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Comparing Whole-life Costs of

incandescent & fluorescent light bulbs

One of the best energy moves anyone can make is replacing incandescent light bulbs with more efficient compact fluorescent (CF) light bulbs. From where you sit, how many of those inefficient little space heaters -- I mean incandescent bulbs -- can you see? How much could you save by replacing them? Let's find out...

Enter correct numbers for your home
number of bulbs:
incandescent bulb watts:
incandescent bulb cost: £
compact fluorescent cost: £
cost per kilowatt hour: £

Calculating Compact Fluorescent Savings. We make many of our judgments on equipment cost alone. Especially if a new product costs MORE than the conventional product, we stay with the safe buy.

(1) Compact fluorescent bulbs last ten times as long as incandescent bulbs, so multiply the number of bulbs to be replaced times the cost of TEN incandescent bulbs, then subtract the initial cost of the CF. This results in the Equipment Savings. Since CFs cost more, their equipment cost "savings" is usually negative, and they usually start the comparison at a disadvantage.

(2) Incandescent bulbs are the drag racers of the lighting world, getting the equivalent of 4 miles to the gallon, but these operating costs cost are hidden in our monthly electric bill, and are harder to know. Nevertheless, we can easily calculate operating savings because we know that a compact fluorescent bulb uses about 20% of the electricity consumed by an incandescent bulb of equivalent luminance. In light bulbs, that results in a sizeable saving.

To calculate Operating Savings, multiply eight times the number of bulbs times the incandescent wattage times the amount you pay for a kilowatt-hour of electricity. Now this number may be hard to determine: electric utilities like to conceal the actual unit price by including service charges and confusing multi-tiered rate structures. To calculate your actual rate, divide the amount you paid the utility for electricity by the number of kilowatt-hours you used.

Where does the eight come from? Well, CFs save 80% of the energy consumed by incandescents, and last 10,000 hours. And we're reckoning consumption in watts but paying for kilowatts. So 0.80 x 10,000 ÷ 1000 = 8.

(3) Calculate your total savings by adding Equipment Savings (usually negative) to Operating Savings (usually a significantly larger positive amount). This result is a fair estimate of the Whole-life Savings. If the compact fluorescents save more than twice their equipment disadvantage, they are a smart buy!


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