Archive for the ‘Climate Change’ Category

Me and Tea.

January 20, 2022

Friends, I have given up putting milk in my tea.

Why? Because as I wrote a few days ago, putting milk in my tea gives rise to annual methane emissions equivalent to almost a third of tonne of carbon dioxide.

On balance, I would rather avoid those emissions than experience the pleasure of putting milk in my tea.

My life in tea

I can still hazily remember being served milky tea with sugar as a child – perhaps I was 6 or 7.

Later on, drinking tea became a habit, and when I was probably 11 or 12, I gave up putting sugar in my tea.

And I have been drinking large amounts of tea each day – maybe 6 cups – ever since.

Around 12 years ago, I was concerned about my son’s seemingly unbreakable attachment to his iPod. To my surprise, he agreed to surrender his iPod if I gave up tea. I agreed, pleased we had reached an amicable bargain.

However I gave him back his iPod after 3 days, because in truth I was – and am – addicted to tea!

So changing the way I drink my tea is changing a life-long habit.

Life-long habits

Carbon dioxide and methane emissions are not very obvious – we generally don’t see them: the gases are invisible and have no smell. And they frequently take place at distant locations such as power stations or farms.

But the emissions are nonetheless real and their long term damage is on a scale that it is scarcely possible to imagine.

Additionally these emissions are entwined with our familiar ways of living.

  • Gas boilers keep us warm.
  • Cars provide mobility.
  • Aeroplanes take us on holiday.
  • Milk and Cheese and Butter taste great.
  • Tea with milk is ‘how normal people have tea’.

So acknowledging the reality of the emissions we give rise to and the harm they cause is hard both intellectually and emotionally.

Writing the article last week it became clear to me that I had to overcome my emotional attachment to milk in my tea.

Breaking these life-long habits is something we will all have to do if we want to create a way of living which does not damage the climate of our children’s future more than we already have.

More than milk

After 10 days I am happy to report that I am enjoying my milk-free tea and have now almost stopped reflexive visits to the fridge each time I make a cup!

I think I taste the tea itself rather more, but it is a very different kind of drink.

I have also been reducing use of butter and cheese and I have found alternatives that are perfectly acceptable in most recipes.

I find it hard to believe my use of dairy products will ever reach zero. But I can easily imagine reducing consumption by 90% or so.

Life is a long journey, and I never thought my journey would take me here: milk-free tea and minimising use of cheese and butter which I love!

It feels strange and unfamiliar.

But here I am.

A Watched Pan…

January 18, 2022

Click on Image for larger version.  A vision of domestic bliss in the de Podesta household. Apparatus in use for measuring the rate of heating of 1 litre of water on an induction hob.

In the beginning…

Friends, my very first blog article (written back on 1st January 2008 and re-posted in 2012) was about whether it is better to boil water with an electric kettle or a gas kettle on a gas hob.

Back then, my focus was simply on energy efficiency rather than carbon dioxide emissions. I had wanted to know how much of the primary energy of methane ended up heating the water. I did this by simply timing how long it took to boil 1 litre of water by various methods.

Prior to doing the experiments I had imagined that heating water with gas was more efficient because the fuel was used directly to heat the water. In contrast, even the best gas-fired power stations are only ~50% efficient.

What I learned back then was that gas cookers are terrible at heating kettles & pans! They were so much worse than I had imagined that I later spent many hours with different size pans, burners, and amounts of water just so I could believe my results!

Typically gas burners only transferred between 36% and 56% of the energy of combustion to the water – the exact fraction depending on the size and power of the burner. Heating things faster with a bigger burner was less efficient. Using a small flame and a very large pan, I could achieve an efficiency of 83%, but of course the water heated only very slowly.

This inefficiency was roughly equivalent to or worse than the inefficiency of the power station generating electricity, and so I concluded that electric kettles and gas kettles were similarly inefficient in their use of the primary energy of the gas. But that using electric kettles allowed one to use the correct amount of water more easily, and so avoided heating water that wasn’t used.

14 years later…

After a recent conversation on Twitter (@Protons4B) I thought I would look at this issue again.

