Written by Rachel BarronThe amount of wind power that the United States can generate has doubled to more than 20 gigawatts in the last two years, the American Wind Energy Association said Wednesday. Renewable-energy policies, such as state mandates that require utilities to get a certain amount of their energy from renewable sources, have helped drive the growth of U.S. wind from 10 gigawatts in 2006. The wind association, also known as AWEA, attributes part of the success of wind specifically to its lower production cost compared with other renewables, such as solar power. Earlier this year, the association said the United States had passed Germany as the world's biggest wind-energy generator. Germany still wears the crown for having the most installed wind-power capacity, but the United States generated more wind power due to stronger winds. AWEA expects the U.S. wind industry to reach a total capacity of about 24.3 gigawatts by the year's end. Such expansion would mark wind as one of the fastest-growing electricity sources in the nation, second only to natural gas, in terms of production capacity. Wind made up 35 percent of the total electricity capacity installed in 2007, according to AWEA. But capacity doesn't equal production. Wind power only provides about 1.5 percent of the nation's electricity. The U.S. Department of Energy in May forecast that wind power could reach 20 percent of the nation's power supply by 2030. AWEA said it is concerned that wind power might not realize its full potential if the federal government does not extend a production tax credit, which is set to expire at the end of this year. The credit pays 2 cents per kilowatt-hour of wind electricity generated from utility-scale wind projects.