Plug-in electric hybrids give off less exhaust than gas guzzlers. And when they’re running on battery power, they generate no fumes at all. But there’s a catch. The power plants that make the electricity you use to recharge the battery produce plumes of pollution. But how much ozone gets produced depends on when you reach for that plug.

Electric power plants that burn fossil fuels release hydrocarbons and nitrogen oxides. And when sunlight hits those compounds, ozone forms. At night, however, well, it’s dark. And the compounds disperse in large part before the sun comes up.

Scientists modeled how much ozone would be generated if the cars were charged at three different times: overnight, just before they’re driven or just after. And they found that drawing electricity from the grid after-hours is best. So to drive green, plug in, lay down and get some ozone-free Zs.