Power is defined as the rate at which energy is transferred or the rate at which work is done. • power = energy transferred / time, P = E/t. • power = work done / time, P = W/t (power, P, in watts, W; energy transferred, E, in joules, J; time, t, in seconds, s; work done, W, in joules, J). • An energy transfer of 1 joule per second is equal to a power of 1 watt. • Students should be able to give examples that illustrate the definition of power eg comparing two electric motors that both lift the same weight through the same height but one does it faster than the other.