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2 Gs

May 14, 2023

Friends, sometime today, 15 May 2023, I will have been alive for about 2 billion seconds. Or as we metrologistas say, two gigaseconds (Gs).

Long periods

We traditionally celebrate our ages in years – orbits around our Sun. And at my age, these milestones pass with tedious regularity. And we take special note of decades, of which we might hope to live for a couple of handfuls. We mark these special birthdays with fear or wild celebration. But these super-long periods are interesting.

Astrologers discuss the significance of the ‘Saturn Return‘ which occurs typically every 29 years and this has something in common with a gigasecond, which is 31.7 years.

I am fortunate to have made it past my first gigasecond – which could be considered to encompass my youth. And  I feel especially fortunate to be alive to experience my second complete gigasecond – which I guess has encompassed what we generally refer to as middle age. But realistically, I am unlikely to be alive to celebrate my third gigasecond, which will pass just after my 95th birthday. This is not a maudlin reflection, this is just statistics. And even considering the oldest people who have ever lived, no one has ever reached four gigaseconds.

And so while it is unusual to measure the length of a human life in seconds – very roughly heartbeats – the gigasecond is a useful unit because our lives can be considered on such a very simple scale: 1, 2, 3…

Friends, whatever your age, I urge you, if you can, to pause to enjoy the day on which I embark on my third gigasecond.

Best wishes


Food and Climate Change Without The Hot Air: A Review

May 11, 2023

Friends, as you may recall I have reduced emissions from my home from about 3.7 tonnes per year to around 0.7 tonnes per year, and this should come down in future years as the electricity supply incorporates a greater fraction of renewable generation.

But my house is only one of the ways in which I emit carbon dioxide. And in the last year I have been working on reducing carbon dioxide emissions associated with my consumption of food. This is a much cheaper endeavour financially, but one which forces me to address habits I have ingrained over the my life. And that makes it hard emotionally.

My minor triumph in this area was giving up milk in tea and coffee (link). Throughout my life I have drunk prodigious quantities of tea and coffee with milk, and so this was initially a real sacrifice. But one year later, I have incorporated this into my lifestyle. And I have been working on reducing the residual meat and dairy in my diet, though rather less successfully.

In order to improve my understanding, last year I bought “Food and Climate Change without the hot air” (FACCWTHA) by Sarah Bridle and read it avidly. I had meant to write a review last year, but somehow despite the slow pace of my life in Teddington, I somehow could not find the time! But a couple of months ago I heard Professor Bridle speak and felt re-inspired.

Admirably (in my opinion), Professor Bridle began her career researching the physics of extra-galactic astronomy, but switched fields to help address climate change. Her research now focuses on using data to help transform food systems.


Friends, working out the carbon dioxide emissions associated with food is a colossally difficult problem. It has all the hallmarks of carbon accounting – which I hate!

Let me give you an example: when one goes to the supermarket to buy – say – a tomato, there is no fixed answer to the question: how much carbon dioxide emission is associated with each tomato. It depends on the time of year, the country of origin, how it was grown, how it was transported to the UK, how we will take it back home from the shop, and how it will be cooked. And the same is true for almost everything we eat.

And so FACCWTHA is not a database in which one can just look up a food stuff. Rather it is a narrative description of the factors which affect the emissions associated with our food choices. Consider it a book written to raise your own consciousness.

In the introduction Professor Bridle writes:

When I first learned about the impact of food on climate change I went vegan for a while. I put my jacket potato into the oven for two hours and waited around smugly with my can of beans, unpacking my suitcase after a transatlantic flight. I drove to the shop 3 km (1.86 miles) down the road just to buy some plant milk, and also probably picked up a pack of green beans flown in from another continent.

Following up on this paragraph, Professor Bridle then compares the emissions associated with her flight, ‘popping’ to the shops by car, and using an oven for two hours.

  • Flight: about 9 kg/day averaged over 1 year
  • Car: about 0.5 kg
  • Oven: about 2.5 kg

The emissions associated with the potato itself (about 0.3 kg) are almost negligible.

In the introduction Professor Bridle explains emissions associated with food amount to about a quarter of global emissions – equivalent to about 10 billion tonnes per year. If we divide that by the world population (around 8 billion), and by 365 we arrive at around 3 kg/day/person. But in wealthy countries, we often have associated emissions much higher than this. So Professor Bridle suggests we use around 3 kg/day as a target.

It might seem that there is no hope: that everything we eat is part of a web of actions and connections that can never be disentangled. And indeed, it is practically impossible to place a definitive amount of emissions on a specific purchase of a specific type of food. But there is hope!

In FACCWTHA Professor Bridle takes us on a tour through typical breakfasts, lunches and dinners and very quickly patterns emerge, and one begins to the learn the things which tend to have low associated emissions and things which tend to have high emissions.

For example, when she returns to analyse baked potatoes in Chapter 11 we are already familiar with the elements of a high emission meal. A single baked potato cooked in an oven and accompanied by ‘lashings’ of cheese could lead to more than 3 kg of emissions (a day’s worth or emissions) whereas the same baked potato cooked in a microwave with low-impact toppings might lead to only 0.3 kg of emissions.

(Cooking tip! Personally when I prepare baked potatoes I microwave them (typically four at a  time) for 15 minutes first before transferring them to a small oven to crisp up).

Click image for a larger version: Graphic illustrating the different emissions associated with a baked potato cooked in different ways and accompanied by different toppings.

As we go through the classic meals of the day, the themes emerge:

  • Food which has been flown to the UK has high associated emissions, but food which has been shipped to the UK can have surprisingly low emissions.
  • Plants of all kinds generally have low associated emissions arising mainly from cooking, transport and the use fertiliser.
  • Dairy products have higher emissions than vegetables and products such as cheese – that require large volumes of milk – can have very high associated emissions.
  • And meat products have high emissions, most especially beef and mutton because of the methane emissions associated with the animals processing of grass.

And although the book is not a database, it does handily summarise the carbon dioxide emissions associated with 1 gram of a variety of foods: I’ve compiled a list at the end of this article.

Most foods give rise to – very roughly – their own mass in  emissions. And since people typically eat between 1 kg and 2 kg of food per day, then it should be no problem to keep below 3 kgCO2 emissions per day.

However there are a few foodstuffs that have such high associated emissions, that in order to keep below 3 kgCO2/day, one would needs to restrict their consumption to being only – on average – a few grams per day.

In practical terms this corresponds to restricting consumption of these foodstuffs, making them rare ‘treats’ rather than regular staples. And these foodstuffs are, unsurprisingly, meat, cheese, greenhouse-grown vegetables, and farmed fish.

For example to restrict emissions to below 1 kgCO2/day on average one would need to eat less than 150 g of beef per week.

Click image for a larger version: Graphic illustrating the different emissions associated with three different evening dinner options.


Friends, FACCWTHA addresses an urgent issue. How do we reduce carbon dioxide emissions associated with what we eat? The book is engagingly written, well-researched with extensive references, and after reading it I found my consciousness had indeed raised, and it was now just up to me.

After reading the book I was reminded of Michael Pollen’s assertion that to eat well we should:

“Eat food. Not too much. Mostly plants”

And I understood that far from being mysterious, reducing carbon dioxide emissions from the food I eat is simple: but it requires that I re-balance my diet. This is hard because I am an old man and my habits are well set. But even at my advanced age, I can happily recommend this book, and suggest that you browse You Tube for inspiration for recipes. Personally, I have enjoyed the easy-vegan style of Will Yeung.

