Assuming the choice was only 100% petroleum or EVs run on 100% coal-electricity there would be some advantage to using EVs because the coal plants could be located away from population centers, thus allowing for cleaner air in our more densely populated areas. It’s also more likely that the coal plants could be monitored more closely and kept “cleaner”.
But all that is fairly irrelevant. We’re close to the point at which coal is producing only 1/3rd of our electricity. Another roughly 1/3rd is from natural gas which produces half the CO2 of coal. So EVs are vastly superior to ICEVs in terms of CO2 emissions. And they will get better and better as we close more coal plants and install more wind and solar.

However the reality as Bob has pointed out is that coal is being rapidly phased out, the generation of electricity from coal in the first 8 months of 2012 is almost 17% lower then in the first 8 months of 2011, coal now represents only 36% of total generation. Over 6 gigawatts of coal plants have been retired already in 2012 and many more are planned, there have been a few new coal plants however in many cases there emissions are up to 50% lower then the 40 year old coal plants they are replacing. If you look at individual states the coal phaseout is even more rapid, revealing that the overall fall is being tempered by a few holdouts. Not surprisingly many of the places with the most rapid reductions in coal are also the places EV’s are being sold in greater numbers. New york for example has seen a 61% drop, California is already at reasonably negligible levels of coal generation but what remains has seen a further 20% fall.

The coal powered EV meme is a common one, a lesser used but even more factually incorrect one is that some people still believe that significant amounts of oil and petroleum products are used for the generation of electricity. Combined these products represent 0.5% of the total electricity generated in the US. This is about 25 times less the the power generated by renewables. Even those low numbers have seen a greater then 25% fall in 2012.

The vast majority of the replacement of coal has been natural gas and renewables. Solar in 2012 has seen generation for the first 8 months of the year rise 131% and wind 17%.

So in summary are electric taxis, powered by dwindling levels of coal and increasing amounts of natural gas and renewables with no tailpipe emission whatsoever in the urban environment a good thing? I would say so.

Renewables are starting to become an appreciable part of our electricity generation.

There are no coal powered electric taxis. And that is a good thing.