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.
Incandescent assumptions:
100 watt incandescent bulb
8 hours/day
365 days/year
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
8 hours/day
365 days/year
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.
Other considerations
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.
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I’d like to compare a CFC bulb with a sodium vapor bulb for outdoor lighting. Can you tell me how to calculate the following, using a 100w SV and and 100w “output” CFC: 1) energy consumption, 2) life of bulb 3) light output in lumens. Any other relevant comparisons. I haven’t been able to find a good side by side comparison anywhere.
Drew: Good question. We live much further south, so it’s not as cold in the winter. Nonetheless, our exterior CFLs start out pretty dim on cold winter nights, though they brighten up as they warm up. I don’t know if it would be similar for you, or if they’d just fail to light up (or get to full brightness) entirely.
Do CFLs work in cold weather?
I’m in northern Illinois. The winters can get brutal, with temps dropping well below zero at times. I’ve got four lights on the front of my house (two by the door, two by the garage door)that are left on all night, as there are no streetlights on my street.
I know the fluorescent shop lights in my garage don’t want to get going in the winter. I’d like to permanently replace the outside lights with CFLs since they are left on all night.
Am I going to have to use CFLs in warm weather and go back to incandescents in the winter?
They seem to save in the usuage, but there sure isn’t any savings from longer life that I have seen. Over the last 18 months I have replaced 10 of the 18 that I have put in so far.
You can get a free CFL bulb from Accent Energy at
https://www.accentenergy.com/lightbulb/
To redeem, fill in information and enter promo code: EARNRWG.
I have cfl’s in any room that I don’t need to work in or read in. The light produced from cfl’s just seems to give me a headache. I know they are more energy efficient but they also take more energy to produce in the first place and contain hazardous mercury materials that are difficult to dispose of. Also to produce the same level of light in a room you have compensate by using table lamps or additional light fittings.
Nickel: I imagine what you are noticing between ones you like and ones you can’t stand is the color temperature (measured in K or Kelvin). If you don’t mind mentioning your favorite brand and if possible model #, I will see what I can dig up on color temp and CRI.
As far as lumens go, I agree that on paper they are equivalent. I don’t know if my wavelength sensitivity is different than normal, or if perhaps the CFL spiral is less efficient in the distribution/throwing of light in a normal application. Either way I admit it’s a minor nitpick since going up a couple watts fixes the brightness perception and still saves a ton of energy.
One other thing I forgot to mention is that CFLs are not very tolerant of vibration and/or “rapid” on-off cycling. I think the standard definition of rapid is 15 minutes. So a bathroom or a closet application will generally shorten (often significantly) the lifespan of a CFL.
I still think CFLs are a great idea, I just like to point out the cons 🙂
I also realized I mispelled incandescent through the whole last post. Doh!
SomeGuy: Light quality varies widely across brands. There are some that I can’t stand, and others that are totally fine. As for the charts being skewed, they are based on a perfectly objective measure – lumens – and here again, I’ve been happy with the results. A 13W CFL gives me 60W worth of light and a 26W CFL gives me 100W worth of light. Here again, you might consider trying another brand.
I agree that overall CFLs are a great way to help conserve energy. I have a couple nits to pick, though.
1) The color characteristics of CFLs still “dim” compared to incandescant. I know there are warm CFLs available and the color of them is definitely better than the older ones that look like fluorescent tubes. It’s still not quite up to par, IMHO. Also, CFLs’ CRI (color rendering index) still sucks compared to incandescant. Have you ever noticed how all color mushes together under the high-pressure sodium vapor (orange) or mercury-halide (whitish-blue) highway lights? That is because they have a CRI in the 20-40 range. I have not seen CFLs above 80. Sunlight is 100 and incandescant bulbs are 95-100.
2) I think the charts matching CFL to incandescent are skewed so the CFL can claim to use less energy. I know it’s subjective but to me a 27W CFL doesn’t match a 100-W traditional bulb. You need to get to 32W or so, again in my opinion.
