1000 days of data

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

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

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

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

Heating Demand

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

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

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

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

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

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

Gas Use 

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

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

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

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

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

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

Electricity from the grid 

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

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

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

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

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

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

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

Carbon Dioxide emissions. 

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

I have assumed that:

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

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

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

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

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

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

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

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

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

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

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

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

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

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

What else?

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

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

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

I’ll let you know how it goes…

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