```Question 84063
You don't have to get involved with joules or units other than those mentioned in the problem.
.
Look at the problem this way. In one hour the bulb consumes 60 watts times 1 hour which
is the same as 60 watt*hours. To convert this to kw*hours, just divide by 1000. When you
do this division you get:
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60 w*hr/1000 w/kw = 0.060 kw*hr
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Units check:
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{{{(w*hr)/(w/kw) = (w*hr)*(kw/w) = (cross(w)*hr)*kw/cross(w) = kw*hr}}}
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So for each hour it runs, the bulb uses up 0.060 kw*hr of energy.
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How many hours does the bulb run per year? At 8 hours per day for 365 days in the year, the
bulb runs 8 times 365 hours which is 2920 hours.
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Units check:
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{{{(hrs/day) * (day/yr) = (hrs/cross(day))* (cross(day)/yr)= hrs/yr}}}
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Then in a year the total consumption is 0.060 kw*hr for each hour times the 2920 hours in
a year for an annual amount of:
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{{{ 0.06*2920 = 175.2}}} kw*hr per year.
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Units check:
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{{{(kw*hr/hr)* (hr/yr) = ((kw*hr)/cross(hr))* ((cross(hr))/yr) = (kw*hr)/yr}}}
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And at a cost of \$0.0683 for each kw*hr the annual cost to run the bulb will be:
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\$0.0683 * 175.2 = \$11.96616 which rounds to \$11.97.
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Units check - using D to represent dollars):
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{{{(D/(kw*hr)) * ((kw*hr)/yr) = (D/(cross(kw*hr)))*(cross(kw*hr)/yr) = D/yr}}}
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Everything checks. Hope this helps you to see your way through this problem.
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