Explaining a unit of electricity

May 28th, 2010 by Gavin Lanoe

Guernsey Electricity Ltd. charges for electricity in units.  One unit is equivalent to using 1000 watts of power (one kilowatt) for one hour.  This is expressed as a kilowatt hour or kW.h.  If a piece of equipment uses 500 watts it would use 1 kW.h in two hours.  (500 watts x 2 hours = 1 kW.h)

Examples:

A kettle boiling uses approximately 2850 watts of 2.85 kW per hour but only for 3 minutes at a time.

An electricity meter shows the kettle using about 2850 watts of power (click on image to enlarge)

You therefore need to boil the kettle 20 times for one hour of usage.

In that hour the kettle will use 2.85 kW.h or 2.85 units.

If using the Super Economy 12 tariff, at today’s electricity prices this would cost you 43 pence on the high rate and 18 pence on the low rate.

An electric kettle draws about as much power as fifty 60 watt lightbulbs.   Consequently you can save energy and money by only filling the kettle with the amount of water you need to boil and no more.

A 100 watt incandescent bulb uses 0.1 kW.h

The bulb must run for 10 hours to use 1 kW.h (100 watts x 10 hours = 1 kW.h)

If using the Super Economy 12 tariff this will cost the domestic customer 1 unit or 15 pence on high rate or 6 pence on low rate for 10 hours of light.

A low energy light (20 watts) equivalent to a 100 watt incandescent bulb uses 0.02 kW.h

The low energy bulb must run for 50 hours to use 1 kW.h

This will cost you 1 unit or about 15 pence on high rate or about 6 pence on low rate for 50 hours of light

A 42” plasma TV uses 0.18 kW.

The TV must run for 6 hours to use 1 kW.h

Where it gets interesting is when you look at Halogen Down lights.

A single halogen down light is usually 50 watts or .05 kW.  The light would be on for 20 hours to use one unit of electricity or 1 kW.h. This is half the power of a 100 watt incandescent bulb.  However, you will typically have four, six or even 10 halogen down lights in a room.  Ten down lights on for an hour is 0.5 kW.h, so for two hours they cost one unit of electricity.  If they are on for five hours per night over a year that’s £120 or about £10 a month to light one room.

A similar article is available on my website.

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