Pupils learn about units of energy and understanding energy bills

10 Learning KS3

Electrical appliances

The scientific unit for energy is the joule, which has the symbol J.

Power is the rate at which energy is used. The unit of power is the watt, which has the symbol W.

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:

energy in kWh = power in kW × time in hours

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?


Energy bills

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:

cost = energy used in kWh × cost of 1 kWh





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