The Hoover Dam produces about 4 billion kilo-watt hours worth of energy in a year. It provides power for Nevada, Arizona, and California which is about 1.3 million people. The dam has a U-shaped structure that has two wings that are each 650 feet long and about 20 stories tall. There are 17 main turbines that produce the energy, 9 on the Arizona side and 8 on the Nevada side. In the United States, hydropower generates nearly nine percent of the total electricity supply. In the Pacific Northwest alone, hydropower provides about two-thirds of the region's electricity supply. Currently, facilities in the U.S. can generate enough hydropower to supply electricity to 28 million households, which is equivalent to about 500 million barrels of oil. In Pennsyvania, there are not many, if there are any, hydroelectric power facilites. Hydroelectric dams require steep rivers and streams, rivers that Pensylvania's Appalachian Mountains don't provide. However, during the 20th century, there was one majior hydro-electric dam, the Holtwood Hydroelectric Power Plant. Built between 1905 and 1910, it was the first hydro-electric dam built on the Susquehanna River. In 1925, it's generators produced 107.2 megawatts of power. In 1987, the Holtwood Hydroelectric Power Plant was declared a historic engineering landmark.