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The growing number of hybrid cars on the road is a sign that more and more drivers are thinking about their impact on the environment. But hybrids won't be the only readily available, environmentally conscious option for long.

The plug-in hybrid -- a new breed of hybrid that will rely more heavily on electricity than gas -- is projected to be available for purchase by 2012. Researchers are developing bigger, longer-lasting hybrid batteries that can charge overnight on your home's electricity and power cars for up to 40 miles at one go.

Developing that battery is one key to the success of plug-ins -- another is a cleaner electricity supply. Powering our cars with clean energy would be a major asset in solving global warming. And plug-in hybrids won't just pollute less; they are predicted to cost half as much to drive as regular cars -- about 6 cents per mile versus 12 cents.

A 2007 joint report from NRDC and the Electric Power Research Institute predicts that with widespread adoption by 2050, plug-in hybrids will have reduced greenhouse gas emissions by more than 450 million metric tons a year (that's like eliminating 82.5 million passenger cars from the road) and cut petroleum consumption by 3 to 4 million barrels a day.

How a plug-in hybrid works

Plug-in hybrids are designed to accelerate and run primarily on electric power for local car trips, making them economical and ideal for errands and commutes. Depending on the model, a plug-in will be able to drive 10 to 40 miles on electricity alone, or on a combination of electricity and gas. As battery power runs down on longer drives, the gasoline engine kicks in. The battery would take about 5 to 6 hours to charge on a standard 120-volt outlet.

Plug-ins will likely be powered by lithium ion batteries, which can hold two to three times more energy than the nickel metal hydride batteries in current hybrids. Engineers are working to improve the power and durability of lithium ion batteries for plug-ins.

Between a hybrid and an electric car

When the plug-in hybrid comes out, it will offer advantages over both the standard hybrid and the electric car.

Like an electric car, a plug-in can fill up its battery from the electricity grid. That means fewer trips to the gas pump and a decrease in greenhouse gas emissions out the tailpipe. But at the point when the pure electric vehicle would sputter to a halt from a low battery, the plug-in's fuel tank serves as a back-up. Regenerative braking and driving with the gas engine recharge the battery on the go, as with a regular hybrid.

Green outlets = greener cars

Plug-in hybrid technology is another essential tool for solving global warming. As we lessen our dependence on coal power and increase our ability to rely on renewable energy sources like wind and the sun, plug-ins themselves will become more and more efficient modes of transportation. Why? Because plug-in hybrids that charge their batteries off solar or wind-powered electricity would release as much pollution into the air as a 74 mpg car. However, plug-in hybrids using coal-powered electricity -- while still far less polluting than today's conventional gasoline cars -- would result in higher greenhouse gas emissions than their standard hybrid counterparts.

The source of the electricity matters, and as the source becomes greener, plug-in hybrids will become even more powerful tools in the fight against global warming.

When can I buy one?

Companies including GM, Toyota, Ford and DaimlerChrysler are working on plug-in hybrids, and consumers can expect to see them reach the dealerships by 2012.

last revised 1/31/2008