To save energy and money, Jackie Smith replaced the 122 Watt incandescent light bulb in her house with a 18 Watt CFL bulb. The expected life of incandescent and CFL bulbs is 1,000 and 10,000 hours respectively. The purchase price of an incandescent bulb is $0.51 and for the CFL it is $5.88. The cost of electricity is 0.06 $ per kWh. What is her total savings over the lifetime of the CFL

Note: 1 watt * 1000 hrs = 1 kWh    

         1 watts * 10,000 hrs = 10 kWh

Incandescent for 1000 hrs

Original cost = .51

  Usage = 122 W * .06  = 7.32

Total cost for 1000 hrs 7.32 + .51 = 7.83

Total cost for 10,000 = 78.3

CFL Bulb for 10,000 hrs

Original cost = 5.88

 Usage = 18  * .06 / kWh * 10 kWh = 10.8

Total cost = 5.88 + 10.8 = 16.68

Gives a total savings of 78.3 - 16.68 = 61.62

Another way of looking at this:

Note: the cost of electricity for the incandescent bulb is:

122 W * 1000 hr * .06 / kWh = 122 kWh * 06 = 7.32  for each KWh

That is 73.2 for 10 kWh   and 5.1 for bulb cost = 78.3

The CFL bulb uses in 10,000 hrs

18 kWh * 10 * .06 = 10.8    cost of electricity

bulb cost = 5.88  for a total cost of 16.68