It seems hard to justify spending $4.00 for a compact fluorescent lightbulb when an ordinary incandescent lightbulb costs 50 ¢. To see if this makes sense, compare a 60 W incandescent bulb lasting 1000 hours to a 15 W compact fluorescent bulb having a lifetime of 10,000 hours. Both bulbs produce the same amount of visible light and are interchangeable. If electricity costs $0.10/kWh, what is the cost - purchase plus energy - to obtain 10,000 hours of light from each type of bulb? This is called the life-cycle cost.

to obtain 10,000 hours of light you would need:

option 1)

10 incandescent light bulbs costing $0.50 each = $5

this incandescent light bulbs will consume 60 x 10,000 = 60,000 watts or 60 kWh = 60 x $0.10 = $6 in electricity

total costs using incandescent light bulbs = $5 + $6 = $11

option 2)

1 fluorescent light bulb = $4

this fluorescent light bulb will consume 15 x 10,000 = 15,000 watts or 15 kWh = 15 x $0.10 = $1.50

total costs using fluorescent light bulbs = $4 + $1.50 = $5.50

you will have $2593.74 in 10 years if you set aside $1,000 at 10%

you will have $9321.91 in 20 years if you set aside $2000 at 8%

you will have $1469.32 in 5 years if you set aside $1000 at 8%

you will have $1296.87 in 10 years if you set aside $500 at 10%

you will have $239.66 in 15 years if you set aside $100 at 6%

you will have $4660.96 in 20 years if you set aside $1000 at 8%