Explanation Determine the total amount of electricity the refrigerator uses a year. W e , total = Total cost of energy Unit cost of energy (I) Determine the number of hours the refrigerator is on per year. Δ t = W e , total W e (II) Here, the total amount of electricity the refrigerator uses a year is W e , total and the power consumed by refrigerator is W ˙ e . Determine the total hours in a year. Total hours in a year = h × D (III) Here, the number of hours in a day is h and the number of days in a year is D . Determine the fraction of the time the refrigerator is on during a year. Time fraction = Δ t total hours per year (IV) Here, the number of hours the refrigerator is on per year is Δ t . Conclusion: Substitute $ 170 / year for total cost of energy and $ 0.125 / kWh for unit cost of energy in Equation (I). W e , total = $ 170 / year $ 0.125 / kWh = 1360 kWh / year Substitute 1360 kWh / year for W e , total and 400 W for W ˙ e in Equation (II)

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Chapter 6.11, Problem 121P

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The fraction of the time the refrigerator will run in a year.

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Chapter 6.11, Problem 120P

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