Marshalls_chris said:
There's more detail if you click through the link on that page to
http://www.earth.org.uk/_gridCarbonIntensityGB.html
To fully charge an Ampera/Volt from empty takes (according to Vauxhall) 12.7kWh, transmission losses are about 2% which takes that figure up to a round 13kWh.
According to today's earth.org.uk figures, if you had charged your car last night between 1am and 5am the average emissions would have been 408g/kWh, i.e. 5.3kg
If you charged it instead yesterday evening it would have taken 491g/kWh, i.e. 6.3kg CO2
1 gallon (UK) of petrol is equivalent to 14.3kg of CO2 (source
http://www.carbonindependent.org/sources_car.htm). That is made up from 10.9kg directly emitted when you burn 1 gallon of petrol plus 3.4kg to cover refining and distribution.
There are probably an infinite number of ways to combine those figures but:
At best a battery range of 50 miles with 5.3kg/charge is 106g/mile. A worst case for battery is 30 miles with 6.3kg/charge which is 210g/mile.
For petrol, best case might be 50 mpg which is 286g/mile. My MX5 does nearer 30mpg i.e. 477g/mile.