Why? Well two things have changed in the last 14 years.

  • Firstly, electricity generation now incorporates dramatically more renewable sources than in 2008 and so using electricity involves ever decreasing amounts of gas-fired generation.
  • Secondly, I am now concerned about emissions of carbon dioxide resulting from lifestyle choices.

Also being a retired person, I now have a bit more time on my hands and access to fancy instruments such as thermometers.

The way I did the experiments is described at the end of the article, but here are the results.

Results#1: Efficiency

The chart below shows estimates for the efficiency with which the electrical energy or the calorific content of the gas is turned into heat in one litre of water. My guess is these figures all have an uncertainty of around ±5%.

  • The kettle was close to 100% efficient:
  • The induction hob was approximately 86% efficient
  • The Microwave oven was approximately 65% efficient

In contrast, heating the water in a pan (with a lid) on a gas hob was only round 38% or 39% efficient.

Click on Image for larger version. Chart showing the efficiency of 5 methods of heating 1 litre of water. 100% efficiency means that all the energy input used resulted in a temperature rise. The two gas results were for heating pans with two different diameters (19 cm and 17 cm).

It was particularly striking that the water heated on the gas burner (~1833 W) took 80% longer to boil than on the Induction hob (~1440 W) despite the heating power being ~20% less on the induction hob.

Click on Image for larger version. Chart showing the rate of heating for each of the 5 methods of heating 1 litre of water. Notice that the water heated on the gas burner (~1833 W) took 80% longer to boil than on the Induction hob (~1440 W) despite the heating power being ~20% less on the induction hob. Notice that up until 40 °C, the microwave oven heats water as fast as the gas hob, despite using half the power!

Results#2: Carbon Dioxide Emissions 

Based on the average carbon intensity of electricity in 2021 (235 g CO2/kWh), boiling a litre of water by any electrical means results in substantially less CO2 emissions than using a pan (with a lid) on a gas burner.

I performed these experiments on 17th January 2021 between 4 p.m. and 7 p.m. when the carbon intensity of electricity was well above averages: ~330 g CO2/kWh. In this case, boiling a litre of water in a kettle or induction hob still gave the lowest emissions, but heating water in a microwave oven resulted in similar emissions to those arising from using a pan (with a lid) on a gas burner.

Click on Image for larger version. Charts showing the amount of carbon dioxide released by heating 1 litre of water from 10 °C to 100 °C using either electrical methods or gas. The gas heating is assumed to have a carbon intensity of 200 gCO2/kWh. The left-hand chart is based on the carbon intensity of 330 gCO2/kWh of electricity which was appropriate at the time the experiments were performed. The right-hand chart is based on the carbon intensity of 235 gCO2/kWh of electricity which was the average value for 2021. Electrical methods of heating result in lower CO2 emissions in almost all circumstances.

Results#3: Cost 

Currently I am paying 3.83 p/kWh for gas and 16.26 p/kWh for electricity i.e. electricity is around four times more expensive than gas.

These prices are likely to rise substantially in the coming months, but it is not clear whether this ratio will change much.

So sadly, despite gas being the slowest way to heat water and the way which releases the most climate damaging gases, it is still the cheapest way to heat water. It’s about 40% cheaper than using an electric kettle.


For the sake of the climate, use an electric kettle if you can.


That was the end of the article and there is no need to read anymore unless you want to know how I made these measurements.


Estimating the power delivered to the water + vessel

  • For electrical measurements I paused the heating typically every 30 seconds, and read the plug-in electrical power meter. This registered electrical energy consumed in kWh to 3 decimal places.
    • I fitted a straight line to the energy versus time graph to estimate power.
  • For gas measurements I read the gas meter before and after each experiment. This reads in m^3 to 3 decimal places and I converted this volume reading to kWh by multiplying 11.19 kWh/m^3.
    • The gas used only amount to 0.025 m^3 so uncertainty is at least 4% from the digitisation.
    • I divided by the time – typically 550 seconds – to estimate the power.

Mass of water

  • I placed the heating vessel (kettle, pan, jug) on the balanced and tired (zeroed) the reading.
  • I then added water until the vessel read within 1 g of 1000 g. Uncertainty is probably around 1% or 10 g.

