Light bulbs may consist of multiple costs. The easiest and most obvious cost involves getting the price of a box of light bulbs and dividing it by the number of bulbs in the box. However, energy costs are also a consideration with light bulbs. The energy cost tells you how much money relates to the amount of kilowatts of energy the light bulb uses. Finding a decently priced light bulb low on kilowatts can save you hundreds of dollars a year.

Purchase the light bulb of your choice. Divide the number of bulbs in the box by the price to find the cost per bulb. For instance, a $4 box of four bulbs is $1 per bulb. Divide the price per bulb by the number of hours each bulb lasts to find how much each bulb costs per hour. Continuing this hypothetical, a bulb lasting 1,000 hours costs $0.001 per hour.

Divide the number of watts the light bulb uses by 1,000. The wattage can be found on the front of the bulb's box. Fill in extra zeros where they are needed. For instance, a 50watt bulb would use .05 kilowatt hours of power. Electric companies charge per kilowatt hour.

Find how much money your electric company charges per kilowatt hour. Multiply this number by the amount of kilowatts per hour your light bulb uses to find how much money the bulb costs per hour. For instance, if your company charges 10 cents per kilowatt hour and your light bulb uses .05 kilowatts per hour, the cost to run the bulb one hour would be $0.005.

Add the cost to run the bulb per hour by the cost of the bulb per hour found in step one to find the total cost of the bulb per hour. For instance, we add the cost to run the bulb per hour, $0.005, by the cost of the bulb per hour, $0.001, and receive $0.006 per hour to run the bulb total. You can find the cost per year by multiplying the cost per hour by the number of hours per year you estimate the light is on. In this instance, keeping the light on for 8,000 hours a year means that single light bulb costs $48 a year.
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