By Timothy Gardner

NEW YORK (Reuters) - Electric cars will not be dramatically cleaner than autos powered by fossil fuels until they rely less on electricity produced from conventional coal-fired power plants, scientists said on Monday.

"For electric vehicles to become a major green alternative, the power fuel mix has to move away from coal, or cleaner coal technologies have to be developed," said Jared Cohon, the chair of a National Research Council report released on Monday called "Hidden Costs of Energy: Unpriced Consequences of Energy Production and Use."

About half of U.S. power is generated by burning coal, which emits many times more of traditional pollutants, such as particulates and smog components, than natural gas, and about twice as much of the main greenhouse gas carbon dioxide.

Nuclear and renewable power would have to generate a larger portion of U.S. power for electric cars to become much greener compared to gasoline-powered cars, Cohan, who is also president of Carnegie Mellon University, said in an interview.

Advances in coal burning, like capturing carbon at power plants for permanent burial underground, could also help electric cars become a cleaner alternative to vehicles powered by fossil fuels, he said.

Pollution from energy sources did $120 billion worth of damage to human health, agriculture and recreation in 2005, said the NRC report, which was requested by the U.S. Congress in 2005 and sponsored by the U.S. Department of the Treasury.

Electricity was responsible for more than half of the damage, the report said.

Electric cars have other benefits such as reducing imports of foreign oil. But they can also have hidden costs

Materials in electric car batteries are hard to produce, which adds to the energy it takes to make them. In fact, the health and environmental costs of making electric cars can be 20 percent greater than conventional cars, and manufacturing efficiencies will have to be achieved in order for the cars to become greener, the report said.

Emissions from operating and building electric cars in 2005 cost about 0.20 cents to 15 cents per vehicle mile traveled, it said. In comparison, gasoline-powered cars cost about 0.34 cents to 5.04 cents per vehicle mile traveled.

The report estimated that electric cars could still cost more than gasoline-powered cars to operate and manufacture in 2030 unless U.S. power production becomes cleaner.

Hybrid gasoline-electric vehicles with batteries that are charged by the driver hitting the brakes scored slightly better than both gasoline-powered cars and plug-in hybrid cars, which have batteries that are charged by the power grid.