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Compact Fluorescent Light Bulb FAQ

10 October 2007

Also known as: “Everything You Ever Wanted to Know About Compact Fluorescent Light Bulbs but Were Afraid to Ask.” If somehow this entry doesn’t answer your question, please leave it in comments and I’ll add it to the list.

What is a compact fluorescent light bulb, and why should I use it?

A compact fluorescent light bulb (CFL) is an energy efficient light bulb that uses a fraction of the energy of an incandescent and lasts seven to ten times longer.

Aren’t they expensive?

Although the initial cost is more than for the old bulbs, a CFL easily pays for itself and more over its longer lifetime of energy savings.

For example, when you replace a 75W incandescent (1,000 hours rated life) with a 20W CFL (8,000 hours rated life), you will get at least a 440 kilowatt-hour (kWh) energy savings over the life of the bulb. With current energy costs ($.065 per kWh) and the additional savings of not purchasing the eight incandescents that you would have bought over the life of the CFL – thats a savings of $32.60 per bulb.

What colors do they come in?

CFLs are available in a range of colors, from the warm yellow light that incandescents give off to a brighter, whiter light. The color of a CFL depends on the Kelvin temperature. To replicate the color of an incandescent, buy a bulb with a Kelvin temperature of 2,700K (warm white). The higher the Kelvin temperature, the “cooler” the light feels and the bluer the effect. The highest Kelvin temperatures (5,000 – 6,500K) replicate daylight and can be better for reading.


I tried CFLs when they first came out and didn’t like them. Has anything changed?

CFL technology has been greatly improved over first generation lamps. Today’s CFLs offer excellent light output, a variety of color choices to effectively replicate incandescent light, no hum or flicker and a variety of shapes and sizes to fit almost any fixture – including indoor and outdoor floods, dimmables, three-ways, decorative candelabra and bathroom globes.


What should I look for when buying a CFL?


You should always look for CFLs that have an Energy Star® label on the box. Introduced in 1992, Energy Star® is a voluntary labeling program used to identify and promote energy efficiency products. Products with the Energy Star® label must meet specific guidelines established by the US Environmental Protection Agency and Department of Energy. If a CFL has the Energy Star® label the manufacturer has met standards in the following areas: Lamp efficiency requirements, retention of light output as lamp ages, Color Rendering Index (CRI), start-up time, rated lamp life and informational requirements on packaging. For more information go to or call 1-888-782-7937




Are CFLs dimmable?
There are CFLs designed for use with dimmers. Make sure the CFL is labeled as dimmable. You CANNOT use a non-dimmable CFL with a dimmer, it will result in early lamp failure. Most dimmable CFLs don’t dim to zero but shut off completely when they reach about 20% light output.



Can I use CFLs outdoors?
CFLs may be used outdoors if they are kept out of wet weather, for example in a garage, are used in an enclosed fixture or are specifically designed to be used outdoors in wet conditions. If the temperature outside is below 32 degrees, the bulb will still start, but will not attain full brightness.



Can I use CFLs in enclosed fixtures?
Yes. CFLs can be used in enclosed fixtures as long as the fixture is not recessed and enclosed. If the fixture is recessed and totally enclosed (for example, a recessed can with a cover over the bulb) it is not a good idea to use a CFL. This combination creates temperatures that are too high, causing early bulb failure.

Can I use CFLs with photocells and electronic timers?
No. Photocells are incompatible with CFLs due to the way they convert radiant energy into electrical current. Timers are also incompatible because they allow a small amount of voltage to cycle through the lamp while it is off, causing the lamp to try and start itself when the proper supply voltage is not present. Both of the scenarios will shorten lamp life and void any warrantees because the lamp is being used for a purpose inconsistent with its design. It is also not recommended to use CFLs in sensor/motion lights.

But I heard that CFLs contain mercury. Isn’t that dangerous?
Yes, CFLs contain four milligrams of mercury.  That is 1/5th of the mercury contained in a watch battery.   And, using CFLs actually reduces the amount of mercury in our environment. The vast majority of our electricity in Minnesota comes from burning coal, and burning coal releases mercury into the environment. Using a CFL, thus using less electricity, thus requiring less coal to be burned – thus releasing less mercury.




