Actress Q’Orianka Kilcher of the film The New World is testing a Honda fuel-cell car.

Which is cleaner, a battery car (running on power from the grid), a plug-in hybrid car (with some power from the grid), or a fuel-cell car running on 100 percent renewable hydrogen? There’s an endless debate about this, and it obviously depends a lot on how the question is asked (and, sometimes, who’s asking it).

Dr. C.E. “Sandy” Thomas, president of Virginia-based H2Gen Innovations, is a major fuel-cell enthusiast, and he says that hydrogen is the winner from every measure, including greenhouse gas emissions. And, in fact, he says that plug-in hybrids don’t even offer any advantage on climate emissions over our current auto fleet.

Dr. Thomas’ evidence is a recent Argonne National Laboratory report, which he says concludes that a plug-in hybrid with 40 miles of all-electric range would have “higher greenhouse gas emissions running on coal-generated power…than a non plug-in gasoline hybrid car.”

Interesting. But an analysis by the Electric Power Research Institute (EPRI) concludes that there will be considerable climate benefits from plug-in hybrids. It says they could reduce carbon dioxide emissions by 474 million metric tons if they have high penetration in the auto fleet by 2050. Even with low penetration, they reduce emissions by 163 million metric tons by then.

The question most people ask, however, is about smog pollution. With electric cars, they say, aren’t you just switching from one source of pollution — the tailpipe — to the coal-burning power plant? But a study by the Los Angeles Department of Water and Power concludes that, over a 100,000-mile life cycle, a standard gas car would produce 3,000 pounds of pollutants, and an EV with power from a coal-fired power plant would produce just 100 pounds.

There’s plenty of evidence that EVs, even running on coal electricity, are far cleaner in terms of smog emissions than are the cars we drive today. Jonathan Dorn, an EPRI staffer, recently produced a paper for the Earth Policy Institute on alternative power vehicles, and he says “even if we burned coal at night to power electric cars, it would still reduce carbon emissions by around 30 percent.” Dorn says that 100 percent conversion of the auto fleet to batteries would require only 10 to 20 percent additional electricity generating capacity.