Question

What is the cost of operating a 3.00-W electric clock for a year if the cost of electricity is $\$0.0900 \textrm{ per kW} \cdot \textrm{h}$? Question by OpenStax is licensed under CC BY 4.0. Final Answer$\$2.37$

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# OpenStax College Physics Solution, Chapter 7, Problem 33 (Problems & Exercises) (1:37)

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This is College Physics Answers with Shaun Dychko. The cost of operating a three watt electric clock for a year is going to be the energy it consumes multiplied by the cost per kilowatt hour or the rate of the energy, electricity bill. So we have three watts converted into kilowatts by multiplying by one kilowatt for every 1000 watts and then multiply by a year and so, converted into hours by the way, because we want to get kilowatt hours of energy used. Then we multiply that by the dollars per kilowatt hour to end up with dollars. So we have one year times 365 days per year, times 24 hours per day and this gives us kilowatt hours in this portion here. So that's the energy used and then we multiply that by 0.09 dollars per kilowatt hour. That gives two dollars and thirty-seven cents will be the cost. The reason the electric utility uses this strange unit of kilowatt hours instead of joules is because it just creates convenient numbers. So you can get a bill that says nine cents per kilowatt hour. If you were to get a bill per joule the numbers would be strange. It would be 0.000005 dollars per joule for example. The numbers wouldn't be what we're used to seeing. We're used to seeing dollars and cents you know, with at the most the ten thousandth place or something like that.