In the months after Nissan’s announcement last year that it would soon introduce the Leaf, the world’s first mass-market electric vehicle, the company embarked on a 24-city “zero-emission tour” to show off the technology. The Leaf’s electric motor draws its energy from a battery pack that plugs into an outlet in your garage. It has no engine, no gas tank and no tailpipe. And during the time the car is on the road, it is truly a zero-emission machine. But at night, in your garage, that battery pack must refill the energy lost to the day’s driving with fresh electrons culled from a nearby power plant. And zero emission it ain’t.