There are several costs associated with using a light bulb in your home. You need to buy the bulb and then you need to pay for the electricity it uses. Incandescent light bulbs work by heating a filament (a thin piece of wire). The filament gets very hot and this gives off light and also a large amount of heat; the bulbs are usually too hot to touch when they are in use. Electricity costs about 15 cents per unit. To work out the total cost of the electricity you have used, multiply the cost per unit by the number of kilowatts and then by the number of hours. This means that if you use 1 kilowatt of electricity for one hour you will pay 15 cents. A 100W bulb uses 0.1 kilowatts of energy when it is switched on. This type of bulb costs about 80 cents to buy and lasts for about 1000 hours. How much does it cost to run one of these bulbs over its lifetime?