Your total lighting cost is determined by the sum of the wattage of your lamps multiplied by the duration of their use. The kilowatt hour is the unit used by electric companies for billing; a kilowatt hour is 1,000 watts run for one hour. Thus 10 bulbs of 100 watts each all on for an hour is one kilowatt. You must estimate or track how long each light is on to get a final cost for your lighting; the closer you can estimate these durations, the more accurate your total will be.

- Skill level:
- Easy

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### Things you need

- Calculator
- Recent electric bill

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## Instructions

- 1
Create a table in four columns: the first for lights, the second for wattage, the third for duration of use and the fourth for watt hours. List each and every light in your household in the first column. Make sure to include exterior lights, such as security floods and pool lights.

- 2
Record the wattage of each of the lights in the second column. Estimate how long each lamp is used each month, in hours, and place that figure in the third column.

- 3
Multiply the wattage of each light on your list by the duration of its use each month, and place the total for each light in the fourth column for watt hours for each light.

- 4
Sum the total watt hours of all the lamps, and divide it by 1,000 to get a sum of the total kilowatt hours used by all of your lighting in one month.

- 5
Refer to your electric bill to see the price per kilowatt hour. Multiply it by the total kilowatt hours used to get your monthly lighting costs. For example, if your total lighting usage each month is 200 kilowatts, and your cost per kilowatt hour is 10 cents, your monthly lighting cost is £13.00.

#### Tips and warnings

- Compact fluorescent lamps use 25 per cent of the wattage of incandescent bulbs for the same amount of light. Using them in your home can lead to considerable savings.