Heating rate with 100% energy conversion

  • Based on the power consumed, I estimated the ideal heating rate if 100% of the supplied power caused temperature rises in the water by using the equations.

  • I assumed the average specific heat capacity of water of the range from 10 °C to 100 °C was 4187 J/ (kg °C)

Measuring the temperature.

  • For electrical measurements I paused the heating typically every 30 seconds, stirred the liquid with a coffee-stirrer for 2 or three seconds, and then took the temperature using a thermocouple probe..
  • For gas measurements it wasn’t possible to the pause the heating because of the way I was measuring the power. So about 10 seconds before the reading was due I slipped the coffee stirrer under the lid to mix the water.

Estimating the rate of temperature rise.

  • For all measurements I fitted a straight line to the temperature versus time data, using only data points below approximately 80 °C to avoid the effects of increased temperature losses near to the boiling point.

Mass of the ‘addenda’.

  • The applied power heated not only the water but also its container.
  • The heat capacity of the 19 cm stainless steel pan (572 g) was roughly 6% of the heat capacity of the water.
  • I chose not to take account this heat capacity because there was no way to heat water with a container. So the container is a somewhat confounding factor, but allows more meaningful comparison of the results.

Efficiency of boiling

  • I estimated efficiency by comparing the actual warming rate with the ideal warming rate.
  • I then calculated the energy required to heat 1 kg of water from 10 °C to 100 °C, and multiplied this by the efficiency.
  • In this way the result is relevant even if all the measurements did not start and stop at the same temperatures.


  • I heated the water in the microwave in a plastic jug which did not have a tight fitting lid. I am not sure if this had an effect.
  • I did notice that the entire microwave oven was warm to hot at the end of the heating sequence.

Would you like milk with your tea?

January 9, 2022

Every blog article starts with a mug of tea.

Friends, I am addicted to tea.

I like all kinds of tea, but my favourite is a basic brew, with milk.

The ritual of settling down with a mug of hot tea is an essential pre-requisite for any kind of concentration – such as writing this article.

This the main use of milk in the household and each week my wife and I consume around 2 litres.

So per year we use roughly 100 litres of milk.

Looking online, I find this corresponds to emissions (mainly of methane) which are equivalent to 315 kg of carbon dioxide per year: almost a third of a tonne!

I think there is a lot of uncertainty in that estimate, and it probably varies from country to country. But taking it at face value, it is a truly colossal impact from a very mundane activity.

Click image for a larger version. The graph shows the global warming impact of emissions associated with production of milk and cheese in terms of the equivalent amount of CO2 emissions which would have the same impact. The data is from Our World in Data

After all the work I have had done on the house, annual heating and electrical emissions have fallen from 3.7 tonnes to about 0.7 tonnes.

So emissions from drinking tea alone have become a significant fraction (∼50%) of general household emissions!

What to do?

As far as I know, the only way to avoid these emissions, is to stop drinking milk –  or to reduce the amount I drink substantially.

The problem

The problem with this solution is that I – like millions of other people – really like having milk in my tea.

I have tried using milk alternatives derived from oats, almonds, and soya. These products look like milk and come in packaging which suggests they are in some way similar to milk.

But they do not taste even remotely like milk.

Additionally, I am emotionally attached to the idea that milk comes from cows that live on farms. For someone who is basically a city-dweller, this connection feels meaningful.

So at the start of this new year I am facing a dilemma.

What to do?

As far as I know, the only way to avoid these emissions, is to stop drinking milk –  or to reduce the amount I drink substantially.

Switching to the plant-based alternatives is just not acceptable, which leaves me with just two options:

  • Abandoning milk in my tea altogether. This is an extreme option, but one I am keeping under review.
  • Currently I am experimenting with a 50:50 mixture of milk with Oat ‘derivative’ product. It is predictably, not as nice as just milk, but it is borderline acceptable. But the emissions are still substantial.

I will let you know how it goes when I have a few more weeks under my belt.

The wider problem

The wider problem with this solution is that I haven’t even mentioned butter or cheese, other dairy-based staples of my diet.

