How many kWh of energy does a 550-W toaster use in the morning if it is in operation for a total of 5.0 min? At a cost of 9.0 cents/kWh, estimate how much this would add to your monthly electric energy bill if you made toast four mornings per week.
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This is Giancoli Answers with Mr. Dychko. Kilowatt-hours are a unit of energy. It's kind of an odd unit, normally we talk about Joules when we're talking about energy. But for utility companies it's just more convenient to say kilowatt-hours because the numbers tend to be, you know, small numbers, like a hundred or a thousand instead of, you know, scientific notation like 5 times 10 to the three Joules...that would be awkward to be writing those type of numbers down. So they have units of kilowatt-hours. So, we'll take -- we know that energy is power multiplied by time, 'cause power is the amount of work done divided by time, and in this case, the work is equal to the potential energy change of the charges. So, we have the power output of 550 watts, and we have the time, 5 minutes, and we're gonna change the watts into kilowatts, by multiplying by one kilowatt for every thousand watts, and then we'll change the time into hours, by multiplying by 5 minutes by one hour for every 60 minutes, and now we have kilowatt-hours. So that's .046 kilowatt-hours. The cost will be .04583 kilowatt- hours per morning, multiplied by four mornings per week, times four weeks per month, times nine cents per kilowatt-hour, and we get the amount of money needed to run the toaster every four mornings a week for a month, which is 6.6 cents per month. And you'll notice that the units cancel here conveniently so we're left with cents per month.
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