Carbon Intensity of foodstuffs

The table below is compiled from FACCWTHA and shows roughly the emissions associated with eating 1 gram of each of the foodstuffs listed.  The columns also show how much of that food can be eaten each day (and each week) to reduce that food item’s impact to 1 kg CO2/day on average.

Category 1 gram of … produces


kg to keep below 1 kgCO2/day kg to keep below 1 kgCO2/week
meat steak 46 0.02 0.152
meat lamb 43 0.02 0.163
drink instant coffee powder 17 0.06 0.412
dairy cheese 16 0.06 0.438
salad tomatoes, heated greenhouse 13 0.08 0.538
meat ham 11 0.09 0.636
sweets milk powder 9 0.11 0.778
meat chicken 9 0.11 0.778
dairy butter 8 0.13 0.875
transport anything 5000 km by air 8 0.13 0.875
fish salmon 8 0.13 0.875
packaging aluminium 6 0.17 1.17
egg egg 5 0.20 1.40
dairy cream 5 0.20 1.40
fish cod 5 0.20 1.40
snacks peanut butter 4 0.25 1.75
drink teabag 3 0.33 2.33
drink sugar 3 0.33 2.33
spread jam 3 0.33 2.33
grain cereal 3 0.33 2.33
spread relish 3 0.33 2.33
vegan Quorn slices 3 0.33 2.33
vegetables frozen oven chips 3 0.33 2.33
sweets cocoa 3 0.33 2.33
snacks peanuts 3 0.33 2.33
packaging plastic 3 0.33 2.33
packaging carboard landfilled 2 0.5 3.50
drink milk 2 0.5 3.50
salad sweetcorn 2 0.5 3.50
beans baked beans 2 0.5 3.50
packaging steel (for cans) 2 0.5 3.50
dairy yoghurt 2 0.5 3.50
fruit strawberries 2 0.5 3.50
fruit raspberries 2 0.5 3.50
snacks almonds 2 0.5 3.50
snacks crisps 2 0.5 3.50
drink orange juice 2 0.5 3.50
grain rice 2 0.5 3.50
beans beans (tinned) 1.8 0.6 3.89
spread vegetable spread 1.5 0.7 4.67
salad lettuce 1.5 0.7 4.67
vegan vegan cheese 1.4 0.7 5.00
packaging carboard composted 1 1.0 7.00
vegan Quorn pieces 1 1.0 7.00
spices spices 1 1.0 7.00
drink beer on tap 1 1.0 7.00
drink wine on tap 1 1.0 7.00
drink plant milk 0.8 1.3 8.75
grain flour 0.8 1.3 8.75
staple bread 0.8 1.3 8.75
packaging re-cycled plastic 0.8 1.3 8.75
vegetables french/green beans 0.8 1.3 8.75
fruit bananas 0.7 1.4 10.00
vegetables potato 0.6 1.7 11.67
beans beans 0.6 1.7 11.67
packaging glass 0.6 1.7 11.67
vegetables carrots 0.6 1.7 11.67
fruit apples (local in season) 0.4 2.5 17.50
vegetables cabbage 0.4 2.5 17.50
transport anything from NZ by ship 0.3 3.3 23.33
fruit oranges (local!) 0.3 3.3 23.33
snacks fizzy drink on tap 0.2 5.0 35.00
transport anything 400 km by truck 0.05 20.0 140.00
drink tap water 0.001 1000.0 7000.00

Air Source Heat Pumps: A Response to Tony A

March 26, 2023

Friends, after writing an article about the possibility of living a carbon zero life (link), I received an extraordinarily long and detailed comment from a property developer, TonyA. In his comment he explains why air source heat pumps are not *the* solution or a simple replacement for gas-fired boilers.

I think Tony A’s comment is really important as his perspective is part of why heat pumps are not already being installed more widely. But I disagree with him, and since this is my blog I thought I would explain why he is – and it’s hard to say this politely – wrong. His comment is reproduced in full at the end of the article, so you can check to see if I have interpreted his comments fairly.

Summarising, TonyA suggests that we:

  • Establish a national high quality insulation service, funded by low-cost loans.
  • Install electrical heating in most homes – radiant panels or convector heaters or similar: – not heat pumps.

What’s missing from Tony A’s comment

What’s missing from TonyA’s comment is any acknowledgement of the urgency of action in the face of the climate change to which we are currently committing our children. This is the reason that any of this matters. So if a solution doesn’t dramatically reduce carbon dioxide emissions, it isn’t a solution.

TonyA states that heat pumps are more expensive than gas boilers: this is true. And he then ridicules the ineffectiveness of the current subsidy system: this is a fair point. But although an individual heat pump costs more than an individual gas boiler or electric heater, heat pumps dramatically reduce the national cost of low carbon heating. And ultimately, that national cost ends up on all our bills. And additionally, using heat pumps results in lower carbon dioxide emissions – which is the point of the entire exercise.

Imagine for a moment that we eventually succeed in building a grid of renewable electricity and that we use heat pumps to heat our homes. Hurrah! This would be a great achievement and would require a certain amount of renewable generation and/or nuclear power.  But if we were to adopt TonyA’s solution – we would need to build a grid three times larger: i.e. three times more solar farms, wind farms and nuclear power stations. That is not a cheap or quick option – and even if we did that, CO2 emissions would not fall as much as using heat pumps.

Really? Let’s look at some numbers

Typical household energy usage is currently 2,900 kWh/year of  electricity and 12,000 kWh/year of gas resulting about 3.4 tonnes of CO2 emissions per year and costing around £2,400/year (at 34p/kWh of electricity and 12 p/kWh gas).

  • Under the TonyA scheme, assuming every household was insulated to need only 60% of current heating requirements, household usage would fall to (roughly) 9,100 kWh of electricity with no gas required. With our current electricity tariffs, running costs would increase to just over £3,000/year and carbon dioxide emissions fall to around 2.1 tonnes per year – a 39% reduction.
  • Under the same assumptions as the TonyA plan, but using heat pumps instead of direct electric heating, household usage would fall to (roughly) 5,000 kWh of electricity. With our current electricity tariffs, running costs would fall to just under £1,700/year and carbon dioxide emissions would fall to around 0.7 tonnes per year – an 80% reduction.

These calculations are very approximate, and assume that the carbon intensity of electricity stays at its current value – around 0.23 kg of CO2 is emitted per kWh of electricity produced. In fact this figure will fall over coming decades. But what ever reduction is achieved, using heat pumps will result in dramatically lower emissions than the Tony A plan.

Click on image for a larger version. Some simple calculations for a typical UK home comparing the cost and carbon emissions of TonyA’s Strategy with one using heat pumps. [Note: this figure was amended on 26/3/2023 due an error in the original spreadsheet]

In short, the TonyA plan – involving electric heating and a vast national retrofit scheme would:

  • Increase costs to consumers,
  • Fail to substantially reduce CO2 emissions,
  • AND require national investment in a grid three times bigger than would otherwise be required. This would delay reductions in CO2 emissions and may not even be feasible.

Please note, I am not against a vast national retrofit scheme. But implementing such a scheme and then installing direct electrical heating would be ineffective in reducing carbon dioxide emissions.

Specific Criticism of heat pumps

Tony A has specific criticisms of ASHPs. He states:

  1. ASHPs are extremely expensive compared with gas boilers,
  2. ASHPs are proving more expensive to run than gas boilers at current electricity prices, especially in poorly insulated homes with outside temperatures below 5 degrees.
  3. The data derived to justify ASHPs as a straight replacement for gas boilers is inappropriately derived from extremely well-insulated larger homes

Let’s look at these in turn.

