Assumptions and data sources

Carbon factors for energy

For Oil and Gas I have used Government conversion factors for company reporting of greenhouse gas emissions [1]. I used the figures for gross calorific value and I added in well to wheel/tank as well as the direct emissions from combustion.

For Electricity, I used [1] for the basic carbon factor. I have assumed that off-peak carbon emissions are 10% little lower, which is in line with carbon emissions from table 12d of SAP 10 [2]. That table is about monthly variation, which I have ignored. However, it does also give different values for differnt tariffs and the low rate 7-hour tariff is consistently lower t han the standard tariff.

Also for electricity I have included an taper of 5% per year. This would reduce emissions by nearly 80% by 2050, which is in line with the UK government's latest Green Book Guidelines [3]

Energy costs

The price per kWh of all types of energy is highly variable. It depends on where you live, who you buy from, and what tariff you are on. Do amend these with your own bills.

Embodied carbon of heating systems

For the embodied carbon of heat pumps I used Finnegan (2018) [4]. This is based on only two studies which are quite out of date and I fear the default I have used, 3000 kg for a mid-size HP is probably on the high side if only because refrigerants have got better since then. However, the carbon savings from coming off gas are so high anyway that the odd tonne is not going to make much difference.

For storage heaters and other heating systems, embodied carbon was based on the weight of some market products and assuming this was mainly steel at about 2 kgCO2e/kg steel.

Embodied carbon in EVs and other other cars

I used data from Carbon Brief for the defaults - but this is highly variable.

Boiler efficiences

Boiler efficiencies are highly variable. They ought to be high e.g. > 90% for a condensing boiler - if it is well configured so that the return temperatures are low enough to achieve condensing. SAP is conservative with its default efficiencies, becuase you are supposed to look up the actual products in the efficiency database. SAP specifies a default of 84% efficiency in winter and 74% in summer for a modern gas condensing boiler with automatic ignition. For an oil boiler, the default is slightly lower: 84% and 72%. Summer efficiencies are lower because a higher proportion of the heat delivered is for water heating and for that the return temperatures will be too high for condensing. However, the overall average is nearer the winter time efficiency

Heat pump efficiency

Heat pump overall efficiencies are highly variable. Poorly configured heat pumps could easily have an SCOP of 2.5 or even less. However, SCOP of 4.0 is not unknown. So far this winter I am getting 3.1 from mine. The figure quoted on my MCS certificate was 2.96. Unfortunately this figure is critical in calculating both carbon savings and bills.

EV efficiency and other cars

I looked at the efficiency reported for some cars and took advice from friends for this.

Vehicle non-energy costs

I took advice from a friend on these. However they are highly variable. EVs typically have lower maintenance costs as well as lower tax. I suggest you look at RAC guidance on costs of running EVs..

Heating system capital costs

I have not used any particular sources for these. For heat pumps, see How much does it cost to install a heat pump. You may have a better idea for your particular situation. It depends partly on the size of the heat pump, and for a first time install there are a lot of other costs.

Solar panel modelling - matching availability with heating demand

The calculation of how much of your heat pump energy could come from your solar panels is based on modelling I have been doing for other projects. The models are too complex to run in this tool and I have approximated the results. So at best this is a reasonable estimate.

Solar panel modelling - if you do not know your export

The annual yield from your solar panels depends on the efficiency of your panels as well as where you live. 800 kWh/year/kWp is about right for my panels but they are fairly old. Yours may be better.

The variation in solar panel yields through the year are taken from SAP tables for the East of England which is where I live, as do a lot of other people. This will not be far wrong for elsewhere in the UK. Annual yield varies much more than the winter/summer ratio.

The calculation of how much you use of your own power is the same as in SAP, based on solar yield, daily use and batter size. This is based on standard consumptions patterns and your self consumption may be higher or lower.


[1] Government conversion factors for company reporting of greenhouse gas emissions

[2] Standard Assessment Procedure 10.2

[3] Green Book supplementary guidance: valuation of energy use and greenhouse gas emissions for appraisal

[4] Stephen Finnegan, Craig Jones, Steve Sharples (2018) The embodied CO2e of sustainable energy technologies used in buildings: A review article. In Energy and Buildings.