1Power is how fast energy is transferred
The power of an appliance is how much energy it transfers each second. It is measured in watts (W): 1 W means 1 joule every second.
powerwatt
Lesson 12.2 notes
Power and energy
Power ratings in watts and kilowatts, energy = power × time, and the kilowatt-hour.
2Power ratings
Every appliance has a power rating, usually on a label underneath. This hair dryer is 1800 W: it transfers 1800 joules every second it is switched on.
power rating
3Kilowatts
Big powers are given in kilowatts (kW): 1 kW = 1000 W. To change watts into kilowatts, divide by 1000: a 3000 W kettle is 3 kW.
kilowatt
4Heating takes the most power
Appliances that heat things, like electric showers, kettles and hair dryers, have the highest power ratings. Lights, chargers and TVs have much lower ones.
5Energy = power × time
The energy transferred depends on the power and on how long the appliance is on: energy (J) = power (W) × time (s). 2000 W for 10 s is 20 000 J.
6The kilowatt-hour
Bills use a bigger unit, the kilowatt-hour (kWh): the energy a 1 kW appliance transfers in 1 hour. energy (kWh) = power (kW) × time (h).
kilowatt-hour
7Power or energy?
A kettle has 30 times the power of a TV, but it is only on for a few minutes. A TV left on all evening can transfer more energy.
Explore: Appliance lab
Switch on an appliance and watch the energy add up. Change the time and see the energy in joules and kilowatt-hours.
Common mistake: Power is not the same as energy
Power is how fast energy is transferred. A 3000 W kettle transfers energy quickly, but a 100 W TV left on all evening can transfer more energy than the kettle does in a few minutes.
Remember
- Power is the energy transferred each second. 1 watt (W) = 1 joule per second; 1 kW = 1000 W.
- energy (J) = power (W) × time (s).
- energy (kWh) = power (kW) × time (h). 1 kWh = 3 600 000 J.
Going further at GCSE