Because of the large amount of raw milk used, each kilogram of cheese is apparently is associated with 24 kg of CO2 equivalent emissions (mostly as methane).

My wife and I eat – and enjoy prodigiously – about 0.5 kg of Davidstow Cheddar each week. This corresponds to around 25 kg per year, and emissions with the equivalent impact of 600 kg of carbon dioxide per year.

Basically the emissions associated with our cheese consumption have an impact roughly equivalent to all the electricity we use to heat and run the house for a year!

Fortunately Our World in Data does not have information about butter. I say ‘fortunately’ because I feel sure it will be bad.

The wider truth is that in regards to my house, all the changes I have made to reduce carbon dioxide emissions have been expensive, but they have not really affected my quality of life.

But it seems that emissions from some of the basics of my diet, foods I love and have eaten all my life, are apparently responsible for more annual carbon dioxide emissions than my entire house!

Reducing these emissions is going to be much tougher and feel much more like a personal sacrifice with a very direct and (at least initially) negative impact on my quality of life.

I guess nobody said it would be easy.

I am going to sit down now with a nice cup of tea to think about this…





Carbon accounts 2021: looking back and looking ahead

January 2, 2022

Click the image for a larger version. Representation of the reduction in domestic carbon dioxide emissions from gas and electricity over the last four years. Also shown are the steps I have taken to achieve these reductions.

[Article Summary: it is actually pretty difficult to estimate carbon dioxide emissions – but it is important to try.]

Friends, it’s New Year’s Day.

And at this still point in the ever-rolling cycle of the years, it seemed like a good time to summarise progress on my project to reduce carbon dioxide emissions from the house.

It may seem like a good time, but actually this is not a good time at all to summarise annual emissions. The best time to do this is 6 months away in the summer. This is because household emissions peak in the winter and fall to practically zero in the summer.

You can see this on the graphs below which show cumulative electricity and gas consumption through the last few years.


To estimate the emissions from gas I have estimated the meter reading which I would have had in the summer of 2018 and used this as a baseline.

I then subtract weekly meter readings from this baseline and convert them to kWh of thermal energy, and then multiply the number of kWh of gas by 200 gCO2 per kWh.

Burning methane gas in a boiler releases around 183 gCO2 per kWh of gas – as documented in this official spreadsheet. (Look up the ‘Fuels’ tab and use cell F42) But some additional CO2 emissions are associated with delivering the gas to my home: compressors drive the gas along pipelines and ships deliver gas across the oceans.

The actual value of these ‘upstream’ emissions is difficult to know precisely, but actual experts suggest it amounts to roughly 24 gCO2 per kWh of gas delivered. So in principle the best estimate of CO2 emissions from gas delivered would be 183 + 24 = 207 gCO2 per kWh. This figure is 4% larger than the figure I used.

However, it is likely that direct methane leaks at wells and in handling plants are underestimated (example). Evaluated over a decade after leakage, methane is 84 times more powerful as a greenhouse gas than CO2. So if even 0.1% of methane leaked on its way to my home, the CO2 equivalent emission would be increased by 17 gCO2/kWh. However some people estimate that actual leakage is more than 1%. If that were so that would practically double the climate impact of using gas to nearly 400 gCO2 per kWh. The unknown magnitude of leaks is just one more reason to stop using methane gas.

Given these uncertainties I have used a figure of 200 gCO2 per kWh as a likely underestimate of true emissions which is not obviously wrong, but which is a convenient round number.

Click the image for a larger version. The graph shows the cumulative emissions of carbon dioxide emissions from domestic gas use over the last three years.

The graph above is based on weekly gas meter readings.

The data form a series of ‘steps’ and it is clear that measuring from one step level to the next gives a better estimate of the yearly emissions than choosing an arbitrary point on the ‘riser’ of the staircase. This implies measuring from summer to summer

The reason is that if the winter is mild before the New Year but cold after New Year, the emissions fall in different years even though they arise from the same winter.

But however one analyses the data, it is clear that there has been no step this winter of 2021/22. We now use gas only for cooking and I hope shortly to stop even this use and make that curve go entirely flat. For ever!


