Bitesize has changed! We're updating subjects as fast as we can. Visit our new site to find Bitesize guides and clips - and tell us what you think!


Electrical appliances


  1. 1
  2. 2
  1. Next

We can calculate the amount of electrical energy transferred by an appliance and how much it costs to run. This is useful for comparing the advantages and disadvantages of using different electrical appliances for a particular purpose.

Electrical energy calculations

The amount of electrical energyelectrical energy: Energy transferred by electricity. transferred to an appliance depends on its power and the length of time it is switched on. The amount of mains electrical energy transferred is measured in kilowatt-hours, kWh. One unit is 1 kWh.

E = P × t

  • E is the energy transferred in kilowatt-hours, kWh
  • P is the power in kilowatts, kW
  • T is the time in hours, h.

Note that power is measured in kilowatts here instead of the more usual watts. To convert from W to kW you must divide by 1,000.

For example, 2,000 W = 2,000 ÷ 1,000 = 2 kW.

Also note that time is measured in hours here, instead of the more usual seconds. To convert from seconds to hours you must divide by 3,600.

For example, 7,200 s = 7,200 ÷ 3,600 = 2 h.

Example calculation

A 2 kW electrical fire is switched on for 3 hours. It uses 2 × 3 = 6 kWh of electrical energy.

Joules, watts and seconds

You also use the equation E = P × t when:

  • E is the energy transferred in joules, J
  • P is the power in watts, W
  • T is the time in seconds, s.

For example, a 20 W lamp lit for 100 seconds transfers 20 × 100 = 2,000 J (2 kJ).


  1. 1
  2. 2
  1. Next

Back to Energy and efficiency index

BBC © 2014 The BBC is not responsible for the content of external sites. Read more.

This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.