Monday, March 9, 2009

Compact Fluorescent Lights Save Less than Many Believe

I saw a piece on CBC Newsworld saying that compact fluorescent lamps (CFLs) don’t save as much money as many people claim. The piece made the explanation seem mysterious and complicated, but it’s quite simple. The problem with the standard analysis is that it ignores secondary effects.

The relevant secondary effect here is heating. All energy absorbed by a light bulb ultimately turns into heat. Even the light produced will bounce around off objects and eventually turn into heat. So a light bulb inside your house is like a mini electric heater. A tiny fraction of the light might escape through a window, but for practical purposes, 100% of the energy heats your home.

Here’s an example of the standard analysis of savings. Suppose that you replace a 60-Watt incandescent light bulb that gets used 6 hours per day with a 15-Watt CFL. Power consumption drops by 45 Watts for 6 hours per day. This saves a total of 8.2 kilowatt-hours per month. Assuming a cost of 12 cents per kilowatt-hour, you save a hair under a dollar each month, or $12 per year.

A real analysis that factors in heating gives a different result. The simplest case to analyze is a house heated with electricity. During the heating months the electric heaters will have to make up the lost 45 Watts of heat that used to come from the incandescent light bulb. This exactly offsets the CFL’s supposed savings. The net savings during heating months is exactly zero.

During months requiring no heating or air conditioning, you will save the dollar each month. However, you may heat your home a little longer into spring if you’re not getting as much heat from light bulbs. This effect should be fairly small.

During months where you use air conditioning, you actually have to spend extra money on air conditioning to get rid of the heat created by light bulbs. So, the savings from using a CFL are more than a dollar per month.

Overall, the yearly savings will depend on how many months you heat and use air conditioning, but in a Canadian climate the total savings per year will be less than the $12 from the simple, but flawed analysis.

Another complication in this analysis comes if you use non-electric heating such as natural gas. As it happens, the total cost per unit of energy from electricity and natural gas for me are both about 12 cents per kilowatt-hour once you take into account all the extra charges and taxes. But, if the natural gas were more expensive than electricity, then the CFL would actually lose money during heating months.

One situation where CFLs are a very clear winner is in outdoor lighting. In this case the heat generation is irrelevant, and the cost savings of CFLs are significant. If you haven’t tried CFLs yet, you may want to start with your outdoor lights.

7 comments:

  1. A real analysis that factors in heating gives a different result. The simplest case to analyze is a house heated with electricity. During the heating months the electric heaters will have to make up the lost 45 Watts of heat that used to come from the incandescent light bulb. This exactly offsets the CFL’s supposed savings. The net savings during heating months is exactly zero.

    Well not quite, because we don't usually put light bulbs in the floor. 45 W of heat trapped near the ceiling is less effective at making your home comfortable than 45 W of heat coming from a baseboard heater...

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  2. Potato: True. I oversimplified. The extent of this effect will depend on air flow in your house. The net effect, though, is that CFL savings would be very small during months requiring heating.

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  3. I was wondering about this a couple months ago, specifically about what happens to the light as it rebounds around the room. I figured it must get absorbed as heat, but I wasn't sure.

    I agree with your point about using CFLs outside, but I've also found they're slow to reach their maximum brightness in cold weather. Could be they've improved since I bought mine.

    I'm in favour of LED Christmas lights in theory, but I still prefer the look of mini incandescent bulbs, even though they are wasteful when used outside. I put them on a timer for about 4 hours a day, and the strings I put up burn about as much energy as two 60W bulbs.

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  4. In response to Potato's comment. The issue is where the heat is going. A new house that has all of it's electrical accessories well insulated will retain the heat quite well. Older houses usually have fixtures directly nailed to the house's frame which can act as a heat sink drawing it directly outside. In northern climates our homes are primarily heated by fridges, stoves, sunlight, our bodies and those evil light bulbs for several weeks before and after summer. Even if the furnace is only operating for the coldest 5 months of the year, we actually need this indirect heat for at least 9.

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  5. And does the CFL even work outside when it is -30 or -40 ?

    And what about the costs for disposal? Our municipality does not want the CFL in the landfill, so I am supposed to get in my car and drive the nonfunctional bulb over to a bin at a designated location. What does that cost?

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  6. Anonymous said...

    And does the CFL even work outside when it is -30 or -40 ?

    Yes, it does. I live in Ottawa and it was that cold for several weeks this year. All CFL's worked.

    I've had them outside for many years and they have lasted well.

    Inside the house is another matter. Apparently they don't take well to being turned on and off frequently.Nor do they like motion so all the ones I have in ceiling fans have burned out early. The outside lights are on for 4 - 7 hours a night and last for years. I'm lucky to get 2 years out of the lights in my kitchen. But there I turn the light on and off all the time and use the fan in the summer.

    I'm opposed to the CFLs for a number of reasons, the loss of the heating the number one (remember, during the cooling season we use electric lights for as little as 2 hours a day due to the longer days.) I'm also opposed to the excess packaging, the extra energy required for disposal (they are considered toxic waste) and the excess cost.

    I find my incandescent bulbs last years (I have a set in my front hall that have been changed only once in the 19 years I've lived in this house.) Only time will tell if CFLs last as long as claimed but I must say the fact that I've changed 4 in the last 6 months indicates they won't.

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  7. Flybaby: The only problem I've had with CFLs outdoors is that they take a while to build up to full brightness when it is cold. This doesn't matter much for the lights at the front of my house that are on all night, but it is annoying to turn on the lights in my garage on a cold day and have to wait a few minutes to be able to see what I'm doing.

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