Energy Use Investigation 7.2:

## Which Light is Most Cost Effective?

### Method 1: Life Cycle Cost

The life cycle cost of a light bulb includes the purchase price and the cost of energy use during its entire lifetime. Since a compact fluorescent bulb lasts 10 times longer than an incandescent bulb, let’s compare one compact fluorescent bulb to 10 incandescent bulbs over a 10,000 hour cycle.

INCANDESCENT: Material cost of ten 60 W incandescent bulbs = \$________

Energy used over life cycle is 60 W x 10,000 hrs = ________ watt-hours

divided by 1000 watt-hours/kilowatt hour = ________ KW-hours

Operating cost over lifecycle = ________ KW-hours

times \$0.10/ kilowatt hours = \$________

Total Cost (material cost plus operating cost) of

ten 60-watt incandescent bulbs = \$________

COMPACT FLUORESCENT LIGHT (CFL): Material cost of one 15-watt CFL = \$________

Energy used over life cycle is 15 W x 10,000 hrs = ________ watt-hours

divided by 1000 watt-hours/kilowatt hour = ________ KW hours

Operating cost over lifecycle = ________ KW-hours

times \$0.10/ kilowatt hours = \$________

Total Cost (material cost plus operating cost) of one 15-W CFL = \$________

### Method 2: Payback Time

Buying the energy efficient light bulb means spending more money at the start, but it also means saving a little money every time you have the light on. A time will come when the savings are enough to pay back the extra cost of the energy efficient light. That time is called the payback time.

To find the payback for choosing a compact fluorescent bulb over an incandescent bulb, make a table showing the total cost for each kind of bulb month by month as time goes by. The payback time comes during the month when the total cost of using an incandescent light bulb catches up to the cost of using the energy efficient compact fluorescent bulb.

At the beginning of the first month the cost is simply the price of the light bulb. For each month after that add on a month’s worth of energy costs. Assume the light is on for four hours a day. Use the wattage of the bulb and the energy cost (10¢ per kilowatt-hour) to calculate the monthly energy cost. Whenever the operating time of the bulb reaches the bulb’s lifetime, add in the cost of a new light bulb. Make a graph of cost versus time. Plot the data for both light bulbs on the same graph. Where the two lines cross is the payback time.