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 |
In the UK, the regulation of electricity generated from renewable sources is complex - every electricity supplier is required to either buy some electricity from renewable sources, or to buy certificates from other suppliers who have bought from renewable sources, or to pay a fee. This system is designed to encourage investment in renewable sources, but it results in the trading of certificates that is almost impossible for the ordinary consumer to understand, since the 'greenness' of electricity can be sold to two or even three different customers by some suppliers.
The National Consumer Council has written a guide , and concludes that many suppliers offering green tariffs are doing little more than meeting legal requirements, and are not delivering the environmental benefits they claim. The guide says that even the best tariffs will reduce CO2 emissions by only a faction (because most of the certificates are still sold on to other suppliers) - the cost of not doing so is put at around £200 for the average household, which (it is judged) few households will be prepared to pay.
The calculator therefore assumes the reduction in CO2 emissions from the best 'green' tariffs to be just 25% - partly from a reduction in CO2 emissions, and partly from the influence on Government policy arising from the demonstration to the Government that some consumers are willing to pay a premium to reduce their CO2 emissions. The best tariffs are defined according to those ranking highly on Ethical Consumer Magazine's Ethiscore (www.ethicalconsumer.org viewed 2 March 2008) as follows:
• Good Energy electricity
• Ecotricity New Energy Plus
• GE 100 electricity tariff
• Ecotricity electricity
• GE 10 Electricity tariff
• N.Ireland Eco Energy green tariff
• RSPB/Scottish & Southern electricity
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 burning oil (also known as kerosene or paraffin).
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].
See Reference List