This calculation is almost identical to the one used for running costs.

The critical difference here is the value being used for the CO² emissions rather than the unit cost of electricity, more specifically how that value has been derived.

Firstly it is necessary to know what forms of fuel are used for energy generation and what proportion of the whole each represents.

Thats the easy bit, most reputable sources agree on these figures for UK energy generation and the variations are very minor.

The amount of emitted carbon per generated kWh of electricity is a much more complex issue. Virtually every potential source of information has a vested interest in making one or more fuel types look better or worse than another. The way emissions are calculated can also have a significant effect on the results, for instance it is possible to claim that a solar panel generates zero emissions while in operation but taking into account the initial manufacture and disposal of the panel over a given life span can result in a figure of 13 - 731g/kWh.

The table below gives a minimum and maximum figure for grams per kilowatt hour based on life-cycle analysis, that is to say the figures include the carbon emitted as part of the production/construction process as well as emissions that result from disposal/destruction.

To derive a default average value for CO² emissions it is necessary to combine the information on emissions with the proportion of an average kWh by fuel type.

For example:
Coal generation represents 33.93% of the total annual energy production [Table 1] therefore we assume that an average kWh is 33.93% Coal generated. Using the figures in Table 2 we can see that an average kWh of Coal generated energy creates 986g of CO², 33.93% of 986g = 334.58g. Repeating this calculation and adding all the results gives the figures in Table 3.

By adding the "Emissions by percentage" from Table 3. we get a figure of 543g of CO² for an average kWh of generated energy.