Wind power is a major potential source of renewable energy, and wind farms are starting to appear across the U.S. and Europe. In 2004 windmills generated 6,740 MW, or 6% of American electricity. The total cost of electricity from windmills is more than from other sources (hydro, coal and nuclear), and windmills do not provide reliable sources of power (as the wind is erratic and the time of supply does not usually meet match the time of demand), so the usual solution is to link the windmill to the electricity grid and sell the power. The Department of Energy is sponsoring a "smart" electric grid; electricity from windmills would feed into the grid. At present they depend on federal tax subsidies because the fixed costs are high.