image for a larger version. The graph shows various temperatures relevant to heat pump operation during the 24 hours of 23rd October 2021. The external temperature; the internal temperature; the flow temperature in the radiators; the temperature of water in the DHW cylinder.

The initial flow temperature is ~34 °C which falls through the day to ~28 °C. This reduction in flow temperature is in response to the increase in external temperature from ~12 °C to ~15 °C.

This so-called ‘weather compensation’ allows the use of lower flow temperatures, which enables the system  to operate with the highest possible COP.

I was surprised that even with my unaltered radiators, flow temperatures of 35 °C were sufficient to warm the house.

24th October 2021

Here is the equivalent data to that for the 23rd – the graphs are similar and I show them only to show that the system seems to be behaving reproducibly.

Click image for a larger version. The graph shows electrical power consumed by the heat pump and the thermal power delivered by the pump during the 24 hours of 24th October 2021.

Click image for a larger version. The graph shows Coefficient of Performance (COP) of the heat pump during the 24 hours of 24th October 2021.

Click image for a larger version. The graph shows various temperatures relevant to heat pump operation during the 24 hours of 24th October 2021. The external temperature; the internal temperature; the flow temperature in the radiators; the temperature of water in the DHW cylinder.


My first conclusion is that the system works. This is quite a relief!

My second conclusion is that the system works slightly better than I had been hoping for.

Being able to heat the house with these low radiator temperatures means that over winter the average COP could be higher than my prior estimate of 3. This means I will use less energy, emit less CO2, and spend less than I had estimated.

My third conclusion is that average heating power was about 1 kW during both days. But the system has plenty of margin to deliver more power either by changing the duty cycle – the pump could stay on for longer – or by increasing the flow temperature.

As soon as the weather settles down to being reliably cold I will begin to carry out experiments to optimise the weather compensation.

My fourth conclusion will be the basis of another article – but I can see already that as far as heating is concerned – I am going to have a very cheap winter.

Keep warm.

1000 days of data

October 9, 2021

Friends, those of you who work with spreadsheets may know that Excelworks out dates in terms of the number of days since 1st January 1900. Or in other versions of Excel, the number of days since 1st January 1904!

Inspired by this arbitrary choice, sometime ago I chose 1st January 2018 as ‘day 1’ for my measurements of energy use around the house.

And from this arbitrary starting date, I have been measuring for just over 1000 days!

So I thought it would be nice to summarise what has happened in the last three years, and to speculate on what the coming winter might hold.

Heating Demand

Our internal thermostats have been set to 19 °C since ‘Day 1’.

So I calculate the demand for heating as the difference between 19 °C and the average weekly outside temperature.

The heating demand (averaged over ±2 weeks) for the last 3 years is shown below.

Click image for a larger version. Graph showing smoothed temperature demand versus day for since the summer of 2018. Also shown is a projection of what the coming winter has in store if it is the same as last year.

I obtained this data from the weather station in my back garden, but you can use the wonderful Meteostat website if you don’t have a nearby station. I used Meteostat to fill in occasional gaps in the data.

So far, this autumn seems to be several degrees warmer than the equivalent period in 2020 i.e. demand is lower.

Gas Use 

Since just before day 1, I have been reading the gas meter once a week. From this I can work out the average rate at which I am using energy (i.e. average power). In this blog I express this as kWh/day.

[To convert kWh/day to watts, multiply the number by 1000/24 = 41.7 e.g. 50 kWh/day = 2083 W i.e. ~2.1 kW.]

Click image for a larger version. Graph showing smoothed temperature demand versus day for since the summer of 2018 as in the previous graph. Also shown are the dates of various interventions, and smoothed gas consumption (kWh/day) plotted against the right-hand axis.

It is clear that gas consumption roughly follows temperature demand. However the Triple-Glazing and External Wall Insulation (EWI) have reduced the gas used to meet a given temperature demand by about half.

In the summers of 2019 and 2020, gas consumption fell to roughly 5 kWh/day, most of which (around 3.5 kWh) seems to have been for hot water, with the balance being used for cooking.

Since the heat pump installation, in July 2021, the gas is only used for cooking (~1.5 kWh/day) , and this will continue until my wife and I can get our heads around installing an electric oven & hob.

Electricity from the grid 

Since just before day 1, I have been reading the electricity meter once a week. From this I can work out the average rate at which I am using electricity from the grid.

