A yellow light for electric cars

In a move that will help cut oil use and greenhouse gases, at least eight big automakers are gearing up to sell new types of electric cars. GM, for example, plans to sell its Volt in 2010, while China wants to become the world's largest producer of plug-in hybrid electric vehicles (PHEVs). And a few entrepreneurs, such as Segway inventor Dean Kamen, are designing novel prototypes.

Leading the charge is President Obama. He wants carmakers to put a million PHEVs on US roads by 2015.

Advocates of these next-gen cars, however, often act like someone who takes for granted that a light will go on whenever they flip a switch. They fail to ask two critical questions:

Will electric utilities be able to build enough power plants for all these electron guzzlers? And, even if they can, will new plants simply burn coal or oil in the same old dirty way – negating to a degree the anticarbon benefits of hybrid-electric cars?

Only a handful of studies have looked at these questions. "It seems that general excitement with the idea of PHEVs springs from the perceived possibility of a 'free lunch,' " states the latest study, done by the Oak Ridge National Laboratory. "There are some concerns, however, about whether this lunch is really 'free.' "

The study says many electric-car owners will likely want to charge their vehicles in the evening during peak usage, rather than at night – even with lower rates. The result may be a rise in coal or oil burning by utilities. And all electricity consumers could see rates rise by 40 percent or more.

In Britain, a similar study by the Campaign for Better Transport found that a move to electric vehicles might require increasing the country's electricity capacity by 2.4 to 3.5 times.

Unless governments can force owners to plug in their vehicles during low-use times, there will likely be more power plants. And unless utilities shift to "clean" coal or abandon fossil fuels altogether – both seen as unlikely soon – electric cars may not be as big a solution to global warming as boosters might hope.

Still, the idea of shifting car pollution from tailpipes to the smokestacks – where it can be more easily cleaned – makes sense, as long as the right questions are answered before this big switch gets flipped.