The CO2 emission factor used is 0.527 kg / kWh [Defra, 14].
This includes an allowance for the 7.5% of losses on the national grid [Defra, 14] and some other inefficiencies that occur before electricity reaches the end user.
Other calculators that use a smaller value (e.g. 0.43 kg / kWh   ) appear not to allow for grid losses.
The average electricity consumption is 4,800 kWh per household .
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).
Mark Linas  makes the following suggestions: small house: 1,650 kWh; medium house: 3,300 kWh; and mansion: 5,000kWh.
Domestic electicity use (excluding heating) is made up of [Defra, 14]:
| Average domestic electricity use |
For selected "green" tariffs, we recommend a reduction of 25%.
Most modern gas meters measure gas in cubic metres (m3). The energy contained in gas is measured in kilowatt-hours (abbreviated to kWh) and for natural gas 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 [DEFRA, 18] .
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  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 , but per meter is 18,000 kWh  (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).
Mark Linas  makes the following suggestions: small house: 10,000 kWh; medium house: 20,500 kWh; and mansion: 28,000kWh.
The factor assumed is 2.96 kg CO2 per litre of oil.
The CO2 emissions from the burning of oil (from source ) is 2.52 kg CO2 per litre (which is equivalent to 3.15 kg CO2 per kg, and 0.245 kg per kWh) 
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, 2]
• 3.0 kg / litre [ML, 6].
The emission factor assumed is 3.26 kg CO2 per kg of coal. This is the value given by DEFRA (2012) 
The emission factor assumed is 0.10 kg CO2 per kg of wood. This is based on the values given by DEFRA (2012) . The direct emission are taken as zero, since the CO2 released is just what was taken up when the trees grew, but there is a small level of emissions due to transport and other overheads.
The emission factor assumed is 3.68 kg CO2 per kg of bottled gas. This is based on the values given by DEFRA (2012) .
See Reference List