Electric cars emit gases indirectly if they use widely available power from fossil fuel electric plants which burn coal, natural gas and petroleum and release greenhouse gases including carbon dioxide.

A comparison of electric cars from manufacturers Tesla and Mini - using data about electricity from coal powered electric plants in the U.S. - shows that they would effectively emit more carbon dioxide than hybrid cars from Honda and Toyota over a travel distance of 100 miles.

The carbon dioxide emissions were derived from the electric vehicles' respective battery capacities and estimated full charge driving distances as well as fuel powered cars' current government miles per gallon ratings.

In a 100 mile drive, an all-electric Tesla Roadster would require an average of 21.72 kilowatt hours of energy, the equivalent of having a medium-sized air conditioning unit operating for 21 hours.

To produce the 21.72 kilowatt hours with coal to power Tesla's Roadster, 47.4 pounds of carbon dioxide would be emitted. When the same amount of electricity is made with natural gas, 23.5 pounds of carbon dioxide are emitted. Natural gas is the second most used fuel for electric power in the U.S. according to the Energy Information Administration. Coal is first, with 48.5 percent of the total in 2007.

To power a Mini E (an all-electric Mini Cooper) the same 100 miles, 23.33 kilowatt hours of electric power are needed which would release 51 pounds of carbon dioxide.

Hybrid cars however - which work with a gasoline engine that charges an electric one - emit less carbon dioxide emissions than those vehicles powered only with electricity from coal power plants.

In a 100 mile drive, Honda's hybrid Insight would release 34.6 pounds of carbon dioxide while Toyota's hybrid Prius would emit 35.3 pounds of the gas.

Carbon dioxide is seen by scientists as a contributor to global warming, although the U.S does not currently regulate the gas. In 2007, estimated carbon dioxide emissions by U.S. electric generators, heat and power facilities increased 2.3 percent compared to the previous year to 5,549.9 billion pounds, the EIA reported.

In comparison, one of the most popular fuel-powered sedans in the U.S. would directly emit more carbon dioxide than the previously noted hybrids and electric vehicles. In a 100-mile drive, a Toyota Camry would emit 62.58 pounds of carbon dioxide. Heading into the high end sports car echelon, a Ferrari F430 would generate 121.25 pounds in the drive.

Electric and hybrid cars are increasingly entering the U.S. market and are seen as the vehicles of the future as awareness about the environment increases, the need to economize on fuel consumption, and even political aims such as increasing the nation's oil independence.

Years from now, cars more closely approaching zero emissions will likely be propelled by electricity from renewable energy sources, which, excluding conventional hydroelectric power, contributed only 2.5 percent of the nation's total electricity in 2007.