Point 1 is true. The trade cost of a 5 kW heat pump is around £3,500 whereas a boiler is closer to £1,200. And the installation is usually more complex requiring typically 2 – 3 days work. These are today’s facts. I think we will see some innovation in this area and I expect heat pump hardware and installation costs to fall. But I doubt they will ever be cheaper than a gas boiler.

But Points 2 & 3 are not true. We are now getting results from multiple real world studies of heat pumps in UK homes. One recent study is this one from a so-called Energy Systems ‘Catapult’ Research Centre. It shows that amongst the 742 installations studied the average seasonal performance factor was 2.8 i.e. averaged over a year, the installations were 280% more efficient than the electric heaters TonyA suggests, and around 330% more efficient than gas boilers. This is already enough to reduce running costs of a heat pump compared with a gas boiler.

If TonyA’s plan of retrofitting all these homes with extra insulation were put into place, the heat pumps would perform even better – negating his own criticism! In my own home, the seasonal performance factor is around 3.5 and could be increased to 4 with further work.

Try listening to the testimony of a customer who had a heat pump installed but without any additional insulation and without changing any of the radiators.


TonyA‘s comments seem at first to be powerful, detailed criticism of a plan which places heat pumps at the centre of our national strategy for domestic heating.

But in fact the plan he proposes – a bold retrofit plan with electrical heaters – would involve MASSIVE additional national investments in renewable energy plant that we just don’t need.

This is already a long article, but I will finally mention that if we are embarking on a massive national retrofit, then when heat pumps are combined with domestic batteries and solar PV, the cost of using a heat pump to heat a home can be reduced dramatically. In my case, neglecting government payments, all my electricity and heating costs amount to less than £400 pounds per year – just over £30/month – and that’s with current high prices.

Below are TonyA’s unedited comments. To me they confirm that in fact heat pumps are the solution for domestic heating in the UK. My mind is open to being changed, but a heating system with more than 300% efficiency is very hard to beat.

Tony A’s Comment in Full

I’m a SME residential property developer and a technical manager for a large housing association, and I’m afraid I don’t share your enthusiasm for air source heat pumps, at least in the way they are being sold to the public as *the* solution and a simple replacement for their gas fired boilers.

There is simply far too much evidence – real evidence, not the right-wing newspapers you refer to, although why the left-wing ones are likely to be any better or lacking in prejudice escapes me – of their limitations and unsuitability for small, badly-insulated British homes:

1. ASHPs are extremely expensive compared with gas boilers, especially when you factor in the need for larger radiators or underfloor heating because ASHPs are designed to run at 60 degrees, and the need for supplementary space and water heating such as electric panel radiators because the performance of ASHPs drops off rapidly and become very expensive to run below 5 degrees. The cost may drop as more manufacturers and installers join the market, but at the moment there is a desperate shortage of installers in particular.

2. ASHPs are proving more expensive to run than gas boilers at current electricity prices, especially in poorly insulated homes with outside temperatures below 5 degrees. Do you actually read the trade or self-build magazines and websites where so many ASHP early-adopters are bemoaning the cost of running the things? It will help if the Government stops forcing providers to link electricity prices to gas, and obviously the ASHP will work much better if the house is properly insulated. Once people realise that they will have to pay for external wall insulation, draught-proofing, possibly new windows, extra loft insulation and insulate their floors to achieve the required rSAP ratings, on top of the ASHP system itself, the cost become prohibitive both in income terms and as a proportion of many houses’ values. Look at the extremely poor take up of the current £5000 grants: once people realise the true costs and the incredible difficulty of finding anyone to do the work, they give up. it was the same with the 2015 Green Deal – bureaucratic, with the funding short-term, and an acute shortage of installers, not helped by the fact that builders were obliged to be trained by and register and pay expensive annual membership fees to join the Government’s validation scheme, all of which duplicates the existing perfectly-valid Building Control system that everyone is familiar with.

Only the Government could dream up such a rubbish system, and I am not making a party-political point here: the Lib Dems and Conservatives oversaw the civil servants who devised the Green Deal and its successors (all failures) but it would probably have been the same under Labour. These were all points made in the recent rather good House of Lords select committee report.

3. The data derived to justify ASHPs as a straight replacement for gas boilers is inappropriately derived from extremely well-insulated larger homes on the Continent, with continental weather patterns of extreme cold for months on end. In our maritime climate and small leaky houses, an ASHP in every single little home is unsuitable for the reasons outlined above and because it is simply overkill. Most UK homes could cope perfectly fine by replacing their gas central heating with a) a serious programme of insulation along Fabric First principles, and b) cheap electric panel radiators and water heaters, with possible ASHP or GSHP at a district heating level for flats and dense city areas, to partly pre-heat the water.

I’m willing to concede that in Scotland and for big detached houses, ASHP or GSHP may be appropriate provided the insulation level is very good. But for the vast majority of people, we simply do not need an ASHP in every home. Demanding this will result in a massive misapplication of limited resources. We should instead first have a national programme of Fabric First insulation and reduction in thermal bridging, funded by low-interest “Net Zero UK” (NZUK) loans from the Government or private providers like Legal and General or pension funds, who seem prepared to accept the very low returns on offer from build-to-rent investment, for example. These loans would be secured against each property’s title, like a mortgage, would transfer with the property on sale, and would be repayable over 50-75 years or earlier if the owner can afford to. Regular mortgage providers would not be allowed to include NZUK loans in their viability assessments. The interest should be very low because the debt will be secured against the nation’s housing stock, which isn’t going anywhere.

The quality of the workmanship for this national programme would be validated by Building Control in the usual way, backed up by a technical survey to establish the works required and by rSAP assessments and air-testing to confirm the property should achieve the required U-value and EPC: this is already required for new-builds, extensions and big projects like External Wall Insulation, and would be simply be extended to the Net Zero Renewal programme for the whole country’s housing stock.

Obviously for such a programme we will need lots more inspectors and assessors, and installers. The current scatter-gun approach where “the market” of atomised householders drives demand for trained labour isn’t working – try finding an EWI or ASHP installer! – so the Government, housing association and private providers probably need to prime the pump with projects such as requiring the upgrading of entire towns and city areas, just as was done with the shift from town to natural gas in the late 1960s and 1970s. Such projects will benefit from hefty economies of scale and will train up cadres of experienced inspectors and installers who will be able to move across the country, set up companies of their own if they have the skills, and train further people as the programme gathers pace nationwide.

This approach could also be extended to net-zero the nation’s commercial property stock: compulsory loans, sweetened by the resulting savings in fuel costs and perhaps by tying council tax and business rates to EPC ratings, that will repayable over decades.

Kettle versus Qooker

January 29, 2023

Friends, have you ever spent time with someone who has just had a Qooker installed?

Discussing topics even tangentially related to the heating of water will result in a torrent of gushing hot water praise for this life-changing water-heating innovation.

And somewhere in the gushing torrent will typically be claims that a Qooker is more energy efficient than heating water using a kettle. Having carried out extensive studies of the boiling of water in domestic settings, I was sceptical. So I asked the oracle that is OpenAI’s ChatGPT about the pros and cons of using a Qooker and using a kettle.

Click image for a larger version. Chat GPT thinks that a Qooker is more efficient than a kettle.

ChatGPT was of the opinion that using a Qooker was more energy efficient than using a kettle. But then ChatGPT is truth-agnostic:

Click image for a larger version. Chat GPT admits it cannot distinguish between ‘true facts’ and ‘false facts’.

So I thought I would make a calculation, and that is what this article is about. In case you don’t have the time to read the whole article, my conclusion is that there is not generally much difference in energy efficiency terms.

The actual answer depends on how much boiling water you use each day, and how much extra water you leave in the kettle each time you boil it. Yes, it’s that tedious.

