We are all told – increasingly often – to turn things off, use less energy, use energy efficient appliances.

But it helps to understand how much appliances and activities use, to know what to concentrate on.

To give an example, it makes no sense to turn off just a lamp in a room where an electric heater has been left on. The power used by the lamp may be 100 watts while the heater draws 2000 watts or more.

The 100W light globe

To start, let’s pick a familiar
energy-using object as a yardstick, say the 100-watt light globe.

How big a yardstick is 100 watts anyway? Let’s assume that a globe is on every night for six hours. That’s about 2200 hours a year.

So to work out the amount of energy used over that year, all we have to do is multiply hours by watts to get the energy in a unit called watt-hours: 2200 x 100 = 220,000 watt-hours.

To make it more manageable, we divide that figure by one thousand to get 220 kilowatt-hours, abbreviated to 220kWh.

To many people, a number like 220kWh doesn’t mean much – so let’s convert it into something familiar– say litres of petrol – energy equivalent.

A litre of petrol contains about 10kWh of energy. A kWh is 3,600,000 watt-seconds which is 3.6 megajoules (3.6MJ; a joule is a watt-second).