From NCPA

According to a recent report by the Union of Concerned Scientists, the amount of greenhouse gases resulting from charging electric vehicles' battery packs varies significantly among the states. Crucially, in some regions of the country, the well-to-wheel emissions level for all-electric vehicles may be higher than the best hybrid and all-gasoline competitors, says the New York Times.

This occurs because many states produce electricity from relatively high emissions sources such as coal. This contrasts strongly with coastal states like California, Washington, New York and Louisiana that put emphasis on other sources.

The relative emissions of these sources can be seen in a comparative example:

• Two identical commuters are both driving a Nissan Leaf in the exact same circumstances, but one is in Los Angeles and the other is in Denver.
• The hypothetical Los Angeles Leaf would be accountable for the release of a low level of greenhouse gases into the atmosphere, similar to a gasoline car getting 79 miles per gallon (mpg).
• The Denver car, on the other hand, would cause as large a load of greenhouse gases to enter the atmosphere equivalent to a 33 mpg car.

The Union of Concerned Scientists' study points out that this phenomenon is not confined to Colorado, but occurs often in regions that are particularly coal-dependent.

• For 45 percent of the U.S. population, an electric vehicle will generate lower levels of greenhouse gases than a gasoline-engine vehicle capable of 50 mpg.
• About 37 percent of Americans live in regions where a Leaf's greenhouse gas emissions would equate to a gasoline-powered vehicle rated at 41 to 50 mpg.
• Meanwhile, the final 18 percent of the population lives in regions where the carbon footprint would be equivalent to 31 to 40 mpg.

These final two values undermine the claim by all-electric vehicle manufacturers that those vehicles outperform any and all gasoline or hybrid vehicles, as a number of those vehicles can outperform these carbon footprint standards.

Source: Paul Stenquist, "How Green Are Electric Cars? Depends on Where You Plug In," New York Times, April 13, 2012.