Power is the rate at which energy is transferred. A more powerful device transfers more energy each second. Power = energy transferred ÷ time, or P = E ÷ t. Because energy transferred equals work done, power = work done ÷ time as well.
Power is measured in watts (W). One watt is one joule per second. A 60 W lamp transfers 60 J every second, so in 10 s it transfers 600 J. Larger powers are given in kilowatts: 1 kW is 1000 W, so a 2 kW heater is 2000 W and transfers 2000 J each second.
Two people can do the same work and have different powers. If two people of the same weight climb the same stairs, the one who takes less time has the greater power, because the same energy is transferred in less time. Rearranging, energy transferred = power × time.