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.

The Leaf should be the first all-electric car off the starting grid, but followers are whirring hot behind it. Chevrolet is introducing the Volt, an electric car supplemented with a small internal-combustion engine that keeps the battery charged. Ford will come out with an electric version of its Focus in 2011, followed by models from Toyota, Volvo, Audi and Hyundai.

A true accounting of the environmental consequences of these cars would have to include the emissions of the power plants that supply their energy. When Department of Energy researchers carried out such an analysis, they found that the results vary considerably with geography.

The researchers split the continental U.S. into 13 regions defined by the North American Electric Reliability Corporation and examined the mix of power sources within each region—generally, a combination of coal, natural gas and nuclear energy, with a smattering of renewable energy thrown in. They then looked at how a new fleet of electric cars would alter that supply. Nuclear and renewables, which together account for less than a quarter of the U.S. electricity supply, are “always on” sources. Their energy gets used up quickly for routine tasks, leaving little to no green energy left over to help charge a burgeoning fleet of electric vehicles. In practical terms, this means that even if you live down the street from a wind farm, its energy is already spoken for before you plug in your plug-in.

With nuclear and renewables taken out of the equation, the researchers concluded that power for the fleets will have to come primarily from coal and natural gas. If you live in a place where natural gas is dominant, electric vehicles will reduce carbon dioxide emissions—in some cases by as much as 40 percent below that of an ordinary hybrid. In regions powered mostly by coal—a much dirtier fuel—electric vehicles will lead to an increase in the amount of carbon dioxide released into the atmosphere. The zero-emission tour may have ended this spring, but the controversy over what zero really means is just getting under way.

Power Sources
Where does your electricity come from? The answer depends on the time of day, day of week and where you live. To determine the sources of energy that will power the coming fleet of electric vehicles, researchers modeled the additional strain that a fleet of electrics would place on the grid. They found that the added demand will likely be met by plants burning fossil fuels. In fact, in the six regions whose numbers are highlighted in yellow, heavy contributions from coal mean that plug-in cars will emit at least as much in the way of greenhouse gases as would an ordinary hybrid.

Three Flavors of Electric Cars 
With no gasoline engine, cars such as the Nissan Leaf run exclusively on battery power. This limits their range to about 100 miles before they require a charge.

Plug-in Hybrid
Vehicles such as the Chevrolet Volt plug into the grid like all-electric cars, but they also include a small internal-combustion engine that can be used to charge the batteries.

Cars such as the Toyota Prius do not connect to the grid. An electric motor powers the car at low speeds, while an internal-combustion engine takes over at high speeds.