Key facts: - The oceans' tides, waves, current, and heat can be used to generate electricity. These resources are renewable, because the moon's gravity drives tides, and winds create waves. Covering 70 percent of the Earth's surface, the oceans collect significant amounts of heat from the sun. - A tidal dam with a capacity of 240 megawatts (MW) has operated in France since 1966; a 100 MW tidal dam has generated power in China since 1987; and a 20 MW tidal dam has operated in Canada since 1984. Several other, smaller ocean energy systems are also in use around the world. Theoretically, ocean waves could produce more than 2 million MW of electricity, according to the US Department of Energy. - The best locations for harnessing wave power are regions with the strongest winds. Areas off the Northwest and Northeast coasts of the United States have good potential for ocean energy.