To estimate the emissions from electricity I have multiplied meter readings in kWh by 230 gCO2 per kWh.

As with gas, it is not obvious how much carbon dioxide is emitted for each kWh of electricity consumed from the grid. Depending on the generation source, the so-called carbon intensity of the electricity can vary significantly. For example, as I write – with low demand and high winds – the carbon intensity of the electricity is just 111 gCO2 per kWh.

The MyGridGB web site maintains a live monitor of carbon intensity, and shows an annual summary of average carbon intensity through the year.

Over the last 3 years the average carbon intensity in the UK has been 245, 222, and 235 gCO2 per kWh. Since these figures are within a few percent of each other I have used a rounded value of 230 gCO2 per kWh for the entire range of the measurements.

Click the image for a larger version. The graph shows the average carbon intensity for each year. The values for eth last three years have been 245, 222, and 235 gCO2 per kWh. The red line at 100 gCO2 per kWh is the target carbon intensity for the year 2030.

Multiplying the number of kWh used by 230 gCO2 per kWh tells me the emissions associated with my use of grid electricity. This is shown below on the same vertical scale as on the gas graph above.

Click the image for a larger version. The graph shows the cumulative emissions of carbon dioxide emissions from domestic electricity use over the last three years. The scale is the same as in the previous graph showing carbon dioxide emissions from gas use.

The graph above is based on weekly electricity meter readings.

During 2019 and 2020 carbon emissions occurred at a regular rate with no seasonal steps. Even, looking closely, I cannot detect the point in November 2020 when solar panels were installed.

But in March 2021 when our Powerwall battery was installed, the curve goes flat as the combination of solar panels and battery was sufficient to take us practically off-grid for the summer.

In September 2021, as solar generation weakened, we began to draw electricity from the grid again, and also began heating with electricity using our air-source heat pump. Currently we are using 20 to 25 kWh/day – more than twice the previous rate. This will probably continue until March

It is hard to estimate precisely, but I think – with colder months ahead – the summer-to-summer emissions will be similar or slightly less than last year.

Gas and Electricity

To estimate the emissions from both gas and electricity use, I have added the data from the two previous graphs together.

Click the image for a larger version. The graph shows the cumulative emissions of carbon dioxide emissions from domestic gas and electricity use over the last three years. The scale is the same as in the previous graphs.

Anticipating data from the spring of 2022, it looks like emissions will have fallen from about 3.6 tonnes in 2018/2019 to (hopefully) only 0.7 tonnes in 2021/22. This is an 80% reduction.

But even though this has already been a tedious article, this is not quite the end of the story.

Embodied Carbon

To achieve that 80% cut in annual emissions, I had to buy things which involved the emission of carbon dioxide – so called embodied carbon.

It is difficult to estimate the amount of embodied carbon in a particular object, but after quite some effort I have come up with the following estimates.


Embodied tonnes of CO2

EWI PU Boards


EWI Mortar


Argon Triple Glazing


Solar Panels




Heat Pump


Air Conditioning




They amount to 10.5 tonnes of embodied carbon. To find out when this embodied carbon has been ‘paid for’ I need to compare the CO2 emissions described above with the so-called counter-factual: the emissions which would have occurred if I had done nothing.

If I had done nothing then my guess is that emissions over the last 4 years would be simply 4 x 3.6 tonnes of CO2 – or 14.4 tonnes.

Actually, CO2 emissions over the last 4 years have been 3.6 + 2.9 + 2.0 + 0.7 = 9.2 tonnes.

So my ‘investment’ of 10.5 tonnes of embodied carbon will have saved 5.2 tonnes of emissions by summer 2022, and should continue to save (3.6 – 0.7) = 2.9 tonnes per year for several years to come. So I should ‘break even’ during the year 2024. Everything beyond that will be pure emissions savings.

Looking ahead

Click the image for a larger version. The graph shows the estimated household emissions from 2018 to 2040. The red line shows the emissions which would have occurred if I had done nothing. The green line shows the emissions according to the current plan. The dotted line shows emissions if the money I pay to Climeworks is not a scam.

