The United States is pushing the use of plug-in hybrids but these cars, which sip gasoline and are low in greenhouse gas emissions, face cost and infrastructure hurdles that will keep them out of the mainstream for years.

The cars have large batteries which can be recharged at home with an extension cord, unlike conventional gasoline-electric hybrid cars which have batteries that are only powered when the driver hits the brake.

The big batteries allow plug-ins to depend more on electricity, the use of which emits less carbon dioxide per unit of energy than petroleum products do, even if the power comes from coal, said Dan Santini, an expert at the Department of Energy's Argonne National Laboratory.

If the electricity comes from nuclear, or perhaps in the future from wind power, the greenhouse gas emissions reduction for plug-ins would be even greater.

The federal government, which is hoping to soon regulate greenhouse gases, is pushing development of plug-ins and President Barack Obama has set a goal for the United States to put 1 million plug-in hybrid cars on the road by 2015. Obama's stimulus plan had $2 billion for development of batteries.

Several car companies like General Motors plan to launch plug-ins and its Chevrolet Volt is expected to go on sale late next year.

Unfortunately, bigger batteries are expensive, which means the price for the Volt will run about $40,000, about double the price of a gasoline-electric hybrid like Toyota's Prius.

Plug-ins are not the perfect solution, they are not going to be the cheapest, said Ewan Pritchard, who manages North Carolina State University's Advanced Transportation Energy Center and who has converted conventional buses to run as plug-in hybrids.

In addition, plug-ins take about four to eight hours to recharge and have limited range, so many owners will want to charge them both at home and at work.

That means a network of charging stations would have to be built for the cars to catch on.

Richard Lowenthal, founder and chief executive of California-based Coulomb Technologies, Inc, which sells charging stations, said only 50 million homes in the United States have a place like a garage to charge a plug-in, while there are 250 million cars in the country.

In addition, the best and cheapest batteries are currently being manufactured in Asia, which has led to criticism that the cars simply swap an addiction on foreign oil for a need for foreign batteries.

Let's say we switch over to batteries. I don't know that we've helped ourselves, Texas oil billionaire T. Boone Pickens told the Alternative Fuels & Vehicles conference in Orlando this week. We'd go from being dependent on Saudi Arabia for oil to China for batteries, said Pickens, who has invested in companies to convert cars to run on domestic natural gas.

The good news is that the United States should have plenty of electricity capacity to power the cars. Britta Gross, an electrical infrastructure expert for GM, said power plants have enough spare capacity to charge plug-ins on all but about the dozen hottest days of the year when air conditioning demand soars.

Mike Waters, an efficiency expert at Progress Energy said a smart grid, or power transmission system that can send information from power plants to customers, could soon send pricing signals to plug-in owners that would encourage them to stop charging when peak power demand nears.

And though the hurdles are high, some companies could clear them as more of the cars are built. The point is to get these vehicles on the road so we can see where the problems are and move from there, Pritchard said.

(Reporting by Timothy Gardner, editing by Marguerita Choy)