November 2021: Heating and Carbon Emissions

Friends, it is December already and I am preparing to hibernate.

But before I curl up and doze, I just thought I would summarise some of the energy statistics for the house in November.

They are actually pretty remarkable: the low-carbon solar-powered world really is here already…


The average November temperature in Teddington was a very typical 8 °C, but the end of the month included several colder days with average temperatures of ~1 °C, and minimum temperatures down to -3 °C.

Click image for a larger version. Daily averages of the internal and external temperatures this November 2021. Also shown is the monthly average (dotted blue line) and the internal temperature measured every 2 minutes (black line connecting the red dots).

However, as the graph above shows, internally our house remained at an extremely stable temperature, barely changing day or night.


Over the month, the house used 609.4 kWh (~20.3 kWh/day) of electricity. This is roughly double our typical use of electricity without heating.

Click image for a larger version. Remarkably, ~21% of the electricity used in November 2021 came from the solar panels. The bulk (~70%) was purchased off-peak (00:30 to 04:30) and stored in the Tesla Powerwall for use during the day. Just 9% was purchased at peak rates after the battery ran out of charge.

This electricity demand was met as follows:

  • 129 kWh (~21%) came from the solar panels.
  • 428.9 kWh (70%) was off-peak electricity from the grid @5p/kWh = £24.02
  • 51.6 kWh (9%) was peak-rate electricity from the grid @16.26p/kWh =£8.39

The shifting of our time-of-use by using the Tesla Powerwall resulted in the average cost of a unit of grid electricity being ~ 6.2p/kWh.

Added to those costs is the standing charge of 25p/day, or £7.50/month.


We are still using gas for cooking and through the month we used:

  • 41.84 kWh (~1.4 kWh/day) @3.83 p/kWh = £1.60

Added to this is the standing charge of 23.85/day, or £7.16/month.

Heating & Domestic Hot Water (DHW)

Over the month, the heat pump used 312.1kWh (~10.4 kWh/day) of electricity – about half of the total 609.4 kWh used through the month.

At an average cost of 6.2 p/kWh of electricity, this cost £19.35.

Using this electricity, the heat pump delivered 1128.6 kWh of heat at an average cost of (£19.35/1128.6) = 1.7 p/kWh.

If a 90% efficient gas boiler had delivered this energy it would have used (1128.6/0.9)= 1254 kWh of gas which would have cost £48.03.

So the heat pump delivered savings of approximately 60% over using a gas boiler.

If we had not used the battery to allow the use of cheap-rate electricity, then 312.1 kWh of electricity would have cost approximately £44.76 – roughly a 7% saving over using gas.

Heat Pump Performance

As the graph at the head of the page makes clear, the 5 kW Vaillant Arotherm plus heat pump performed well, providing heating and DHW uncomplainingly even in the cold weather.

Click image for a larger version. COP data from last 12 days of November 2021. The blue dots are hourly averages and the large yellow dots show daily averages. The data include the domestic hot water and anti-legionalla cycles which heat water above 55 °C. The trend line indicates that COP is typically between 3 and 4 for external temperatures between 0 °C and 11 °C.

The key measure of the performance of a heat pump is its coefficient of performance (COP). This specifies the ratio of the heating energy delivered divided by the electrical energy consumed.

The graph above shows how the COP varied hour-by-hour and day-by-day through the last 12 days of November.

It is clear that the COP falls at lower external temperatures. This is because the heat pump has to work harder to deliver the heat across a bigger temperature difference.

  • Outside the external temperatures are lower
  • Inside the heat pump increases the temperature of the water flowing through the radiators to deliver more heat.

More specifically,

  • When the external temperature is ~ 10 °C, the water flowing through the radiators is at ~ 32 °C, a temperature difference of ~22 °C. With this small temperature difference the heat pump can operate with a COP in excess of 4.
  • When the external temperature is ~ 0 °C, the water flowing through the radiators is at ~ 42 °C, a temperature difference of ~42 °C. With this temperature difference the heat pump can only achieve a COP of ~ 3.

Some hourly readings show COP values much less than the trend line. These are hours in which ‘odd’ events occurred, such as de-frost cycles on the heat pump heat exchanger, or re-heating the hot water tank at 55 °C.

Carbon Dioxide Emissions

Since this is just a monthly analysis, I will consider only the fuel costs and neglecting the embodied carbon in the heat pump etc.. But please be assured, in the fuller analysis this is fully accounted for.

During November MyGrid GB reported the average carbon intensity to be 235 gCO2/kWh whereas Carbon Intensity reported the average to be 191 gCO2/kWh. The graph below shows the hour-by-hour data and the curves look similar but appear to be offset by ~ 45 gCO2/kWh. I don’t know which one is correct but I am going to calculate with the Carbon Intensity figures because they allow me to download half-hourly data for the whole month. The difference between the estimates can be added as a constant. Apologies for the confusion.

Click image for a larger version. Hour-by-hour Carbon intensity data from Carbon Intensity and MyGridGB They appear to differ by a constant additive number of ~ 45 gCO2/kWh. I do not know which one is right.

Analysing the data, I find that the 4 hours of off-peak electricity had an average carbon intensity of 141 gCO2/kWh versus 191 gCO2/kWh for the other 20 hours. (This would be ~ 186 gCO2/kWh versus 246 gCO2/kWh for the MyGridGB estimates.)

Using Carbon Intensity figures I estimate that:

  • 428.9 kWh of off-peak electricity @141 gCO2/kWh released = 60.47 kg CO2
  • 51.6 kWh of off-peak electricity @191 gCO2/kWh released = 9.86 kg CO2

for a total of 70.33 kgCO2 released from electrical use through the month. (92.46 kgCO2 using the MyGridGB data)

The effective carbon intensity would be 146 gCO2/kWh (or 191 gCO2/kWh if the MyGridGB figures were correct)

The heat pump used 312.1kWh of electricity and so released 45.57 kgCO2. Dividing this by the 1128.6 kWh of heat delivered by the heat pump (£45.57/1128.6) = 40 gCO2/kWh. Using the MyGridGB estimate this would increase to 53 gCO2/kWh.