I think there are a wide range of use cases where a Qooker might well be more energy efficient than a kettle. However, what Qookers actually save is time, and I think that is why people who own them like them so much.

What is a Qooker?

A Qooker is a device that preheats around 3 litres of water to just over 100 °C and holds it in a pressurised, insulated container – like a vacuum flask – under a kitchen countertop. Other brands of water-heating tap are available.

Click image for a larger version. Illustration of a Qooker with publicity photograph .

When boiling water is required – such as for making tea or coffee, or filling a saucepan – water at around 100 °C can be dispensed immediately via safety-tap.

Energy Efficiency

The energy-saving potential arises from the fact that the tap dispenses just the amount of hot water required. This is in contrast with a kettle which usually requires some amount of extra water be boiled each time boiling water is required.

However, in order to realise this benefit, the Qooker has to keep around 3 litres of water in a pressurised container at around 108 °C – and some of that heat leaks out constantly into the kitchen.

So the question to answer is the relative magnitude of these heat losses (boils extra water versus losing heat 24/7). I decided to write a spreadsheet. Obviously I asked CHatGPT to do this first but the result wasn’t very helpful.

Click image for a larger version. Chat GPT’s suggested spreadsheet  didn’t take account of the fact that kettles and Qookers waste energy in different ways: one by keeping water hot for an extra time, the other by heating extra water.

So I wrote my own spreadsheet. You can download it here and it’s key features are shown in the image below.

Click image for a larger version. It calculates the extra costs of using excess water and compares then with the constant losses from teh Qooker.

My conclusion is that while it is possible to use a conventional kettle more efficiently than a Qooker, in most common circumstances, the Qooker is likely to be more efficient.

For the scenario illustrated above – boiling 10 cups of water for tea/coffee and preparing a one litre saucepan of boiling water – both Qooker and kettle use 0.42 kWh to heat the water, but the Qooker ‘wastes’  0.24 kWh/day keeping the water hot and and the kettle ‘wastes’ 0.35 kWh/day boiling extra water.

Over a year the saving of 0.11 kWh/day would add up to a saving of around 40 kWh/year, around 10 kg CO2/year, and around £14/year. The financial saving on a £1,000 + investment is negligible, and in carbon terms (and financial terms) the money would be much better spent on insulation!

Which raises the question…

Why do people love their Qookers?

People love their Qookers. Their relationships can be almost as profound as their relationship with their Air Fryers. And as far as I can tell, the reason is that aside from their emotional investment in their tap, the Qooker saves time.

Think about the difference between using a computer where opening a file takes many seconds – and the windows are slow to refresh. One learns to live with such computers, but after one has used a faster computer, returning to using the old computer seems painful.

Similarly, I think this sense of instant availability can feel magical after a lifetime of waiting for the kettle to boil. Two minutes per cup of tea; ten minutes per day; an hour a week; two days a year; a big fraction of person’s life could be taken up waiting for the kettle to boil.

People’s devotion is nothing to do with energy saving, and certainly nothing to do with cost savings – which are all offset by the need to regularly replace filters.

So will I be getting one? No. It’s just one more thing I don’t need.


Friends, I ignored lots of things in this article.

  • I ignored the fact that the lost heating energy isn’t really ‘wasted’ for either kettles or Qookers: it all goes into heating the house.
  • I ignored the mass of the kettle which must be re-heated each time the kettle is boiled.
  • And I ignored embodied carbon dioxide.
  • And I ignored safety concerns. 


Appalling error by Octopus Energy.

January 24, 2023

Update: Within 24 hours of publishing this, Octopus sorted out the issue. Of course it should never have happened but mistakes do happen and I am grateful that they (eventually) sorted it out.

Apparently the error arose from a firm ware update to the smart meter which clashed with a meter reading, and Octopus’s system didn’t catch the error. 

Worryingly, several other readers of this blog and followers on Twitter have reported the same problem at exactly the same timeAnyway. I am older and wiser!

Friends, I am a customer of Octopus Energy who seem like a decent company with an agenda to push forward the energy transition.

But everyone makes mistakes. And I have just been sent a bill for a single month which is in error by more than £1,600. Yes, it is approximately one thousand six hundred pounds too much on a bill expected to be around £100.

This is an obvious error. Octopus’s own numbers do not add up. And yet they have still billed me for this incorrect amount.

I am sure this will be corrected eventually but I still feel frightened simply to see that amount indicated as an amount that I owe. Indeed, I am shaking as I type this.

The important question here is not my liability: it is this: If Octopus can make such a gross error and not notice it, how can we have have confidence that they – and other energy companies – are not making errors routinely but at a level at which people just might not notice?

What’s the problem?

Here are some excerpts from my bill for 12th December 2022 to 11th January 2023.

Click on Image for a larger version. Excerpt from Octopus Energy bill claiming that I used £1,873.30 of electricity between 12th December 2022 and 11th January 2023.

The excerpt above compellingly suggests that I owe Octopus energy £1,873.30 for electricity used between 12th December 2022 and 11th January 2023.

The excerpt below emphasises this. It shows the breakdown of that consumption between peak and off-peak hours. Most of the consumption arose because I apparently consumed 4,458.6 kWh of electricity during peak hours during this single month.

To understand how ridiculous that is, last year I used 3,323 kWh for the whole year! It’s an obvious error – but it gets worse.

Click on Image for a larger version. Excerpt from Octopus Energy bill suggesting that I used 4,458.6 kWh of electricity during peak hours between 12th December 2022 and 11th January 2023. For comparison , last year I used 3,323 kWh for the whole year!

Pleasingly, Octopus send a detailed breakdown of electricity showing consumption every 30 minutes during the entire month. the 31 pages each look something like the page below:

Click on Image for a larger version. Octopus send details of energy consumption for every 30-minutes throughout the month. Here is the page showing consumption on 12th December 2022.

From this you can see that on 12th December 2022, we downloaded energy to charge our battery using off-peak electricity. But on this very cold day, the battery had discharged by around 2:00 p.m. and we then consumed full price electricity for the rest of the day.

This cold day involved a relatively high consumption of electricity – 37.4 kWh/day – to operate the heat pump. But even if we did this every day it would still only amount to only 1,160 kWh for the whole month.

I went through each page and added up the energy used: It looks like this:

Click on Image for a larger version. Adding up the amount of electricity consumed through the month teh answer comes to 750 kWh at a cost of £104.94 – an average cost per unit of around 14.0 p/kWh.

Looking at the days individually and adding up the cost on each day, Octopus’s analysis suggest that I used around 750 kWh of energy in this period, or around 24 kWh/day. This is very much in line with what my manual meter readings suggested and what I was expecting.

It is not in agreement with the statement on the bill that I used 4,458.6 kWh of electricity during this period at a cost of £1,730. So Octopus’s own bill simply does not add up.

Octopus Response

I wrote to Octopus on 13th January pointing out this error and a representative replied to tell me that Octopus were aware of the issue and I would not be billed.

However it is now 24th January and I now have been billed! I am now alarmed and distressed.

Click on Image for a larger version. Octopus told me they were aware of the issue and I would not be billed.

How has this come about?

I think I know how this has occurred. I think it arises from a single erroneous reading of my meter.

I subscribe for £12 a year to the Powershaper service which gives me access to my own half-hourly electricity readings! I think these data are read from the same database used by the electricity companies for billing.

Looking through these readings I noticed an anomaly on 11th January 2023. In a single half hour between 7:00 and 7:30 a.m. I apparently used 4,295 kWh.