Click image for a larger version. Graph showing smoothed temperature demand versus day for since the summer of 2018 as in the first graph. Also shown are the dates of various interventions, and smoothed electricity consumption (kWh/day) plotted against the right-hand axis. Also shown is the small amount of electricity which was exported to the grid.

It is clear that electricity consumption was – through 2019 and 2020 – roughly independent of temperature demand.

Solar panels were installed at around the same time as the EWI (~day 660) but this did not substantially affect the amount of electricity we drew from the grid until we installed a battery (~day 810). After that, we drew very little electricity from the grid during the period March to September.

After the heat pump installation (~day 930), we began heating the hot water with electricity rather than gas. But heat pumps require only about 30% of the energy which a boiler would use to heat water.

Looking to the winter ahead, we expect solar generation to fall to around 2 kWh/day on average, but electricity use to rise above our normal ~ 10 kWh/day because the heat pump will be used for space heating (varying with the temperature demand) as well as hot water (~1 kWh/day).

Last winter gas consumption peaked at 50 kWh/day. If the heat pump operates with a coefficient of performance of 3 – which seems a safe guess – then this should require around 50/3 ≈17 kWh/day of electricity.

Carbon Dioxide emissions. 

From the data above it is possible to roughly estimate the corresponding carbon dioxide emissions.

I have assumed that:

  • Each kWh of gas consumption results in 0.2 kg of CO2 emissions.
    • This is fixed by the chemistry of methane combustion.
  • Each kWh of electricity imported from the grid results in 0.24 kg of CO2 emissions
    • This is an average of the last three years carbon intensity (Link).

There is some uncertainty in the figures above, but the assumptions are pretty uncontroversial. These represent estimates of actual amounts of CO2 which entered the atmosphere due to ‘my’ actions.

How one should deal with exported solar electricity is more controversial. Some people point out that because of the way electricity is ‘dispatched’, solar generation directly displaces gas-fired generation. Thus each kWh of my solar generation avoids the emission of  0.45 kg of CO2 emissions from a gas-fired station.

One might argue that such exports are therefore equivalent to negative emissions even though no CO2 is actually removed from the atmosphere.

With this assumption the daily carbon emissions are summarised in the graph below.

Click image for a larger version. Graph showing estimated household carbon dioxide emissions per day since the summer of 2018. Also shown are the dates of various interventions, and the expected emissions for the coming year. As discussed in the text, avoided emissions due to exports of electricity are counted as negative emissions.

The graph shows that – subject to the uncertainty of the projection – since 2018:

  • Winter emissions will have fallen from 25 kg/day to 5 kg/day – a 5-fold reduction
  • Summer emissions will have fallen from emitting ~3 kg/day to avoiding ~1 kg/day of someone else’s emissions.

Click image for a larger version. Table shows estimated carbon dioxide emissions in tonnes for the last three years (period July to June) along with a forecast of the emissions in the coming year.

The net effect of all these changes is gas emissions are now negligible. Electricity emissions were roughly halved by installing solar panels and a battery, but in the coming year they will probably return to roughly their previous value because of the electricity used to operate the heat pump.

There are two interesting things to note about the forecast aside from the fact that it’s a forecast and we don’t know what the winter will be like.

Firstly, this assumes the heat pump COP will be 3. My hope is that it will be better than this because the well-insulated house should require such a small amount of heating that I should be able to lower the flow temperature in the radiators to 40 °C. At this temperature the heat pump has a specified performance closer to a COP of 4 even with an external temperature of -5 °C.

[Aside. As of 9th October 2021, I am still waiting excitedly for the weather to get cold enough that the heat pump will switch itself on so I can test this! The insulation appears to be good enough that the internal temperature is still greater than 19 °C (~ 20 °C) without any heating!]

Secondly, 90% of the CO2 emissions now arise from electricity. So as the grid gets greener (we hope) in coming years, these CO2 emissions should naturally reduce. If the target 100 gCO2 emissions per kWh is reached in 2030, the overall household emissions will fall to under 0.5 tonnes.

What else?

There are still one or two things I could do to reduce household CO2 emissions. But at this point my intention is just to measure how these existing interventions perform for a year or two.

In 2018/19, household CO2 emissions comprised the largest category of ‘my’ emissions, and now they are more similar to emissions from other activities: consumption, transport and investments (i.e. my pension)

In the coming year I hope to turn my attention to these much trickier categories.

I’ll let you know how it goes…

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