1 W is 1 J per second. So, for example, a 20 W electric lamp uses 20 J of electrical energy every second to stay alight.

Electrical appliances have electrical rating plates on them. These show the power rating of the appliance in W or kW (1000W). These are useful to tell you how much energy your appliance will use when switched on.

Energy suppliers (companies that provide electricity and gas) use the kilowatt hour as a measure of energy use, shown as kW hour or kWh.

One kWh is the same as the amount of energy used by a 1 kW appliance for 1 hour:

If a 0.79 kW electric oven is used for 2 hours to cook a roast lunch, can you calculate the energy used in kWh?

Household or domestic fuel bills include information about the energy used, including:

- the number of kW hours used
- the cost of each kW hour
- the total cost of the energy used

Gas is usually cheaper per kWh than electricity.

The cost of the energy used can be calculated: