Electric cars have no tailpipe emissions, but their overall impact on the environment includes the electricity generated to recharge their batteries.

The carbon emissions associated with that power vary greatly, depending on the local utility that provides it.

Now an updated study from the Union of Concerned Scientists calculates that in many states, electric cars now have lower "wells-to-wheels" emissions per mile than they did just a year ago.

DON'T MISS: New Study Doesn't Say 'Electric Cars Aren't Green' (Headlines To The Contrary)

So how'd they do that? The answer is that two things have changed.

First, the sales-weighted mix of electric cars sold has gotten slightly more efficient from December 2010, when the first one was sold, to June 2014.

Electric-car wells-to-wheels emission equivalencies in MPG, Sep 2014 [Union of Concerned Scientists]

An increase in the electric range of the Nissan Leaf from 73 miles (2011-2012 models) to 84 miles (2014-2015 models) accounts for part of this increase.

So does the May arrival of more efficient new models like the BMW i3, which has the highest efficiency rating of any car sold in the U.S. this year.

That means it takes fewer watt-hours to drive 1 mile than it used to.

ALSO SEE: 2014 BMW i3 Electric Car Rated At 81 Miles, 124 MPGe

Second, the UCS updated its "State of Charge" study in September, to use more recent data on the state-by-state mix of generating sources and the carbon dioxide--a greenhouse gas--associated with the production of each kilowatt-hour.

The previous edition of the study used 2009 plant emissions and generation data from U.S. eGrid report. The latest version updates that by one year, to 2010 data; each was the most up-to-date breakout at the time.

U.S. electric power generation by fuel type [Energy Information Agency, U.S. Dept of Energy via UCS]

The UCS points out that data from 2011 and 2012 (not yet included in the eGrid model) indicates a further reduction in coal generation.

The net effect is that the MPG equivalency figures for electric cars have risen from both more efficient use of every kilowatt-hour, and lower carbon emissions associated with each of those kWh.

In California, which buys up to half of the nation's plug-in cars, a gasoline car would now have to deliver 95 mpg (up from 78 mpg) to be as low-emission as the average plug-in electric car.

And the UCS notes that the wells-to-wheels emissions per mile for plug-in electric cars has fallen in virtually every U.S. region.

Three more states have entered the "Best" category, meaning the emissions from an electric car in those states are lower than those of a 50-mpg Toyota Prius--the most efficient gasoline car you can buy without a plug.

Even some northern Plains states (Iowa, Minnesota, Nebraska, North Dakota, and South Dakota) moved from "Good" (31-40 MPG equivalent) to "Better" (41-50 MPG).

MORE: EPA's Coal-Plant Emissions Rules: This Will Be The Big Fight

A variety of studies shows that electric cars are always cleaner than gasoline and diesel cars unless they're recharged using electricity produced largely or entirely from coal.

Today, coal is less than 40 percent of the average U.S. generation mix, and that percentage is expected to fall as utilities switch more of their mix to natural gas, while simultaneously boosting the capacity of renewable sources: wind, solar, and hydroelectric.

Electric-car wells-to-wheels emission equivalencies in MPG, Sep 2013 [Union of Concerned Scientists]

But the percentage of coal varies greatly from state to state; California and some Northeast states have the cleanest grids.

Illinois, Ohio, North Dakota, West Virginia, and Wyoming presently have the dirtiest grids--but even North Dakota improved on this year's study.

[hat tip: Rik]

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