Compact fluorescent bulbs cost $10 per bulb and last 10 years. Tungsten bulbs cost $0.50 per bulb and last 1 year

Question

For a given amount of light, a tungsten bulb draws 75 W
and a compact fluorescent bulb draws 10 W. If the cost of electricity is $0.10/kWh and the
bulbs are operated for 2000 h/yr, what amount of money is saved by use of a compact
fluorescent instead of a tungsten bulb over a 20-year period?

Answer 1

Over 20 years, you would use two compact fluorescent bulbs and twenty
incandescent bulbs. The cost of the compact fluorescent bulbs would be $20 ($10 x 2),
while the cost of the incandescents would be $10 ($0.50 x 20). The yearly energy cost of a
compact fluorescent bulb is (10 W) x (2000 h) = 20 kWh; the monetary cost is 20 kWh x
$0.10/kWh, or $2 . The yearly energy cost of an incandescent bulb is (75 W) x (2000 h) =
150 kWh; the monetary cost is 150 kWh x $0.10/kWh, or $15
Over a twenty-year period, the cost of the compact fluorescents will be $20 + 20 x $2 =
$60 . The cost of lighting by incandescent bulbs will be $10 + 20 x $15 = $310 . The
compact fluorescent would save $250 over this interval.
No matter which way we look at things, the compact fluorescent bulb is cheaper. This could
also be seen in yearly costs: Compact fluorescent bulbs cost $10/10 + $2 = $3, while
incandescent bulbs cost $0.50 + $15 = $15.50