Power is the energy transferred divided by the time taken:
power = energy transferred ÷ time taken
Power is in watts (W), energy transferred in joules (J) and time taken in seconds (s).
Example 1: a motor transfers 600 J in 20 s. Power = 600 ÷ 20 = 30 W.
The equation can be rearranged. Energy transferred = power × time, so a 2000 W kettle running for 30 s transfers 2000 × 30 = 60 000 J. Time taken = energy transferred ÷ power, so a 500 W heater that transfers 10 000 J runs for 10 000 ÷ 500 = 20 s.
Example 2: a lamp transfers 1200 J in 20 s, so its power is 60 W. In 60 s it transfers 60 × 60 = 3600 J.