This is more than my entire annual usage last year in a single half-hour period! It corresponds to electricity consumption at a rate of 8.59 MW – megawatts! – for the entire half hour. This is not physically possible, and is obviously an erroneous reading.

Click on Image for a larger version. Octopus told me they were aware of the issue and I would not be billed.

I think Octopus have overridden their automated systems to correct this error on the day-by-day analysis, but failed to correct the consequental error on the monthly bill.

So how can Octopus have made such an error?

I just don’t know how sophisticated billing systems which must surely be audited (?) could possibly make such an error.

I am sure that Octopus will eventually correct this. But I am frankly appalled that it is even technically possible to issue a bill which is so grossly in error.

So check your bills!

Weather Compensation: Experimental Tweaking

October 21, 2022

Friends, as I mentioned in my previous article, I have no real idea how to actually operate my 5 kW Vaillant Arotherm plus heat pump – or to check how well it is operating. That’s because there is no readable manual for the controller and the App does not do what it says it does.

But since I have an independent monitoring system, I have begun a series of experiments to tweak the heat pump weather compensation settings, and see what happens!

If ‘Reading the Manual is like taking a course in theoretical heat pumps, then this is more like a course in experimental heat pumps.

This is quite a technical article, and it is nearly 1500 words long. So if you are not really interested in heat pump arcana I would recommend giving this one a miss. On the plus side, it does have some nice graphs :-).

Weather Compensation

Weather Compensation is the idea that when the weather is mild, one can heat water in radiators or under-floor heating to a low temperature – perhaps just 25 °C. But when the weather is colder, and the heating demand is greater, one can increase the temperature of the hot water to perhaps 40 °C or 50 °C to meet the heating demand.

Using weather compensation to match the output of a heat pump to the heating demand contrasts with using a thermostat for the same purpose.

Click on image for larger version. The heating supplied to a dwelling can be changed to match demand in two ways. In a traditional thermostat-based system, the radiator flow temperature is fixed and switches on and off to maintain a constant indoor temperature. In contrast, using weather compensation the radiator flow temperature is adjusted.

In a thermostat-based heating system, the flow temperature to which water is heated is pre-set: in boilers it is often as high as 70 °C, and for heat pumps it might be 50 °C. And then to match heating to demand, the thermostat switches the heating source on and off intermittently to maintain the desired temperature.

Weather compensation is particularly valuable when using heat pumps because the coefficient of performance (COP) of the heat pump varies with both flow temperature and environmental temperature. But it can be tricky to adjust the settings for any heat pump, but especially one with no decent manual!

Weather Compensation in action

The graph below shows data taken every two minutes during the week from 00:01 on 11th October 2022.

  • The red curve shows the outside temperature
  • The grey dots show the instantaneous flow temperature.
  • The green curve shows the flow temperature averaged over 1 hour
  • The orange curve shows the internal temperature

Click on image for larger version. Weather compensation in action. When the outside temperature falls, the flow temperature in the radiators increases to maintain the internal temperature.

Notice that when the outside temperature falls, the flow temperature in the radiators increases to maintain the internal temperature.

But on day 4 of the period shown in the graph above, I changed the setting of the Weather Compensation from the curve labelled ‘0.6’ to the curve labelled ‘0.5’ in an attempt to lower internal temperature of the house. I’ll explain more about these labels below.

The graph below shows that average for the 4 days before the change was 21.0 °C and the average for the 3 days after was 20.76 °C: so it does seem to have had a small (0.24 °C) effect, but I will need to continue experiments – see the end of the article for an update.

Click on image for larger version. Graph shows the internal temperature of the house detail averaged over a period of 1 hour. The weather compensation parameter was changed on Day 4 and it does seem to have slightly lowered internal temperature.

It is striking to me how stable the internal temperature is given that – as I understand it – it is based entirely on measuring the temperature OUTSIDE the house – not INSIDE it!


To evaluate the COP, one needs to work out the ratio of the heat delivered to the electrical energy used, over some set time period.

The hourly averaged COP is shown in the graph below. The times when the COP is greater than 4 correspond to times when the difference between the flow temperature and the outside temperature is small, and so not very much heat is being delivered with these high COP values.

Click on image for larger version. Graph shows the hourly averaged COP. Considering only use for DHW the average COP was 3.1 and considering only use for space heating the average COP was 3.9. Overall, considering both DHW and space heating across  the entire period the average COP was 3.7. These averages are shown as dotted lines on the figure.

With a little spreadsheet untangling it is possible to extract the data corresponding to periods when the heat pump is heating DHW and periods when it is heating water for space heating. For DHW the average COP for heating water to 50 °C was 3.1 and for space heating the average COP was 3.9. Overall, considering both DHW and space heating across the entire period, the average COP was 3.7.

Electrical and Thermal Power

Calculation of COP requires evaluation of both electrical power consumed and thermal energy delivered. The graphs below show both these quantities measured every 2 minutes throughout the week or so under consideration.

Click on either image for larger version. Graphs show hourly averages of electrical and thermal power. The DHW cycle runs once a night using cheap rate electricity. The separation of the two uses of the heat pump is not quite perfect: sorry.


So far I have just showed a week or so of data. Now I will explain what I hope to achieve with some ‘tweaks’ First let me explain, about how Vaillant implement Weather Compensation.

Their scheme is illustrated in the figure below. The flow temperature of water in the radiators is set depending the temperature outside. The sensitivity of the weather compensation is set by picking a curve labelled by a number from 0.1 to 4. For example, when the outside temperature is 5 °C,

  • the curve labelled 0.6 would result in a flow temperature of about 34 °C but
  • the curve labelled 0.5 would result in a flow temperature of about 32 °C

Click on image for larger version. The flow temperature of water in the radiators is set depending the temperature outside. The sensitivity of the weather compensation is set by picking a curve labelled by a number from 0.1 to 4. When the outside temperature is 5 °C, the curve labelled 0.6 would result in a flow temperature of about 34 °C but using the curve labelled 0.5 would result in a flow temperature of about 32 °C.

On Day 4 I adjusted the weather compensation from 0.6 to 0.5. To see if this tweak is working we can look at the second figure in this article in this article which I have reproduced below.

Click on image for larger version. On Day 4 the weather compensation setting was changed from 0.6 to 0.5. If we look at cold spells before and after the change it does look as though as the flow temperature is perhaps a degree or two than one might otherwise have expected.

If we look at cold spells before and after the change it does look as though as the flow temperature is perhaps a degree or two cooler than one might otherwise have expected. And since this article has taken a day or two to prepare, I now have a couple more days data on the internal temperature with WC curve 0.5. It does indeed seem to have maintained an internal temperature about 0.23 °C cooler than using WC curve 0.6.

Click on image for larger version. Updated version of the second graph in this article with 3 extra days data. The graph shows the internal temperature of the house in detail averaged over a period of 1 hour. The weather compensation parameter was changed on Day 4 and it does seem to have slightly lowered internal temperature.


My main conclusion is that the weather compensation adjustment does seem to be sort-of working. I will continue experiments and let you know how they go.

My second conclusion, is that observing these effects is really hard and it takes hours of analysis to unearth this kind of insight!

My third conclusion – which you may have already spotted – is that my 5 kW heat pump is just too big. It only needs to output 1,500 W to maintain a temperature of just over 20 °C in my home when the outside temperature is 5 °C i.e. with 15 °C of demand. This seems to indicated that a 3 kW heat pump would have been adequate to heat the home down to (say) – 5°C.

This oversizing is probably responsible (at least in part) for the rapid cycling on and off of the heat pump – exactly what weather compensation was supposed to avoid!


Vaillant Arotherm Plus Heat Pump: The good, the bad and the ugly.