What should I do if a CFL breaks?
Cleaning up a broken CFL is easy and safe. First, open the windows in the room where the bulb broke to help disperse any vapor that may escape. Next, carefully sweep up the fragments and place them in a plastic bag. Do not use your bare hands or a vacuum cleaner! Next, wipe the area with a damp disposable paper towel to pick up any glass fragments. Seal the plastic bag and take the fragments with you the next time you recycle any old bulbs.

How do I dispose of CFLs?
Minnesota state law requires that CFLs be recycled because they contain mercury. They should also never be incinerated. Check with your local hardware store to see if they collect burned-out CFLs. They may charge a small fee for drop-off. If your hardware store does charge a fee, your local utility may offer coupons to help reduce the cost. You should also be able to take them to your county’s hazardous waste site for free. Be sure to call ahead of time for drop-off rules. Log onto or for more information about lamp recycling in your area. You may also call 1-800-CLEAN-UP.

12 Comments leave one →
  1. Rich permalink
    27 March 2008 11:23 am

    One advantage of incandescent light bulbs over CFLs is the heat they generate. The Mall of America (MOA) does not heat the main part of the building relying instead on the greenhouse effect from it’s glass cieling, incandescent light generated heat and body heat of occupants.

    Can you tell me how much additional work / load is placed on my furnace (perhaps as a measure of BTU’s per SQ Foot) to make up for the amount of heat not generated by the change in light bulbs? Assuming of course a standard sized house and that every bulb has been replaced.

  2. Jorge E. Rhor permalink
    18 April 2008 11:49 am

    How do you measure de amount of mercury that goes in each lamp, during the manufacturing process and after the lamp is finished. Can weighing the bulb before sealing and weight it after is filled with argon and dosed with the mercury give you what much mercury in miligrams is in each bulb?

    L.I.T.E. Consulting

  3. 21 May 2008 11:28 am

    CFLs do generate heat, but the amount is negligible – especially because the electricity used with light bulbs is more expensive and more carbon intensive than the natural gas you would use to heat your home. It would be inefficient to try to use incandescents to heat your home instead of using light bulbs that save you money and energy with little to no impact on your comfort.

  4. Umm permalink
    1 November 2008 7:12 pm

    “especially because the electricity used with light bulbs is more expensive and more carbon intensive than the natural gas you would use to heat your home.”

    That’s making an awful lot of assumptions, like that my home is heated by natural gas (it could be heated with electricity, coal, fuel oil, propane, or wood) and that the electricity one uses comes from coal. Even though half the nation’s electricity is produced using coal, there are people who live near hydroelectric dams who pay $0.02/kWh for their power.

  5. 3 November 2008 8:16 am

    Well, this blog is for the Minnesota Energy Challenge, so all of our assumptions are based on facts about Minnesotans’ energy use. The vast majority of Minnesotans use natural gas to heat their homes, although it stands that even compared to fuel oil, propane and coal, electricity is still more carbon intensive. I’m not sure about wood.

    And about 60% of Minnesota’s electricity is produced by burning coal, so while in states like California heating one’s home with incandescent bulbs may be less carbon intensive, that is certainly not the case here in the Land of Ten Thousand Lakes.

    However, even if one used coal to heat one’s home and lived in a state where the vast majority of one’s electricity was produced by hydroelectric power, it would still take a vast amount of bulbs to heat one’s home. It would be far easier and cheaper to use a more efficient furnace.

  6. 22 January 2009 11:24 pm

    This is interesting. The news story today about CFL lights and the dangers from UV light. I will be following this with some great interest.

    Thanks for the great post!!!


  1. Real Simple’s Best CFLs « Challenge for Change
  2. Ask the Experts « Challenge for Change
  3. LEDs: Are we there yet? « Challenge for Change
  4. What do I do if I hate CFLs? « Challenge for Change
  5. 2010 in review « The Minnesota Energy Challenge Blog
  6. I’ve written 600 posts on energy efficiency? « The Minnesota Energy Challenge Blog

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