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Electricity

The CO2 emission factor used is 0.309 kge / kWh, taken from BEIS (2018) [1] . This includes an allowance for the 7.8% of transmission/distribution losses on the national grid [1]. There is a more recent (2019) document [2] with a smaller factor of 0.277 kge / kWh. However, the 2018 value is preferred here because much of the reduction is due to the switch from burning coal to burning wood pellets, with the burning of wood pellet being treated as zero CO2 emission. There are strong arguments that large scale burning of wood cannot be considered to be zero carbon [3][4].

The average electricity consumption is 4,800 kWh per household [5] . A smaller than average household is taken arbitrarily to be 3,000 kWh (i.e. roughly two-thirds of the average), and a larger than average household to be 7,000 kWh (i.e. roughly 50% more).

A value of 2000 kWh per person per year is used for student accommodation in a hall of residence [6].

(Mark Linas [7] makes the following alternative suggestions: small house: 1,650 kWh; medium house: 3,300 kWh; and mansion: 5,000kWh.)

Domestic electicity use (excluding heating) is made up of [8]:

Average domestic electricity use (excluding heating)	%
Cold appliances	18%
Cooking appliances	15%
Wet appliances	15%
Lighting	19%
Consumer electronics	19%
Domestic ICT	9%
Other	5%
Total	100%

'Green' electricity

For selected "green" tariffs, we recommend a reduction of 25% in the CO2 emission factor - see "Green electricity" (document 16).

Average versus marginal emission factors

Part of the electricity supply is generated from low carbon renewable sources (wind and solar power) and part from high carbon sources (fossil fuels). When someone switches on an electric light, or plugs in an electric vehicle for recharging, the additional demand on the national grid is met by burning more fossil fuel since this where the spare capacity lies.

So the average emissions for electricity production is not a good guide to decisions on whether a switch should be made from burning fossil fuel to powering by electricity.

Instead, a more accurate way of accounting would be to allocate the fixed low carbon electricity on a per-capita basis as zero or low emissions, and then electricity use above this would be calculated at the marginal rate.

For simplicity, for the time being, the calculator uses the average emission rate for electricity. but people should be wary of claims that electric vehicles are zero carbon because in fact, in most cases, the electricity used is derived from fossil fuels.

Another consequence of marginal emission accounting is that installation of solar panels or other renewable generators reduces emissions at the marginal rate, not the average rate.

Natural gas

Most modern gas meters measure gas in cubic metres. The energy contained in gas is measured in kilowatt-hours (abbreviated to kWh) and for natural gas, this is 11.2 kWh per cubic metre.

Older gas meters measure gas in hundreds of cubic feet - 100 cubic feet equal 2.83 cubic metres. So the energy contained in gas measured by an older gas meter is 31.7 kWh per 100 cubic feet.

The CO2 generated by burning natural gas is 0.185 kg / kWh [9] .

In 2006, the total UK gas supplied was 1,047,000 GWh, but of this 79,400 GWh was 'Energy industry use' and 12,000 GWh was 'Losses' (see source [5] Table 4.1). These total inefficiencies were 91,400 GWh, i.e. 8.7%, and so the CO2 emissions need to be adjusted by this amount from 0.185 to 0.203 kg / kWh.

The average UK annual gas consumption is 16,000 kWh per household [5], but per gas meter is 18,000 kWh [5] (a larger amount as not every household has a supply of natural gas). A smaller than average household is taken arbitrarily to be 12,000 kWh (two-thirds of the average gas meter), and a larger than average household to be 27,000 kWh (50% more).

A value of 5000 kWh per person per year is used for student accommodation in a hall of residence [6].

(Mark Linas [7] makes the following alternative suggestions: small house: 10,000 kWh; medium house: 20,500 kWh; and mansion: 28,000kWh.)

Heating oil

The factor assumed is 2.96 kg CO2 per litre of oil.

The CO2 emissions from the burning of oil (from source [8]) is 2.52 kg CO2 per litre (which is equivalent to 3.15 kg CO2 per kg, and 0.245 kg per kWh) [8]. But this needs to be adjusted for the fossil fuel used in the extraction of oil and in refinery inefficiency, which together gives an inefficiency of 15% (see car sources page), giving a figure of 2.96 kg CO2 per litre.

Other sources give 2.5 kg/litre (NEF) [10], and 3.0 kg/litre [7].

Coal

The emission factor assumed is 3.26 kg CO2 per kg of coal. This is the value given by DEFRA (2012) [11]

Wood

The emission factor assumed is 0.10 kg CO2 per kg of wood. This is based on the values given by DEFRA (2012) [11]. The direct emissions are here taken as zero for home wood burning on the basis that the wood is sourced sustainably, and the CO2 that is released is just what will be taken up when the trees regrow, but there is a small level of emissions due to transport and other overheads . Whether it is correct to count wood as zero emission when done on an industrial scale is a highly contentious issue according to Biofuelwatch [4].

Bottled gas

The emission factor assumed is 3.68 kg CO2 per kg of bottled gas. This is based on the values given by DEFRA (2012) [11].

Heating by hydrogen?

The UK Government is proposing that homes are heated by hydrogen, as part of its "Net Zero 2050" strategy. Hydrogen is currently maufactured from natural gas (see document 97), and so a switch from heating by natural gas would not reduce CO2 emissions significantly.

References

[1]	BEIS (2018) 2018 Government GHG Conversion Factors for Company Reporting: Methodology paper for emission factors: final report https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/726911/2018_methodology_paper_FINAL_v01-00.pdf

[2]	BEIS (2019) 2019 Government Greenhouse Gas Conversion Factors for Company Reporting: Methodology paper for emission factors: final report https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/829336/2019_Green-house-gas-reporting-methodology.pdf

[3]	Congress Says Biomass Is Carbon-Neutral, but Scientists Disagree (2018) Scientific American https://www.scientificamerican.com/article/congress-says-biomass-is-carbon-neutral-but-scientists-disagree/

[4]	Biofuelwatch https://www.biofuelwatch.org.uk/ (accessed 9.10.22)

[5]	BERR Energy Trends December 2007 www.berr.gov.uk/files/file43304.pdf or www.carbonindependent.org/files/file43304.pdf

[6]	Bezyrtzi et al (2006) Estimation of the Carbon Footprint of Student Halls of Residence in the University of Strathclyde https://pure.strath.ac.uk/ws/portalfiles/portal/240026/strathprints006691.pdf

[7]	Mark Lynas (2007) Carbon Counter (Collins)

[8]	DEFRA (2007) Act on CO2 Calculator: Public Trial Version Data, Methodology and Assumptions Paper www.defra.gov.uk or www.carbonindependent.org/files/actonco2-calc-methodology.pdf

[9]	DEFRA (2007) Guidelines to Defra's GHG conversion factors for company reporting www.defra.gov.uk or www.carbonindependent.org/files/conversion-factors.pdf

[10]	National Energy Foundation www.nef.org.uk/greencompany/co2calculator.htm (viewed 26.1.08)

[11]	DEFRA (2012) 2012 Guidelines to Defra / DECC's GHG Conversion Factors for Company Reporting https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/69554/pb13773-ghg-conversion-factors-2012.pdf

First published: 2007
Last updated: 8 Dec 2023