October 19, 2022

Friends, it’s been just over a year now since we had our 5 kW Vaillant Arotherm Plus heat pump installed.

The Good

In short, I love the heat pump: it is super quiet; uses low GWP propane as a working fluid; and can even heat water to 70 °C if I should ever desire.

And it is has worked well with a seasonal average COP (sCOP) of 3.6 in its first heating season during which the internal temperature of the house has been steady at ~21.5 °C 24/7. And we have lots of pleasantly hot hot water.

But, not everything is good. And this article is about the things that are seriously bad and the things that are downright ugly.

The Bad

There is no User Manual! There is lots of excellent engineering documentation and installation instructions, but I personally would appreciate a relatively short document that explained how to adjust various aspects of the heat pump.

Click for a larger version. Engineering documentation for the Vaillant Heat Pump. But no user manual.

For example, if one uses a thermostat, it is relatively easy to programme a setback period overnight where the temperature is lower. But it would nonetheless be nice to have instructions.

But if you switch to using weather compensation instead of a thermostat? There is no explanation of how the weather compensation interacts – if it all – with the thermostat setting. And the settings are four or five layers down in a menu system that is labrythine in its obscurity. A manual would be helpful.

The SensoComfort controller looks great: sleek and black. But using it is a nightmare. Each time one attempts to achieve a particular task one has to decide whether it should be looked for under a variety of confusing menu headings:

  • Installation
  • Basic System Diagram Configuration
  • HP control module configuration
  • Heat Pump 1
  • HP control module
  • Circuit 1
  • House
  • Domestic Hot Water

There is no logic to this and it’s just guesswork every time because there is no manual!

The Ugly

30 May 2023: See also my review of the update app here.

The Vaillant SensoAPP does just about have some basic functionality.

For example, it allows one to set a period of absence or trigger a boost to the domestic hot water. However, it frequently fails to do even these basic tasks, commonly reporting a variety of errors.

But the one thing I would like the App to do would be tell me the Coefficient of Performance (COP) of the heat pump. The COP tells the owner or an engineer, how well the heat pump is working.

The COP is the ratio of the amount of heat delivered to the house, to the amount of electrical energy used to operate the pump. Typically COP lies in the range 2 (poor) to 5 (outstanding) and this provides the most significant measure of a heat pump’s performance.

Ideally, the App would report the COP for hot water and for space heating separately. But actually there is just nothing!

In my opinion it is scandalous that the App does not report the COP.

There are signs that the App should be able to show the COP, but then using the available data it gives erroneous answers. In short, when it comes to monitoring heat pump performance, it is literaly useless.

Let me explain.

The App offers no direct readout of COP– which is disappointing – but the ‘Information’ screen on the SensoAPP appears to offer the opportunity to see the electrical consumption and the thermal output (called the ‘environmental yield’) for both space heating and domestic hot water.

Click image for a larger version. The information page on the Vaillant sensoApp looks like it should have all the information one needs to calculate the COP.

Using these data it should be possible to evaluate the COP. Sadly this is not the case.

In an attempt to do this I downloaded the weekly data for the electrical consumption and checked it against the completely independent MMSP monitoring system I have installed.

The weekly consumption information screen for DHW looks like the figure below. It is highly suspicious to me that the data appear to be exactly whole numbers of kWh every day – but the screen tells me that I used 8 kWh of electricity that week for domestic hot water, and that is the figure I recorded.

Click on the figure for a larger version. The electrical consumption for domestic hot water in Week 41 of 2022 as reported by the Vaillant sensoApp. Notice that the daily consumptions are all exact numbers of kWh.

My weekly MMSP data runs Saturday to Friday while the Vaillant data runs Sunday to Saturday, so we might not expect the data to be identical, but the data (below) are similar. I was hopeful when I saw this correspondence.

Click on the figure for a larger version. The blue curve shows the weekly electrical consumption (kWh/day) as self-reported by the Vaillant App. The red curve shows the same quantity as measured by an independent monitoring system.

Over the 61 weeks since installation the Vaillant reported consumption of 2,147 kWh – 4.3% less than the MMSP system. Not great agreement.

However, if one looks at the thermal data – the environmental yield – the data are both dodgy and missing.

How can they be both dodgy AND missing? As the screen grab below shows, the graph suggest the environmental yield is an exact whole number of kWh every day – something which is very unlikely. This makes the data seem dodgy to me.

But in this case we can also add up the daily yield very easily – it comes to 8 kWh that week. However the App does not do that summation for me – it simply states that the total heating over the entire installation time is 721 kWh. The weekly data are just missing!

Click on the figure for a larger version. The environmental yield for domestic hot water in Week 41 of 2022 as reported by the Vaillant sensoApp. Notice that the daily yields are all exact numbers of kWh.

While I can add up the data in the bar chart above quite easily, this not possible for other screens such as that below – which again simply states the total yield over the entire installation period.

Click on the figure for a larger version. The environmental yield for space heating in Week 41 of 2022 as reported by the Vaillant sensoApp. Notice that the daily yields are all exact numbers of kWh.

This means that it is impossible to work out the COP.

I did try working out the overall COP since installation, but the results were not believable. The Vaillant self-reported average COP is 2.0 whereas the MMSP monitoring system indicates an answer closer to 3.51 .

The good, the bad and the ugly

Summarising, the heat pump is fantastic, and works well.

But I only know that because I have an independent monitoring system.

If I didn’t have independent monitoring I would literally have no idea how well the heat pump was working.

And there has been no improvement or new software updates in the last year

Overall, this is shockingly bad.

More Climate Communications

August 2, 2022

Friends, I have been out again in search of inspiration for how to encourage changes that will reduce carbon dioxide emissions.

In this article I am just noting down things that happened and reflecting on the interactions. It seems that there is widespread ignorance and misunderstanding of Climate Change coupled with bewilderment about what will actually make a difference.

These notes are for two quite contrasting days. As usual, a solid 99% of people just passed by. Of those who stopped, there was quite a bit of positivity, but also some bizarrely misinformed opinions.

I am still learning…

Thursday, July 28.

10:58 Set up outside CarpetRIght in Teddington.

11:05 John stopped by with many positive ideas. He suggested I put posters in the library and in the local community noticeboard on Broad Street which I could access through the library. He suggested my board should show the savings to be gleaned from using low carbon energy, such as solar power. John lived locally and his street had a WhatsApp group that could share information. It made me think that maybe the ‘Neighbourhood’ app might be useful.

11:12 When John walked back the other way he held up his fist and said “Viva La Revolution

11:23 A mother and her two children (a boy aged 12 and a girl aged 9) stopped by. She was very supportive but the children were obviously disinterested. I asked her what things I could do to communicate better, and she suggested my box should have more colour on it. The children then suggested there should be graphics showing a healthy earth and a poorly earth. The boy suggested I should use Instagram. The mother said they were going with the children to Bordeaux in France next week, and that they would see some of the evidence of wildfires. 

11:34 Three teenagers walked by and waved. But didn’t stop to talk

11:35 A lady caught my eye and said she was in a rush for work, but wished me good luck.

11:38 Man on a bright yellow Suzuki motorbike stopped at the traffic lights, and caught my eye. He nodded.

11:45 A regular at the Sidra café where I have my morning coffee stopped by. I said I was just trying to find out what people thought, and get out of my Twitter bubble. He told me that “almost everyone will agree with me, but that almost no one knew what to do“. I said that seemed a succinct précis of what people had said

11:48 A gentleman, quite elderly, with a straw hat stopped by and asked me “What’s going on with all this weather then? Are we going to get more of it?”. I said we probably were going to get more of it, one or two days like that every year and then slowly becoming more common. I asked him if he thought there was anything we could do about it, and he told me it was “...all those gases going up in the atmosphere“, a phrase he repeated several times. As he left he said “We’ll just have to see what happens then…“.

