Find the light bulb wattage on the left hand side. Across the top find the hours per day it is used. Where that row and column meet is the cost of powering that light bulb for one year.
An old 75 Watt bulb running 5 hours a night costs $37 a year. Definitely worth changing to a more efficient option.
It also works for other items - if you know their power usage. A stereo that uses 10 Watts in standby mode all day will cost $24 a year even without playing any music (might be worth turning off at the wall). A laptop computer that uses 20 Watts and is used for 4 hours per day will cost just $8 to run.
For this calculator I've assumed 27 cents per unit of electricity. If your price is different then the estimates may vary.
Also, if the exact wattage isn't in the table, use one that is close to it as an estimate. Or if you've got a calculator you can do the exact calculations yourself. Here's the formula:
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