A 70 W outdoor security light burns, on average, 10 hours a day. A new bulb costs $5.00, and the lifetime is about 1000 hours. If electricity costs $0.10 per kW·h, what is the yearly price of “security,” per light?
What will be an ideal response?
70 Watts times 10 hours per day is 700 Watt-hours per day or 0.7 kW-hour/day. Multiplying this by 365 days per year gives 255.5 kW-hours/year of electricity use at $0.10 per kW-hour, for a total annual electricity cost of $25.55. If the light is on 3650 hours per year, one will use an average of 3.65 bulbs (each lasting 1000 hours, or 100 days at 10 hours per day) each year, at an average annual cost of $5*3.65 = $18.25/year. So the total annual cost to operate the light is about $18.25 + $25.55 = $44.
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