Adding extra batteries to hybrids, and a plug to charge them, is a good way to save gas, replacing it with electricity from the grid. Indeed, drivers could commute to work and back using almost no gasoline. Such "plug-in" hybrids have garnered support from those who hope to reduce consumption of foreign oil.

But it hasn't been completely clear that replacing gasoline with electricity produced largely from fossil fuels would help reduce greenhouse-gas emissions. And some have feared that using more electricity would drive up levels of sulfates, ozone, particulates, and other pollutants in the air.

A study released today by the environmental group National Resources Defense Council and the Electric Power Research Institute helps clear up these issues, showing that

plug-ins, once they're on the market, will significantly cut greenhouse gases. They'll also decrease other pollutants, on average, across the United States.