Looking further ahead, the tonnes of carbon ‘debt’ I have incurred seems less significant. And carbon dioxide emissions avoided by 2040 amount to 60 tonnes.

Additionally since March 2021, I have been paying Climeworks £40/month to permanently remove 50 kg/month (0.6 tonnes/year) of carbon dioxide. If they are actually doing this – and I have no real way of knowing! – then our household is very nearly carbon neutral.

However none of what I have discussed accounts for emissions arising from consumption, or travel, or from my pension investments – all of which are likely to be quite significant.

So there is still lots to do in the new year.

I love Greta Thunberg

December 30, 2021

Click image for a larger version. My son gave me a Christmas Tree decoration in the likeness of Greta Thunberg.

Friends, love is a strong word.

Back in 2012 I wrote that I loved James Hansen. If you haven’t heard it, I strongly recommend his TED talk.

I wrote:

When I hear him speak I feel I am listening to a human being who understands enough to feel compelled to shout ‘Fire’ in the ‘cinema’ of the modern world. He feels that no matter what the consequences, we must face up to the climate challenge ahead. Being prepared to be arrested for his insistence that the US government should listen to what the science (they have paid for!) has to say seems like an act of great bravery to me.

And today I would like to declare a similar – but different – admiration for Greta Thunberg. Greta is not after all, a world-leading scientist.

The fact that my son gave me a Christmas Tree decoration in the likeness of Greta Thunberg – but not James Hansen – is testament to their different roles. He also gave me a book of Greta’s speeches and not a copy of James Hansen’s papers (e.g. this one from 1981).

Greta Thunberg has unintentionally become a global cultural phenomenon. But having read her book, I can assure you it is not because of her oratory. It is because of her unflinching honesty.

Reading her words addressed to old people like me, I do not feel inspired: I feel shamed.

I will leave you with a quote from the book: Greta’s speech to the UK Parliament in 2019. I hope you too will be as affected by her honesty as I have been.

UK Parliament 2019

23 April 2019:

Is my microphone on? Can you hear me?

Around the year 2030, 10 years 252 days and 10 hours away from now, we will be in a position where we set off an irreversible chain reaction beyond human control, that will most likely lead to the end of our civilisation as we know it. That is unless in that time, permanent and unprecedented changes in all aspects of society have taken place, including a reduction of CO2 emissions by at least 50%.

And please note that these calculations are depending on inventions that have not yet been invented at scale, inventions that are supposed to clear the atmosphere of astronomical amounts of carbon dioxide.

Furthermore, these calculations do not include unforeseen tipping points and feedback loops like the extremely powerful methane gas escaping from rapidly thawing arctic permafrost.

Nor do these scientific calculations include already locked-in warming hidden by toxic air pollution. Nor the aspect of equity – or climate justice – clearly stated throughout the Paris agreement, which is absolutely necessary to make it work on a global scale.

We must also bear in mind that these are just calculations. Estimations. That means that these “points of no return” may occur a bit sooner or later than 2030. No one can know for sure. We can, however, be certain that they will occur approximately in these timeframes, because these calculations are not opinions or wild guesses.

These projections are backed up by scientific facts, concluded by all nations through the IPCC. Nearly every single major national scientific body around the world unreservedly supports the work and findings of the IPCC.

Did you hear what I just said? Is my English OK? Is the microphone on? Because I’m beginning to wonder.

During the last six months I have travelled around Europe for hundreds of hours in trains, electric cars and buses, repeating these life-changing words over and over again. But no one seems to be talking about it, and nothing has changed. In fact, the emissions are still rising.

When I have been travelling around to speak in different countries, I am always offered help to write about the specific climate policies in specific countries. But that is not really necessary. Because the basic problem is the same everywhere. And the basic problem is that basically nothing is being done to halt – or even slow – climate and ecological breakdown, despite all the beautiful words and promises.

The UK is, however, very special. Not only for its mind-blowing historical carbon debt, but also for its current, very creative, carbon accounting.

