A machine for generating mechanical power in rotary motion from the energy of steam at temperature and pressure above that of an available sink. By far the most widely used and most powerful turbines are those driven by steam. In the United States well over 85% of the electrical energy consumed is produced by steam-turbine-driven generators. Individual turbine ratings historically have tended to follow the increasing capacity trend but have reached limits imposed by material and machine design considerations. The largest unit shipped during the 1950s was rated 500 MW. Units rated about 1100 MW were in service by the close of the 1960s, and ratings up to 1300 MW saw frequent application in the 1970s and early 1980s. Units of all sizes, from a few horsepower to the largest, have their applications.