3) Mercury contamination from CFL breakage. The EPA even has a post about it: http://www.epa.gov/mercury/spills/. I won’t go through it all but highlights include getting everyone out of the room, opening a window or door for at least 15 minutes, and turning off HVAC. The next time you vacuum after the initial cleanup you are supposed to turn off the HVAC again. Now hopefully this is just overly safe bordering on neurotic, but wow!
Although I have CFLs in all rooms… I cannot wait for LED lighting to come down in price. I’ve been looking for an LED equivalent to a 60W incandescent for a while and CCrane just started selling them… at $120 a pop!
http://www.ccrane.com/geobulb/index.aspx
Once they come down in price, I believe that they will replace all blubs, incandescent and CFL.
Chris: Apparently you didn’t read to the end. I already addressed all of those points in the last two paragraphs.
My electricity bills have really dropped since we switched many of our lights to CFLs. I think I’ve changed out 30 (THIRTY!!) bulbs to CFLs over the past year or so. It will be really interesting when I can start seeing those year-over-year changes.
I’m especially excited about changing out the bulbs in our can lights in our basement. When we flip on the basement lights, 9 bulbs light up. At 75 to 100 watts each, that’s major power usage (and makes the family room really hot in the summer). I have now switched 6 of those bulbs for CFLs.
Look for good deals on the bulbs themselves and you can cut the costs a lot. I got a 10-pack of 65-watt replacement bulbs at Costco last year for $13 — with a $10 instant rebate from our power company. That came out to $0.30 a bulb and they are nice bulbs. I just got a multipack of CFL floods there, too, for about $2.70 per bulb vs. the $7 they cost at Wal-Mart or the hardware store.
Also, CFL recycling is becoming more available. IKEA and now ACE Hardware are recycling CFLs to properly manage the mercury issue.
It’s true that if you use an air conditioner in the summer that a CFL will help keep your house comfortable (and if you don’t use an air conditioner then its even more important, which is our situation) – but don’t discount the heat benefit of an incandescent in the winter.
It may not save much on your heating bill since the heat doesn’t circulate, but it can make a huge difference in the small area that you are using it. We keep our house around 55° in the Winter (no kids yet) so having an incandescent bulb in a nearby reading lamp can provide a little extra warmth. And since we’d only have it on when we were sitting right next to it anyway…
@Will’s Commenter: By using CFL’s instead of regular lightbulbs, we reduce the amount of mercury coming out of coal power plants so much so that the tiny, minute amount in CFL’s is negligible. Less energy needed=less coal burned=less pollution=less mercury in the air.
Great post, a good walk through on the high level basics of the savings that are possible.
Great point by the commenter above regarding the disposal issues…
Also, I know it wasn’t the specific focus of the article, but if this had been a holistic perspective on CLFs, then I think it’d be worth noting at the bottom of the article that the math used assumes a fairly high level of usage of the bulbs (8 hours a day). If we assume a more realistic amount of usage, say 4 hours, then those savings sure do dip down.
And it also costs money to buy those bulbs, so one might not come out ahead in the first year.
Finally, if you’re going to claim less heat during the summer as icing, then you have to consider the heat in the winter for regular bulbs as icing as well. Or just ignore that factor totally.
CFL’s are very good. The only issue seems to be their disposal. I put up a post about
ways to save money by going green on my blog recently, and I received this comment, which turned out to be right on the money:
[…] I am less enthusiastic than most about compact fluorescent bulbs. This is due to the fact that the ones currently available contain significant amounts of mercury. If one of these bulbs should break inside of a person’s home, it could cause a challenging disposal situation. It is my belief that the technology should progress to a point at which the mercury levels are low or nonexistent before people changeover their entire homes. Another consideration is that as these bulbs burn out, they will most likely be thrown away as though they are normal rubbish and landfills will have incredibly high levels of mercury in their soil as a result.