According to DUKES 3.4 the UK uses 38,287 thousand tonnes of oil in road transport (2005) and Miliband tells us car are responsible for 60% of road transport emissions so must use some 23 million tonnes. At 45 megajoules per kg, 23 million tonnes of oil represents 1018 joules or 288TWh. This is the primary energy, internal combustion engines are some three times less efficient than electric motors so to make sure we're not comparing apples with oranges the figure should be reduced to 96TWh. This compares with a total electricity supply of 409TWh (DUKES 5.2) and so represents 23% more electricity.

To get to Miliband's 12% we have to assume electric cars actually use less energy to deliver the same energy service (the transportation). This is actually quite feasible as the electric drive train is more efficient and the vehicle mass can be significantly reduced.