Power:
The rate of doing work is called power, and it is defined as the work done in unit time. As a formula, this would be:
power = work done/time Taken
Power is expressed in several different units, such as the watt, ergs per second, and foot-pounds per second. The most common unit of power in general use in the United States is the horsepower. One horsepower (hp) is equal to 550 ft-lb's or 33000 ft-1b/min. In the metric system the unit of power is the watt (W) or the kilowatt (kW). One hp is equal to 746 watts; and 1 kW = 1.34 hp.
Example:
To compute the power necessary to raise an elevator containing 10 persons a distance of 100 ft in 5 s (assuming the loaded elevator weighs 2500 lb), proceed as follows:
Power = work done/Time Taken = 2500 x 100/5 = 50,000 ft-lb's/sec
Since 1hp = 550 ft-lb's/sec then required hp = 50,000/550
= 90.9 hp (67.81 kw assuming no friction losses)