Whichever estimate is correct, this is truly low-carbon heating.

If a 90% efficient gas boiler had delivered this heating it would have released 251 kgCO2.

So the heat pump emitted around 20% of the CO2 which would have been emitted by using a gas boiler.

Even if we had not used the battery to allow the use of cheap-rate electricity, the carbon savings would still be dramatic.


As I wrote the other day, I am relieved to find that all the investments I have made in my home are working – the house is emitting just a small fraction of the CO2 it emitted previously, with absolutely no loss of quality of life.

  • The triple-glazing and external wall insulation reduce heating demand by half
  • The solar panels are still delivering 20% of our electricity demand in November!
  • The battery allowed me to time-shift the use of low-carbon off-peak electricity.
  • And the air source heat pump operated with more than 300% efficiency even on the coldest says.

Time to snuggle up…

10 Responses to “November 2021: Heating and Carbon Emissions”

  1. Peter S Says:

    Have you tried increasing the efficiency of your heating by using radiator fans? I live in an apartment that sometimes get a bit too cold and since I’m unable to control neither the temperature nor the max flow rate of the incoming water, I’ve been experimenting with getting more heat out of what I’m given.

    I’ve suspended a bunch of old computer fans underneath the radiator, blowing upwards, and it kind of works. Although I only get about a 0,5 °C increase in indoor temperature so far, the temperature differential between in and outflowing water from the radiator went from 45/38 °C without fans, to 45/35 °C with fans. And yesterday when it was really cold I got a fairly substantial increase from 55/44 to 55/39 – so it seems like I can increase the heat output by almost 50% which is kind of neat since the fans only consume about 5 watts. I could probably make a much better installation as well by for example ducting more airflow in behind the radiator.

    Your situation is of course very different from mine since you have to pay for the extra heat you pull out of the water – but it might enable you to lower the water temperature for a given heat output and thereby increase your COP.

    • protonsforbreakfast Says:


      Hi. Yes, I have indeed thought of that, but I have not yet tried it.

      I wrote about how radiators work back in September

      and my calculations slightly surprised me at just how much heat was transferred by convection. So forcing the convection could make a fair difference – especially with low flow temperatures in the radiators.

      There actually many products available specifically for this purpose

      but any fan would do.

      The measurements you have made are inspiring in their simplicity: Delta T has increased from 11 °C to 16 °C – extracting 45% more. Measurements like these will keep you warm and help pass the long winter evenings.

      Best wishes


  2. Dan Grey Says:

    Hi Michael, great post! You’ve probably written this up, but how does your DHW work? What size and type of tank, and when do you tell heat pump to heat it up?

    • protonsforbreakfast Says:


      Good Evening.

      I have a 200 litre ‘Unistor’ tank which is heated to ~ 55 °C once each day at 2:00 a.m. to take advantage of Octopus Go cheap rate.

      Once a week I run an anti-legionella cycle which heats the cylinder to ~ 65 °C.

      If we have children at home we ‘boost’ the hot water when it is required. The 5kW Arotherm plus pump is set to ‘eco’ which limits normal operation to 3.6 kW which more or less half-fills the tank with heat in an hour. Over that time-scale domestic temperatures are unaffected.

      We have a blending vale on the cylinder outlet to avoid the possibility of scaldingly high temperatures at taps after the anti-legionella cycle.

      Best wishes


      • Dan Grey Says:

        Thank you! Two things I wonder about:

        How much higher would the DHW CoP be higher if the tank heating occurred in the warmest part of the day, typically mid-afternoon? (Theoretically; setting aside grid carbon intensity and ToU costs)

        Could overall efficiency be increased if the heating could be driven from the PV panels when their output is sufficient? Rather than doing a round-trip through the Powerwall.

        The second could make the first viable, at least on sunny days.

      • protonsforbreakfast Says:

        1. Time of day for DHW heating
        Basically yes, but I don’t think it’s a big effect. Why?

        The mains water temperature does not change much during the day so the basic amount of heating required remains the same.
        The air temperature might change by (say) 10 °C and so the COP for heating to 50 °C might change, but by how much?
        Once a week I heat the water by an extra 10 °C to 60 °C for the anti-legionella cycle. This 10 °C change in Delta T will probably be roughly equivalent to the roughly 10 °C change in air temperature. As the graph below shows, the COP penalty for this 10 °C is small. The COP changes from around 3.3 to 2.9

        2. Direct Heating vs using a heat pump via battery.
        Basically No.
        Direct PV ===>>> water heating = 100% efficiency.

        PV ===>>> Battery ===>>> Mains = 90% efficiency
        Mains power to heat pump ===>>> water heating = 300% efficiency.
        Overall efficiency = 90% x 300% = 270%

        So it is not even close.

      • protonsforbreakfast Says:

        Also, regarding weather compensation, did you catch this article?


  3. Dan Grey Says:

    I can’t seem to reply directly to your comment so am doing it here

    “Direct Heating vs using a heat pump via battery” -> so there’s no route from the solar PV through the inverter to the heat pump? It has to be driven from the mains?

    • protonsforbreakfast Says:


      Hi. The logic for use of solar electricity is this:

      1. Use it meet immediate domestic load
      2. If domestic load is met use excess solar to charge the battery.
      3. If domestic load is met and battery is charged, export excess solar to the grid.

      I will send you a direct message (@dan_grey) with an image of day with some solar.

      Best wishes


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