To calculate the consumption of an electrical appliance in kWh, you have to take into account three factors:

1. the capacity of your electrical appliance, expressed in watt.
2. the number of hours that the appliance is in use in one day.
3. the number of days per year when the appliance is in use.

The calculation is as follows:

[number of  hours’ use] x [number of days’ use] x ([capacity of appliance expressed in watt] / 1,000) = number of kWh

The capacity should be divided by 1,000 to convert the number of watts into the number of kilowatts. This finally gives us the number of kWh (kilowatt hours).

Calculation for the energy consumption of a radio alarm

A radio alarm is on all the time and therefore uses energy continuously.

Annual energy consumption of radio alarm: 24 x 365 x (10 watts / 1,000) = 87.6 kWh

Calculation for the energy consumption of a vacuum cleaner

In the table below we assume that the vacuum cleaner is used for two hours once a week.

Annual energy consumption of vacuum cleaner: 2 x 52 x (2,000 watts / 1,000) = 208 kWh

Take a look at the list giving the consumption of the most common electrical household appliances.