Some devices transfer energy faster than others. Power is the rate at which energy is transferred, or the rate at which work is done. It is found using power = energy transferred ÷ time, or power = work done ÷ time. Power is measured in watts (W), and 1 watt is 1 joule per second. A kilowatt (kW) is 1000 W.
A 2 kW kettle (2000 W) transfers 2000 J of energy every second, so it heats water faster than a 1 kW kettle. A more powerful device does not necessarily transfer more energy overall. It transfers the same energy in a shorter time.
Example: two students climb the same flight of stairs, so they do the same work. The student who takes less time has the greater power. A motor that transfers 600 J in 3 s has a power of 600 ÷ 3 = 200 W.