This post has been updated below.

One criticism of electric cars is that they often just replace one source of carbon pollution with another. Instead of a combustion engine that burns gasoline, you get a plug-in vehicle that depends on electricity from burning coal. In extreme cases, this can look like a dismal trade-off. One recent report from the Innovation Center for Energy and Transportation estimated that, in many parts of China, a Nissan Leaf powered by coal-fired power plants could actually produce more carbon emissions per mile than a comparable gas-powered car. Bad news.

(Felix Ordonez/Reuters)

How is that possible? Petroleum refineries use a lot of electricity — Norby estimates that it takes about 6 kilowatt-hours of electricity per gallon of gasoline. [Update: This isn’t quite right, see below.] He then tacks on the electricity used for extraction, refining, and shipping and comes up with a “conservative” estimate of about 8 kwh per gallon. If that’s the case, then a gas-powered car that gets 22 miles per gallon would use about 40 kwh of electricity to go 100 miles. An average electric car, by contrast, would only use about 30 kwh of electricity to go the same distance. In other words, not only does the all-electric car use no gasoline — it would appear to use less electricity, too.

Now, before EV backers get too excited, there are all sorts of ways to complicate these numbers. Fuel-economy standards in the United States are rising fast and most new cars get significantly better mileage than this (in 2010, the average new passenger car got about 33.7 miles per gallon). You’d also have to look at what type of electricity was being used in each case (natural gas vs. coal, for instance). What’s more, this likely doesn’t change the analysis for China mentioned above, since that report did do a full life-cycle analysis that included a rough estimate for refinery-related emissions.

Still, it’s a striking stat — and one you’d expect to hear a lot more often. Sebastian Blanco at Autoblogreen notes that Nissan has tried to bring it up, but only very occasionally.

Update: It looks like Norby’s analysis is a little off. Thanks to commenter dmccabe1 for bringing this to my attention, any my apologies for not catching it earlier.

According to this Argonne study — and this analysis by the Department of Energy’s Jacob Ward—it takes about 6 kwh of energy to refine a gallon of gasoline, not 6 kwh of electricity, as I originally stated. Now, some of the energy inputs that a refinery uses (such as natural gas) could be used to generate electricity instead if we shifted away from regular cars to electric cars. But even in that case you wouldn’t get a full 6 kwh of electricity — probably less than half that. (Note, however, that you still need additional energy to extract and transport the gasoline.)

All told, electric cars still look pretty good in comparison, especially since they’re not burning any gasoline and putting additional carbon in the air, but it looks like it’s not, strictly speaking, true that “It takes more electricity to drive the average gasoline car 100 miles, than it does to drive an electric car 100 miles.” This probably explains why Nissan abandoned its sticker. The energy comparison still looks favorable for electric cars, but the talking point isn’t nearly as neat and clean.