How to calculate the cost of running electrical appliances

The power used by an electrical appliance is expressed in Watts. All electrical appliances should display their nominal power consumption in Watts; this will either be on a plate of some kind or embossed into the casing.

• The cost unit of electricity is known as the kilowatt-hour (KWh).
• The numbers displayed by your electricity meter indicate how many kilowatt-hours your household has used.
• 1KWh is equivalent to the amount of energy used by a 1000 Watt appliance (or load) running for one hour.

In theory it is very easy to calculate how much your appliances cost to run. Firstly, you will need to determine how much your electricity supplier is charging per ‘unit’ or KWh. This should be shown on your bill.

This is a simplified version of the power / cost formula.

Here is an example:

My electricity supplier charges 18 pence per unit (KWh). I have just run a 650-Watt microwave oven for 15 minutes.

650/1000 = 0.65 it has been running for 15 minutes 15/ 60 = 0.25. 0.65 x 0.25 =0.1625 and 0.1625 x 18p = 2.9p The microwave has used 2.9 or 3 pence worth of electricity.

Some electrical loads will draw more power than they use on an AC supply. Things like washing machines, vacuum cleaners, power tools and fluorescent lighting. This is because they have what is known as inductance and present an “inductive reactance” to the supply. Electrical engineers refer to “reactive power” and poor “power factors” resulting in customers paying for large amounts of unutilised or ‘reflected’ power. If you have a large number of such loads it is worth getting an electrical engineer to check that you have adequate power factor correction for the sake of economy.