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Lightbulbs.xls

This is an example of how much money you can save by changing an ordinary lightbulb for an energy saving one.

I have a 100 watt lightbulb in my kitchen.

I can replace it with an energy saving bulb of 20 watts power.

I need to work out how much energy I will save in a year. Then I can convert that into money.

Energy use is usually measured in kilowatt hours (kwh). A kilowatt is 1000 watts and a kilowatt hour is a 1000 watts used for an hour.

My lightbulb is 100 watts, which in kilowatts is 100/1000, which is 0.100. It therefore uses 0.1 of a kilowatt hour in an hour.

I make an estimate of how many hours it is on during the year. Usually we have lights on more during the winter than the summer, so I think how much I use it summer and winter. I reckon I have it on for about two hours each day during the summer, and about eight hours each day during the winter. The average of this is five, so I reckon that on average during the year I have it on for five hours a day. There are other ways to estimate the usage - you need to use whatever seems most reasonable to you.

So my bulb is on for 5 hours every day, which is 1825 hours a year. During each hour it uses 0.100 kilowatt hours, so the total for the year is:

1825 * 0.100, which is 182.5 kwh.

Now I look at my electricity bill and it tells me that I am paying 8.57 pence for each kilowatt hour I use. So the overall cost of that bulb for a year is

182.5 * 8.57 = 1564.025 pence. Divide that by 100 to get pounds - £15.64.

Using an energy saving bulb for the same time would take 20 * 1825 = 36500 watts, which is 36.5 kilowatts. The price for that would be 36.5 * 8.57 = 312.805 pence, or £3.13 (rounding to the nearest penny).

The saving then is £15.64 - £3.13, or £12.51 per year.

Obviously in the first year, you need to take the cost of the bulb into account. It was £3.95, so even in the first year, I have saved £12.51 - £3.95 = £8.56.

Now you can calculate the saving made by changing all the bulbs in your house. The Lightbulbs.xls spreadsheet, downloadble here, makes it easy.