Every domestic appliance has a power rating, shown on a label in watts (W). Power is the energy transferred each second, so one watt is one joule per second. We use the equation power = energy transferred ÷ time, which can be rearranged to energy transferred = power × time.
The power rating tells you how quickly the appliance changes energy from one store to another. A 2000 W kettle transfers 2000 J every second from the electrical supply to the thermal store of the water. A 500 W kettle transfers energy more slowly, so the same water heats up more slowly. A phone charger with a rating of about 5 W transfers far less energy each second.
You can use a joulemeter to measure the energy an appliance transfers. A 60 W lamp left on for 10 s transfers 600 J. A higher power rating does not mean a better appliance, only that it transfers energy faster.