A reader named Mary recently asked the following:
Could you please tell me how much it costs to run a 100 watt incandescent lightbulb for 8 hours/day over the course of a year? Could you then compare the cost for a year with a compact fluorescent lightbulb? Which wattage CFL would to replace a 100 watt incandescent bulb? I’m new at this. Thanks for your response.
It’s no secret that I’m a huge fan of CFLs, so I think that this is a great question.
Here’s the answer…
For starters, a 26 watt CFL is the approximate equivalent of a 100 watt lightbulb, so right there we can see that the CFL will consume 26% as much energy. If you’re curious about other wattage equivalents, check the chart below.
Now let’s work through the math so we can come up with some hard numbers.
100 watt incandescent bulb
100 watts corresponds to 0.1 kilowatts. At 10 hours/day that works out to:
0.1 kW * 8 hours/day * 365 days = 292 kWh
I just checked our latest power bill, and we are currently paying $0.108/kWh for electricity, so that one incandescent bulb would consume $31.54 worth of electricity per year.
Compact fluorescent assumptions:
26 watt compact fluorescent bulb
Doing the same math as above, we have:
0.026 kW * 8 hours/day * 365 days = 75.9 kWh
At the rate for electricity, that works out to $8.20 per year — a savings of $23.34 per year just for switching out one light bulb. And that’s considering just the cost of electricity.
Given that most CFLs are rated to last an estimated 10x longer than incandescent lights, you’ll come out even further ahead if you can get them for less than 10x the price of an incandescent bulb. Since CFL pricing has come down dramatically in recent years, you’ll actually come out way ahead.
Another thing to consider is that CFLs run much cooler, so they don’t add as much to your A/C burden during the summer. It’s hard for me to put an exact number on this, so let’s just consider it to be icing on the cake.
The flipside, of course, is that heat produced by incandescent bulbs can (at least in theory) help when it comes to heating your home in the winter. But as far as I’m concerned, you should use your lights for light and your heating system for heat. In reality, an incandescent bulb has a somewhat negligible effect on overall comfort in a cold home as there is no way of effectively circulating that heat throughout the house.
Of course, the numbers above will vary based on the number and wattage of bulbs that you are replacing, how heavily they’re used, and how much you pay for electricity. But this should give you a good idea of to work things out for yourself.