Power is the rate at which work is done, P = w/t, or work per unit time. The watt (W) is the unit for power and is equal to 1 joule (or newtonmeter) per second. Horsepower (hp) is another familiar unit of power used primarily for rating internal combustion engines. A 1 hp machine does 550 ft⋅lb of work in 1 second and 1 hp equals 746 watts.
Power is the rate at which:
potential energy is converted into kinetic energy 

work is done 

friction is overcome 

input force is transferred to output force 