Power Power is the time rate of using, or delivering, energy Power= Energy /Time(P=E/t) In metric system, we use units of watts for power, where one watt is one joule per second In the United States, automobile engines and electric motor are often rated in horsepower. 1 horse power= 550 ft- lb/s=745.7 wattsPower: Power is the time rate of using, or delivering , energy. Power = Energy / Time (P = E / t) In metric system, we use units of watts for power, where one watt is one joule per second. In the United States, automobile engines and electric motor are often rated in horsepower. 1 horse power = 550 ft∙lb/s = 745.7 watts