11:54 A lady called who works in the theology department at the local University stopped by. “Air-conditioning on the buses and trains” she said “that’s the solution“. She said she was out on the 40 °C day, and that the solution was to wear damp T-shirts and trousers. I suggested that some people might think it immodest for ladies to wear a wet T-shirt but she was unabashed.

12:10 An elegantly-dressed lady pushing her mother in a wheelchair stopped by. She said it wasn’t a popular thing to say but she wasn’t sure that it wasn’t the best thing for people to just destroy themselves, leaving just a few small tribes on Earth, and then people would be forced to live sustainably.

She said that what people want is to know what is the right thing, but something which will influence others. We discussed the idea of a Personal Carbon budget. For example, people could have a lifetime budget of 100 tons of carbon emissions by flying, which they could then sell, or buy from other people. She seemed to like this idea.

Her mother intervened to say that when she was younger, flying was very expensive and so people went by boat, and she had come to England from Canada on a freighter which had just a few cabins for passengers. “Flying was very expensive. Slower is better.” she said.

The lady thought that “Human beings will pay if they have too. Raising the price of things will eliminate frivolous use of things e.g., weekend trips”.

She said that  People are creative” and told me that in Lebanon people had rigged up makeshift solar systems with batteries to get by when there was no mains electricity and that people there cooked collectively. She said this was also done in Morocco and Algeria. 

She told me to look out for the ”The Boy Who Harnessed The Wind” a Netflix film about renewable energy in Africa

12:22 A lady carrying her lunch back to her office spoke as she passed, saying “Thank you for doing this.”, although I am not sure she knew what I was doing!

12:24 A gentleman passing by didn’t stop but looked at me, and raised his eyebrows and said “Alright!”

12:29 Colleagues from NPL passed by, and one person told me that his wife’s sister’s husband (!) had asked them if they had heard about this “Protons for Breakfast” site. So it does seem that news is travelling.

12:42 An ex-colleague from NPL stopped by. He was working on life-cycle impact assessments, and said he had done did a life-cycle assessment of the Audi E Tron (an EV) and that it was worse than petrol cars because of the massive embodied energy in the battery.

12:52 A lady was walking slowly past with a stick. I caught her eye and commented that it was warm now. She nodded and said “As long as it doesn’t get too warm…

12:58 A man who worked at a local e-bike shop stopped and suggested I should get a giant banner and block the entire road. He said he had heard that the materials mined for batteries involved child labour. I told him I was pretty sure that involved the cobalt and not the lithium and that modern batteries use no cobalt or nickel. 

13:00 An expert in Climate Change science and an ex-colleague from NPL stopped by – remember this is Teddington!. She told me she’d read a book of random facts about climate change, but had had to stop because it was too depressing.

13:15 A lady passed by without stopping but said she had had lots of conversations about climate change with friends. I told her to keep talking.

13:17. E-bike man passed  by again having bought his lunch, and he said that he had seen an article on associated press that morning about fugitive methane

13:26. Stopped

Tuesday, August 2nd.

11:03 I ventured away from Teddington to nearby Hampton Hill and set up outside a parade of shops near a local Tesco.

11:25 Interesting interaction with two air-conditioning servicing engineers who had just emerged from Tesco with their lunch.

The younger one engaged me and said “We’re part of the problem mate”. he said, “We’re 7% of emissions.”.  I said he was also part of the solution. We’re all going to need air-conditioning in the future, and installing heat pumps is going to be a big business. He acknowledged that, but said that the refrigerant was harmful, and the less harmful it was, the worse it was as a refrigerant.

Then his older and fatter colleague came out. He said “I’m not going to get drawn into this because we’ll end up having an argument.” And then he got drawn into it. He said there may be some emissions he said, but there is also a natural cycle. I told him that actually it’s all human emissions.

He then went on about ice ages, and the massive emissions in the Victorian era when there was no global warming. I told him emissions in the Victorian era were low: around 1 billion tonnes of CO2 per year and that now emissions are 36 billion tonnes per year.

He then went on to say “Anyway it’s not us, 72% of emissions come from China”. I pointed out that it wasn’t that high and that the US and China pollute similarly. He wandered off saying “I knew we’d have an argument.”

After that interaction, two people at the bus stop spoke to me and said “Sorry you had to have that, he was like that in Tesco!”. Then they said “Good luck!”

11:35 A man about 60 years of age with long hair approached me and asked me what I was an expert in. I said I was an expert in measurements of temperature and explained how I had made the most accurate temperature measurements ever made. He said he had studied particle physics so I included some slightly more technical details. He told me he was in the film and graphics business but didn’t do much these days, just built websites.

I asked him his take on the climate crisis and he said he wasn’t concerned. He said global warming was 0.2°C, and that NASA had edited the data for specific years. I told him that wasn’t true but he insisted and named the individual years in question. Then he had to get on the bus   

11:41 A young man with a ponytail and some over-ear headphones nodded to me. Then an elderly man with smart grey hair and wearing cool summer shorts said good morning. 

11:51 A black man stopped by and said “He didn’t know what to say because things may get political”. He said first he was from Africa and then changed his mind – he sounded very English to me, like he came from Henley or similar – but he did trade with Africa. He said that the UK was exporting polluting old cars to Africa. I said that was shameful and then I asked what he thought we should do about the climate crisis. 

He said that heating has always been terrible in the UK, and that when he was a child (he said he was 61) he would go out to the library to get warm during the day, and when he was at home they would wear cardigans and pullovers. Now he said his children want to wash a T-shirt then put it in the dryer so they can wear that same day. He thought the price rises that are happening at the moment, although they were very hard on people, might help people value energy more.

He said he couldn’t understand how people in Africa, Nigeria in particular were still using diesel generators so much, when Solar is so cheap. Then he had to go and catch a bus. 

11:59  An elderly man called stopped by, he said it was really warming up. I said yes, but that you had to be something like our age to understand just how different the climate was now to how it was when we were young. He said the government needed to sort it out, and I said I didn’t think the government would be able to sort out their own bedroom, and he laughed and agreed.

I asked him what he thought could be done. He asked me “What could we do?” And I told him that our house in Teddington had been off-grid for a quarter of a year. I said there were things that people could do, but it would take the people who had the money to do them first. He said he was really impressed by news of my house and he would tell his friends. As he left he said good luck to me.

12:35 A lady was just waiting for a bus, but caught my eye and said she hoped I was getting lots of interest. I said I”n fact No, most people just didn’t want to think about it” She said that she had just joined Extinction Rebellion and was waiting to go on the first training course.

13:00  After 25 minutes with no interactions I thought it was time to call it a day.

Domestic Thermal Storage 3: Concrete

July 23, 2022

Friends, this is the third and final article comparing different types of thermal storage.

In previous articles I looked at the humble domestic hot water (DHW) cylinder and the use of a phase-change materials (PCMs) to store heat.

In this article I will look at the use of ‘very big lumps of something very hot‘ as a thermal store.

This brute force technology is implemented in a so-called Zero Emission Boiler (ZEB) from the Tepeo company, and in a behemoth of a storage device akin to a miniature version of the ‘Sand Battery’ that got me started on this, the Warmstone store from the Caldera company.

Zero Emission Boiler (ZEB)

A ZEB thermal store is a device about the size of washing machine that is conceived as replacement for a gas boiler in settings where a heat pump cannot be installed.

