One of the biggest arguments against electric cars — that they aren’t that green if they're charged on a coal-heavy grid — just got a big boost from a Carnegie Mellon study. But its findings don’t have much to do with the real world, because EVs — mostly on the east and west coasts, not the dirty Midwest — are plugging into far cleaner sources of power. The study concludes:

We find that [battery electric cars] have higher life cycle air emissions damages than gasoline [hybrids] in the recent grid scenario, which has a high percentage of coal generation on the margin. In particular, battery electric vehicles with large battery capacity can produce two to three times as much air emissions damage as gasoline [hybrids], depending on charge timing. In our future 2018 grid scenarios that account for predicted coal plant retirements, [battery electrics] would produce air emissions damages comparable to or slightly lower than [hybrids].

Might as well just buy a hybrid, right? Not so fast. The Carnegie Mellon study looked at the 13 states covered by the PJM Interconnect distribution service: Delaware, Illinois, Indiana, Kentucky, Maryland, Michigan, New Jersey, North Carolina, Ohio, Pennsylvania, Tennessee, Virginia, West Virginia and the District of Columbia. There’s a lot of coal on that grid. But if we look at where electric cars are actually sold, the picture brightens considerably.

Tesla Model S chargingA Tesla Model S charging in California's Muir Woods. That's one clean grid. (Photo: Robert Couse-Baker/flickr)

Nearly half of battery electric cars are sold in California, which offers generous subsidies, a robust charging network and super-valuable perks, like being able to drive solo in the HOV lanes. And what kind of grid does California have? A very green one. Coal was only 6.4 percent of generation in 2014, compared to 44.5 percent for much cleaner natural gas, a whopping 20 percent renewables (8.1 percent wind, 4.2 percent solar, 2.5 percent biomass) and 8.5 percent nuclear.

Southern California Edison, which offers incentives for EV owners to charge at night (as does San Diego Pacific Gas and Electric), offers this encouraging assessment: “Most of the resources used to power our grid are found close to home, and approximately 20 percent of the power we deliver to our customers is generated from renewable energy sources like wind and solar. When you fuel your vehicle, you are buying 100 percent North American-made energy instead of the imported oil that powers most conventional vehicles today.”

Tesla Roadster charges on windThis Tesla Roadster is charging on zero-emission wind, and it doesn't get cleaner than that. (Photo: The NRMA/flickr)

And the savings to EV customers charging on electricity amount to 25 to 50 percent, Edison said.

The best study we have of how EVs fare in California and other regions with low coal emissions is from the Union of Concerned Scientists in 2012. It’s encouraging, in no small part because the areas of the country with the best emission patterns are also those where EV sales are concentrated:

Nearly half (45 percent) of Americans live in regions where an EV has lower global warming emissions than a 50 mpg gasoline-powered vehicle, topping even the best gasoline hybrids on the market. Charging an EV in the cleanest electricity regions, which include California, New York (excluding Long Island), the Pacific Northwest, and parts of Alaska, yields global warming emissions equivalent to a gasoline-powered vehicle achieving over 70 mpg.

The Carnegie Mellon study does account for anticipated coal plant closings, but that process is only going to accelerate. And that’s the great thing about electric cars — they get cleaner as the grid gets cleaner. Gas cars just get dirtier — because their efficiency deteriorates and their tailpipe emissions get worse.

Even in the Midwest, coal is going the way of the dodo. And as this video from Minnesota makes clear, greens in the Heartland want to drive electric:

Jim Motavalli ( @jmotavalli ) writes about cars, technology and the environmental world to anyone curious enough to ask.