Every domestic electrical appliance has a power rating, shown in watts (W). Power is the energy transferred each second. One watt means one joule of energy is transferred every second, so 1 W = 1 J/s. The equation is power = energy transferred ÷ time.
A higher power rating means the appliance transfers more energy each second. A 2000 W kettle transfers 2000 J every second from the electrical supply to the thermal store of the water. A 60 W lamp transfers only 60 J every second, so in 10 s it transfers 600 J. The kettle changes the stored energy of its water much faster than the lamp does.
The energy an appliance transfers depends on both its power rating and how long it is switched on. A joulemeter can be connected to an appliance to measure the energy transferred. In the same time, a higher-power appliance gives a larger reading on the joulemeter.