It is generally placed wherever the gas boiler was, but being essentially a large lump of concrete, it is extremely heavy – 370 kg – and must be placed only on the ground floor of a house.

Click on the image for a larger version. A ZEB thermal store in an extraordinarily uncluttered kitchen as pictured on the Tepeo website.

The difference between a ZEB and the other thermal storage devices I have described is in the amount of energy which can be stored – a ZEB can store 40 kWh of thermal energy – around 5 times more than a typical DHW cylinder. This is enough thermal energy to not just provide hot water to a house (typically 5 to 10 kWh/day) but also to heat an entire home via its central heating system.

One can imagine a ZEB as being a centralised version of old-fashioned electrical storage heaters. It stores thermal energy by electrically heating a block of ‘high-density concrete’ to an astonishing 800 °C.

At these high temperatures, heat loss is significant, but my estimates suggest that ~150 mm of insulation around a 40 cm cubical block, should limit heat losses to ~ 5.6 kWh/day or around 14%/day.

Extracting thermal energy from a ZEB at 800 °C into water flowing at (say) 50 °C is tricky. Slightly to my surprise, energy cannot be extracted rapidly enough for this to instantly heat water and so it cannot be used to replace a combination boiler. It must still be used with a DHW cylinder as an intermediate store of hot water. However it seems likely to me that Tepeo will solve this problem in the next few years.

Below is a YouTube video  in which Robert Llewelyn discusses the ZEB he has had installed in his own home.


A Warmstone thermal store is something like a miniature version of the ‘Sand Battery‘ that claimed to store heat inter-seasonally. But instead of storing 8 MWh like the ‘sand battery’ – it stores ‘only’ 0.1 MWh or 100 kWh.

It achieves this large capacity by heating a large mass of material – probably concrete of some description – to 500 °C. But it uses vacuum installation to reduce the heat losses to just 4.8 kWh/day – or 5% per day – half the fractional losses of a DHW cylinder or PCM material.

Unsurprisingly the device is large and heavy, weighing 1.8 tonnes and standing 1.8 metres tall with a diameter of 1 metre, so this too large for it to pretend to be a domestic installation. The company imagine it being used in large homes which have outhouses or large gardens.


In the first article I looked at thermal storage in a DHW cylinder. This is the default thermal storage that many people still have in their homes – holding about 8 kWh of thermal energy.

In the second article I looked at Sunamp’s PCM storage which can operate in practice like a DHW cylinder – but is typically less than half the physical size while storing similar amounts of thermal energy.

In this last article I looked at two companies looking to ‘go large’ and store one to two days use of thermal energy for a home. To achieve this they have used large masses of stored material electrically-heated to high temperatures. Surprisingly, despite the large masses and high temperatures, the rate at which water can be heated by these devices is still (currently) limited and so they must both still be used with a DHW cylinder.

All the technologies beyond the basic DHW cylinder all feature computer technology which allows Apps to control the storage and allow integration with smart home controllers – something which is apparently very important, but is an area in which I have absolutely no interest: sorry.

What I learned in writing these articles is the very simple lesson that there is no magic to thermal storage technology. There are no magic materials and there is no magical insulation. To store more energy one simply needs a large mass of material, heated to a high temperature, and kept as well insulated as possible.

Summer Solstice 2022: Solar PV update

June 21, 2022

The Summer Solstice seems like a good a time to take a look at the first half-year of generation from our 12 solar panels and the effect of our Tesla Powerwall battery.

Last year at around this time I wrote that – having been off-grid for 90 days – I felt like I was ‘floating’.  And then just a couple of days later I came back down to Earth after several consecutive days of unseasonably dull weather led to me have to buy some full-price electricity! In mid-summer!

This year we have been off-grid for only 50 days so far and I will discuss the reason for the difference below.

In case you don’t have time to read the article, it’s performing pretty much as expected and very similarly to last year.

Let’s begin.

Solar Generation.

One of the difficulties in communicating data from a solar PV system is its variability. The data obviously change seasonally, but also on daily, weekly and monthly time scales. So I have chosen to present the same data in several ways.

Let’s start with a basic chart showing the average daily generation (in kWh) over the last two years.

Click image for a larger version. Average daily solar generation (kWh) for each month of this year and last year.

This chart shows that generation this year is generally a little better than last year, with the exception of last May, which was enchantingly sunny.

Now let’s look at the daily data, and different averages.

Click image for a larger version: you’ll need to do this to see anything! This chart shows the daily generation this year, and the 5-day running average  of generation this year and last year. See the text for details of the other data on the graph.

The graph above is complicated, showing how various quantities (expressed as kWh/day) change versus day of the year.

Primarily it shows daily solar generation as a light green line. Notice the day-to-day variation: even in June, daily generation can fall to 3.8 kW/day even when the average generation is ~15 kWh/day.

Also shown are the 5-day averages of solar generation this year and last year. The 5-day average shows the smoothing effect of the use of a 13.5 kWh battery. When the 5-day average falls below demand, then it’s likely we will need to buy some electricity from the grid. You can see that this happened in mid-summer last year.

Notice that the 5-day averages show peaks and troughs that can last for weeks.

My expectations for the system are shown as a dotted green line (just a simple mathematical guess)  and as yellow monthly data points estimated using a multi-year European database. Generation is broadly in line with expectations.

The graph also shows nominal household demand as two red-dotted lines.

  • The horizontal line (10 kWh/day) corresponds to daily demand throughout the year.
  • The demand peaking in winter represents the electricity used by the heat pump to heat the house. This demand has only been present this winter – previously, heating was supplied by a gas boiler.

So the generation has interesting day-to-day variability, but when the 5-day average exceeds the average demand, then with the aid of the battery, we stand a good chance of being able to be off-grid for a sustained period.

Looking at the graph, the heating demand was still non-zero in April – and this delayed the point at which we were able to go off-grid.

The graph below shows actual (rather than nominal)  domestic use of electricity and the electricity drawn from the grid.

Click image for a larger version: This chart shows the ±7-day averages of the electricity we use in the house for heating and the electricity we draw from the grid. The difference between the two curves is supplied by the solar PV/battery system. Periods when the house has been off-grid are highlighted in red.

Another way of looking at the data.

Plotting day-to-day generation tends to emphasise the variability of the data.

Plotting cumulative generation through the year tends to de-emphasise the variability and highlight the similarity of generation from one year to the next.

The graphs below show cumulative generation versus day-of-the-year for this year and last year. The upper graph shows the whole year and the lower graph the first half of the year only.

Also shown as dotted lines are the MCS suggestion for likely annual generation and an empirical curve based on the MCS figures that I use to guide my expectations.

Click image for a larger version: This chart shows the cumulative generation this year and last year. See the text for details of the other data on the graph. Broadly speaking, the data are very similar.

Click image for a larger version: This chart shows the cumulative generation for the first 6-months of  this year and last year. See the text for details of the other data on the graph. Broadly speaking, the data are very similar. The very dull period in midsummer last June can be seen as a flat portion on the 2021 generation curve.

Remember that year-to-year variability in generation is typically ±5% (link), so that yearly variations of ~200 kWh on an expected generation of 3,780 kWh is quite normal.


Overall, everything is proceeding as expected. The combination of 12 x 340 Watt Q-Cells panels and the 13.5 kWh Tesla Powerwall battery has been astonishing.

  • We save money in summer – because we are off-grid.
  • We save money in spring and autumn because solar PV is still significant.
  • We save money in winter because we can buy cheap-rate electricity and use it during the day.

The reality of running the house – with fridges and freezers and computers and washing machines and air-conditioning and even tumble dryers – solely from the sunshine for several months a year, still warms my heart.

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