Question

An article in Scientific American pointed out that replacing a 75 watt incandescent bulb with an 18 watt compact fluorescent bulb would save about 75% in the cost of electricity. Electricity is generally priced per kilowatt-hour (kW-h). Using the price of electricity where you live, and the fact that an incandescent bulb has a typical life of only ~750 hours, calculate how much money you would save over the life of one 18 watt compact fluorescent bulb (~10,000 h) versus an equivalent number of 75 watt incandescent bulbs.
(Note: You must look up the cost of electricity, and the cost of buying incandescent and compact fluorescent bulbs at a local retailer, e.g. Shop Rite, Wal-Mart, Lowes, etc.)

Answer 1

According to the Energy Information Administration report of Oct 28, 2011, In USA, the average price for electricity is about 12 cents per kilowatt-hour.

A 75-watt incandescent bulb would consume 75000 Wh of electricity for 10,000 hours and an 18 Watt compact fluorescent bulb would consume 18000 Wh of electricity for 10,000 hours.

The cost of electricity when 75W bulb is used for 10000 hours =

75000 Wh x (1h/1000 Wh) x (0.120 $/ KWh) = 9 $

The cost of electricity when 18W bulb is used for 10000 hours =

18000 Wh x (1h/ 1000Wh) x (0.120$ / KWh) =2.16 $

Therefore, electricity savings = 9- 2.16 = 6.84 $ during the life time of the compact fluorescent bulb.

Also,

Cost of buying a typical 75W incandescent bulb at Walmart = 6$ (Please note that below values may change depending on the brand)

Cost of buying a typical 18 W compact fluorescent bulb at Walmart =8 $

Therefore total savings by using the CFL = 6.84 - 2 $= 4.84 $