Energy experts tell us to replace regular incandescant lightbulbs with CFL bulbs

Question

Energy experts tell us to replace regular incandescant lightbulbs with CFL bulbs, but it does seem hard to justify spending $15 on a lightbulb when others are so cheap. Assume that a 60W incandescant lightbulb cost 50¢ and has a lifetime of 1000 hours. A 15 W CFL bulb produces the same amount of light as the 60W incandescant bulb. The CFL costs $15 and has a lifetime of 10,000 hours. Compare the Life-Cycle costs of 60W bulbs to 15W bulbs. The Life-Cycle cost is the cost of an object plus the cost of fuleing and mainting it over its useful life. Assuming electricity costs 10¢ per kWh, which bulb has a more expensive Life-Cycle cost? Make sure you are measuring over the same period of time.

Explanation / Answer

First, find out how much energy the incandescent bulbs use:

(60 watts) x (1000 hours) = (60,000 watt hours)

60,000 watt hours (Wh) = 60 kilowatt hours (kWh)

Now, figure how much it costs for the energy. You know how many hours the bulb runs, and you know the cost per hour, so...

(60 kWh) x ($0.10/kWh) = ($6.00)

However, you need to add the cost of the bulb, since it doesn't just appear out of nowhere. You DO have to buy them.

$6.00 + $0.50 = $6.50

So, every incandescent bulb costs $6.50 per 1000 hours, energy included.

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Now, you need to do the same thing for the fluorescent bulbs. Calculate the amount of energy consumed by these bulbs.

(15 watts) x (10,000 hours) = (150,000 watt hours)

150,000 watt hours (Wh) = 150 kilowatt hours (kWh)

Once again, it's time to figure how much it costs for the energy. You know how many hours the bulb runs, and you know the cost per hour, so...

(150 kWh) x ($0.10/kWh) = ($15.00)

Once again, add the price of the bulb into the total cost.

$15.00 + $15.00 = $30.00

So, over the course of 10,000 hours, the total cost of the fluorescent bulb is $30.00.

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Now, time to add up all of the total costs. First, compare the two bulb prices.

Incandescent: $6.50/1,000 hours
Fluorescent: $30.00/10,000 hours

At first glance, it appears as if the Incandescent bulb is much cheaper. However, you need to convert the incandescent into the same time frame as the fluorescent. The incandescent costs $6.50 per 1000 hours. But, you need 10 incandescents to match the time frame of the flourescent. So:

($6.50 per 1,000 hours) x (10) = ($65.00 per 10,000 hours)

Now, compare the costs.

Incandescent: $65.00/10,000 hours
Fluorescent: $30.00/10,000 hours

Over the course of 10,000 hours, it is clear that the fluorescent is the cheaper of the two. By purchasing fluorescent bulbs, you are saving 35 dollars, or 46.2%, with every bulb.

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