William W. answered • 06/18/21

Experienced Tutor and Retired Engineer

Cost = cost of bulbs + cost of the electricity

Let C_{1} = cost of using the energy savings bulb and C_{2} be the cost of using the conventional bulb.

26W = 26/1000 kW = 0.026 kW and the electricity used by an energy efficient bulb is 0.026(10,000) kW-hrs = 260 kW-hrs.

100W = 100/1000 kW = 0.1 kW and the electricity used by a conventional bulb is 0.1(10,000) kW-hrs = 1000 kW-hrs.

1 energy efficient bulb is required for 10,000 hrs

10000/750 = 13.333 = 14 conventional bulbs are required for 10,000 hrs.

C_{1} = (cost of 1 bulb) + (cost of electricity) = 4.20 + 260(0.20) = $56.20

C_{2} = (cost of 14 bulbs) + (cost of electricity) = 14(0.54) + 1000(0.20) = $207.56

Cost savings in using energy efficient bulbs (for 10,000 hrs) = $207.56 - $56.20 = $151.36