1. Figures in the original table are expressed in kgCO 2 kWh -1: here they are scaled up to tCO2 MWh -1 - so figures are numerically identical, but the presentation here simplifies the calculations, and is better suited to the likely scale of wind energy developments.
2. Electricity is not a primary fuel, but a means of transporting energy from the point of generation to the point of use. Overall emissions from electricity generation depend not only on the primary fuel used, but also on the efficiency of conversion: for most systems, much of the energy is dissipated as heat. Among fossil fuels, natural gas is relatively 'clean', followed by oil with coal being the most polluting in terms of carbon missions. Electricity generated from renewable sources is zero-rated, as is electricity from nuclear sources. For renewable technologies and nuclear sources, there are carbon emissions associated with construction, servicing, maintenance, decommissioning and transportation of fuel. These may be relevant when examining marginal cases.
3. A common emission factor is used for all electricity supplied from public supply network. This emission factor does not vary from year to year. The long-term marginal factor assumes that, over a long time period (a decade or more) avoided electricity use will displace generation at a new Combined Cycle Gas Turbine ( CCGT) plant.
4. This is the emission factor for Fossil Fuel Mix, a 5 year average emission factor calculated using CO 2 emissions for 2002 and 2003 National Atmospheric Emission Inventory (Baggott et al., 2007) and for 2004 to 2006 from the Digest of the United Kingdom Energy Statistics (2007).