Electric cars are promoted as the chic harbinger of an environmentally benign future. Ads assure us of "zero emissions," and President Obama has promised a million on the road by 2015. With sales for 2012 coming in at about 50,000, that million-car figure is a pipe dream. Consumers remain wary of the cars' limited range, higher price and the logistics of battery-charging. But for those who do own an electric car, at least there is the consolation that it's truly green, right? Not really.

For proponents such as the actor and activist Leonardo DiCaprio, the main argument is that their electric cars—whether it's a $100,000 Fisker Karma (Mr. DiCaprio's ride) or a $28,000 Nissan Leaf—don't contribute to global warming. And, sure, electric cars don't emit carbon-dioxide on the road. But the energy used for their manufacture and continual battery charges certainly does—far more than most people realize.

A 2012 comprehensive life-cycle analysis in Journal of Industrial Ecology shows that almost half the lifetime carbon-dioxide emissions from an electric car come from the energy used to produce the car, especially the battery. The mining of lithium, for instance, is a less than green activity. By contrast, the manufacture of a gas-powered car accounts for 17% of its lifetime carbon-dioxide emissions. When an electric car rolls off the production line, it has already been responsible for 30,000 pounds of carbon-dioxide emission. The amount for making a conventional car: 14,000 pounds.

While electric-car owners may cruise around feeling virtuous, they still recharge using electricity overwhelmingly produced with fossil fuels. Thus, the life-cycle analysis shows that for every mile driven, the average electric car indirectly emits about six ounces of carbon-dioxide. This is still a lot better than a similar-size conventional car, which emits about 12 ounces per mile. But remember, the production of the electric car has already resulted in sizeable emissions—the equivalent of 80,000 miles of travel in the vehicle.

So unless the electric car is driven a lot, it will never get ahead environmentally. And that turns out to be a challenge. Consider the Nissan Leaf. It has only a 73-mile range per charge. Drivers attempting long road trips, as in one BBC test drive, have reported that recharging takes so long that the average speed is close to six miles per hour—a bit faster than your average jogger.