A compact fluorescent lightbulb costs about $6.00 and has a typical lifetime of

Question

A compact fluorescent lightbulb costs about $6.00 and has a typical lifetime of 10,000 h. The bulb uses 20 W of power but produces illumination equivalent to that of a 75-W incandescent bulb. An incandescent bulb costs about $1.50 and has a typical lifetime of 1000 h. Your family wonders whether it should buy fluorescent lightbulbs. Calculate the amount of money your household would save each year by using compact fluorescent lightbulbs instead of the incandescent bulbs. (Assume that the household lights are on for an average of 8.0 hours per day, the number of bulbs in use is 6, and the cost of electricity is $0.111 per kWh. Use 365 days = 1 year.)

Explanation / Answer

compact fluorescent bulbs save in two ways. Lifetime and cost per hour. 365*8 = 2920 hours bulbs are on per year. 2920/1000 = 2.92 incandescent bulbs needed at each light fixture each year 2920/10000 = 0.292 fluorescent needed at each fixture each year 2.92*1.50*6 = $26.28 to replace incandescent bulbs each year 0.292*6*6 = $10.51 to replace fluorescent bulbs each year 2920 hrs*0.02kW*0.111$/kwhr*6 = $38.89 to operate all fluorescent bulbs each year. 2920hrs*0.075kW*0.111$/kwhr*6 = $145.85 to operate all incandescent bulbs each year. 145.854 + 26.28 - 38.8944 - 10.512 = 122.7276 So, you save $122.73 per year. (with cost of bulbs averaged out over life of bulbs) Note that you pay for energy in your electric bill. kW is power kWhr is power times time. Power is energy divided by time, so kWhr is energy divided by time times time = energy. If only the fluorescent bulbs would be bright right away instead of taking a minute to get bright, everyone would use them.

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