This article from 2013 is, IMO, a very balanced summary of the question of how much 'greener' EVs are, or might be:
http://www.bbc.co.uk/news/magazine-22001356
It makes the point that Norway with its predominance of hydroelectric generation can, as @Crispin observes, achieve a bigger benefit than the UK whose generation mix is mostly fossil fuel based. Still, they put a 10% advantage in the UK for EVs at present, which is significant and will grow assuming the mix continues to shifts over time towards renewables or indeed nuclear (which of course it may not, for some time, but that depends on a lot of factors).
Meanwhile a lot of very exciting development going on in battery technology, both for vehicles themselves and for home energy storage which will smooth grid demand... and indeed for grid storage at some point, possibly. so looking at the comparative green economics of today's EVs isn't necessarily valid for the near future.
Just in the interests of being transparent: all the cars in Velma's household are currently internal combustion - but (cover your ears, Velma) I'm watching the EV sector with interest.