At $0.095/kWh, what does it cost to leave a 25-W porch light on day and night for a year?
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This is Giancoli Answers with Mr. Dychko. The cost is gonna be the amount of energy used multiplied by the number of dollars per unit of energy. And when we're talking about power utilities, energy is in units of kilowatt-hours. Because kilowatt-hours tends to result in numbers that are easy to write down. Whereas, if you used units of Joules for energy, then you'll have scientific notation, typically. And so that will be just clumsy to write down scientific notation on a power bill all the time. So, when you get a bill, it's easy to write down a number like 219 kilowatt-hours instead of, you know, three times 10 to the 8 Joules or something. So, energy is power multiplied by time, or you can think of this as the work done by the electricity. So this is 25 watts -- we have to convert it into kilowatts, so we multiply by one kilowatt for every thousand watts. And then, we'll multiply by the time which is 365 and a quarter days in a year, but that needs to be in unit of hours, 'cause we need kilowatt- hours in our final answer. So, we need to multiply this by 24 hours per day. So now we have: kilowatt-hours per year, I guess, because that's left behind. And so, the cost is gonna be 219.15 kilowatt-hours in a year, times .095 dollars per kilowatt-hour, which is about 21 dollars to leave this 25-watt porchlight on, 24 hours a day for an entire year.
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