But where should I plug it in? (Rebecca Cook/Reuters)

A new report from the Union of Concerned Scientists provides an answer: It depends on where in the country you’re plugging in your electric car. Let’s say you charge your shiny new Leaf in the Midwest, where a large percentage of electricity comes from coal. In that case, the vehicle would still emit more carbon emissions than the newest, most efficient hybrids. (It would still, however, be cleaner than the typical new compact car getting 27 miles per gallon.) Electric vehicles powered by coal are only a so-so option for averting climate change.

On the other hand, an electric vehicle that’s charged out West or in the Northeast or even in South Carolina — places where cleaner natural gas, hydropower or nuclear power play a bigger role in electricity generation — is more environmentally friendly than even the most efficient hybrids. And a plug-in electric car charged with wind- or solar-generated electricity emits virtually no carbon. Here’s a map breaking down the charging options:

There’s a lot of variance between states. For example, the report (pdf) notes that the Rocky Mountain region, which includes Colorado, has the most carbon-intensive electric grid in the country. An electric vehicle charged in this area would produce as much greenhouse-gas pollution as a conventional car that gets about 34 miles per gallon — like, say, the Ford Fiesta.

The UCS report, which is generally bullish on electric cars, tries to emphasize two points. The first is that 45 percent of Americans live in the regions that are “best” for charging an electric car. So in large swaths of the country, electric cars appear to be the most eco-friendly option. But the report also emphasizes that, even in the Midwest, electric vehicles still generate less carbon pollution than “the average new compact gasoline-powered vehicle” sold on the market today. That’s largely due to the fact that electric motors tend to be more efficient than the traditional combustion engine.

There have been previous studies of the “green advantage” of electric cars, but most of those have approached the topic from a broad standpoint. The UCS report is the first to examine the effects of plug-ins in different regions of the country.

The report also notes that electric-vehicle owners can expect to save $750 to $1,200 a year in fuel costs, compared with an average gasoline-powered vehicle that gets 27 mpg. Whether that actually compensates for the extra up-front cost of the electric vehicle is a more complex calculation — see Daniel Altman’s post here for more on that.

Paul Stenquist of the New York Times, who was the first to report on the UCS study, has more commentary here. Among other things, he notes that climate considerations aren’t the only advantages of electric cars — among other things, they’re also better-shielded from wild swings in the oil markets.