Since 1990 the UK has achieved a 37% reduction of its territorial CO2 emissions, according to the Global Carbon Project. And that does sound very impressive. But these numbers do not include emissions from aviation, shipping and those associated with imports and exports. If these numbers are included the reduction is around 10% since 1990 – or an an average of 0.4% a year, according to Tyndall Manchester.

And the main reason for this reduction is not a consequence of climate policies, but rather a 2001 EU directive on air quality that essentially forced the UK to close down its very old and extremely dirty coal power plants and replace them with less dirty gas power stations. And switching from one disastrous energy source to a slightly less disastrous one will of course result in a lowering of emissions.

But perhaps the most dangerous misconception about the climate crisis is that we have to “lower” our emissions. Because that is far from enough. Our emissions have to stop if we are to stay below 1.5-2 °C of warming. The “lowering of emissions” is of course necessary but it is only the beginning of a fast process that must lead to a stop within a couple of decades, or less. And by “stop” I mean net zero – and then quickly on to negative figures. That rules out most of today’s politics.

The fact that we are speaking of “lowering” instead of “stopping” emissions is perhaps the greatest force behind the continuing business as usual. The UK’s active current support of new exploitation of fossil fuels – for example, the UK shale gas fracking industry, the expansion of its North Sea oil and gas fields, the expansion of airports as well as the planning permission for a brand new coal mine – is beyond absurd

This ongoing irresponsible behaviour will no doubt be remembered in history as one of the greatest failures of humankind.

People always tell me and the other millions of school strikers that we should be proud of ourselves for what we have accomplished. But the only thing that we need to look at is the emission curve. And I’m sorry, but it’s still rising. That curve is the only thing we should look at.

Every time we make a decision we should ask ourselves; how will this decision affect that curve? We should no longer measure our wealth and success in the graph that shows economic growth, but in the curve that shows the emissions of greenhouse gases. We should no longer only ask: “Have we got enough money to go through with this?” but also: “Have we got enough of the carbon budget to spare to go through with this?” That should and must become the centre of our new currency.

Many people say that we don’t have any solutions to the climate crisis. And they are right. Because how could we? How do you “solve” the greatest crisis that humanity has ever faced? How do you “solve” a war? How do you “solve” going to the moon for the first time? How do you “solve” inventing new inventions?

The climate crisis is both the easiest and the hardest issue we have ever faced. The easiest because we know what we must do. We must stop the emissions of greenhouse gases. The hardest because our current economics are still totally dependent on burning fossil fuels, and thereby destroying ecosystems in order to create everlasting economic growth.

“So, exactly how do we solve that?” you ask us – the schoolchildren striking for the climate.

And we say: “No one knows for sure. But we have to stop burning fossil fuels and restore nature and many other things that we may not have quite figured out yet.”

Then you say: “That’s not an answer!”

So we say: “We have to start treating the crisis like a crisis – and act even if we don’t have all the solutions.”

“That’s still not an answer,” you say.

Then we start talking about circular economy and rewilding nature and the need for a just transition. Then you don’t understand what we are talking about.

We say that all those solutions needed are not known to anyone and therefore we must unite behind the science and find them together along the way. But you do not listen to that. Because those answers are for solving a crisis that most of you don’t even fully understand. Or don’t want to understand.

You don’t listen to the science because you are only interested in solutions that will enable you to carry on like before. Like now. And those answers don’t exist any more. Because you did not act in time.

Avoiding climate breakdown will require cathedral thinking. We must lay the foundation while we may not know exactly how to build the ceiling.

Sometimes we just simply have to find a way. The moment we decide to fulfil something, we can do anything. And I’m sure that the moment we start behaving as if we were in an emergency, we can avoid climate and ecological catastrophe. Humans are very adaptable: we can still fix this. But the opportunity to do so will not last for long. We must start today. We have no more excuses.

We children are not sacrificing our education and our childhood for you to tell us what you consider is politically possible in the society that you have created.

We have not taken to the streets for you to take selfies with us, and tell us that you really admire what we do.

We children are doing this to wake the adults up.

We children are doing this for you to put your differences aside and start acting as you would in a crisis.

We children are doing this because we want our hopes and dreams back.

I hope my microphone was on. I hope you could all hear me.

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


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