Power is the energy transferred each second, so we calculate it by dividing the energy transferred by the time taken: power = energy transferred ÷ time taken, or P = E ÷ t. Power is in watts (W), energy in joules (J) and time in seconds (s).
The equation can be rearranged. Energy transferred is E = P × t, so a bigger power for the same time transfers more energy. The time taken is t = E ÷ P.
Worked example: a lamp transfers 1800 J in 30 s, so P = 1800 ÷ 30 = 60 W. A 3000 W kettle that transfers 360000 J takes t = 360000 ÷ 3000 = 120 s.