Emissions factors

The UK government's method for calculating carbon dioxide emissions associated with burning fuels is contained in the Annexes to the Guidelines to DEFRA's GHG conversion factors.

• Annex 1 contains carbon dioxide emissions resulting from burning fossil fuels for any purpose.  The kWh factors refer to the heat content of the fuel, not the amount of electrical energy that might be produced from it.

• Annex 3 contains the method for estimating the carbon dioxide emissions associated with electricity generation.  These are annual averages.  They vary with the mix of fuels used in generation, and the efficiency of the power stations. 

DEFRA's electricity emissions factors do not take account of the continuous, hour-by-hour variations in the carbon content of electricity.  These can be viewed in real time at Elexon's Balancing Summary Page.

Heat rate curves

The efficiency of a thermal power generator tends to reduce as output is turned down below maximum.  This decline is normally shown on the heat rate curve or Willans line. Examples of real and modelled heat rate curves are found here.