Q: I have compact fluorescent (CFL) bulbs in the ceiling fan in the kitchen. Should I turn this fixture off when I leave the room for only 5-15 minutes?
Asked by Paula Giese, ’67, Naperville, Ill.
How a CFL Works
When he was a civil and environmental engineering teaching assistant at Stanford, Will Bishop explained electricity fundamentals to everyone from freshman liberal arts majors to electrical engineering PhD candidates. So even though Bishop is designing battery packs for Tesla Motors now, it’s not surprising that he knows a little something about how CFLs work, too. He’s also enthusiastic:
“CFLs are pretty freaking cool in that they use some interesting technology. You have chemical engineering, you have glowing phosphorus coatings, you have a cool bulb shape . . . and you have some much more sophisticated electronics going on [than in an incandescent bulb], and it just all makes for a more efficient bulb.”
It’s a multi-step process for CFLs to produce light. First, a part called the ballast (the fat part near the base of most CFLs) sends precise amounts of electric current through argon gas and mercury vapor in the CFL’s glass tube. The current causes the gas and vapor to emit ultraviolet light (that we can’t see), which in turn causes a phosphor coating in the bulb to emit visible light (that we can see).
While it takes more energy to get this process going than it does to light up an incandescent bulb, the start-up phase is very brief, lasting about 1/120th of a second, and CFLs typically use only one-fourth of the energy of an equivalent incandescent bulb once they’ve started up. That’s why you may see an 18-watt CFL labeled as a “75-watt equivalent.” The amount of light a CFL emits depends in part on how long its tube is, so that funky twisted shape you see in many CFLs allows the manufacturer to cram more light-emitting tubing into a smaller, traditional bulb-shaped space.
Power versus Energy
You might think that the high-energy start-up phase would result in increased electricity costs for turning CFLs on versus leaving them off. Understanding why it doesn’t requires knowing a little bit about the difference between power and energy.
Using water as an analogy, Bishop describes power as “how much water you’re putting out at any time. If I turn on the sink or turn on a fire hose, how fast is the water coming out?" Turning on a CFL means blasting that faucet for a fraction of a second (a lot of power) and then turning it down to a trickle.
Energy is different. Bishop says, “How much energy you’re using is just how much water you end up putting out. I could turn on the fire hose for ten seconds but it might not be as much as leaving a leaky faucet on for a week.” Energy use (measured in kilowatt-hours) is what the electric company charges you for. So, if you left a CFL on all the time, leaking water slowly, you’d end up with more total water (energy) used than a CFL that had brief bursts when you turned it on but was off most of the time. Your monthly electric bill would be higher as a result.
The end result is that, according to the Department of Energy, turning a CFL off for even five seconds will save energy versus leaving it on. I tried a quick home experiment with a Kill A Watt EZ electricity monitor and a CFL, and I found that turning the CFL on and off more often did indeed use less total energy than leaving it on for longer periods of time.
CFL Usage Lifetime
Many common CFLs are rated to need replacement after they’ve been on for 10,000 cumulative hours. That number may be something of an exaggeration, but nevertheless, how long the bulb actually lasts depends a great deal on how often it’s turned on and off.
A 1998 study conducted by the Lighting Research Center at Rensselaer Polytechnic Institute examined how a CFL’s useful life was affected by its duty cycle, i.e. how often it was switched on and off. The study showed that bulbs that were switched on and off every five minutes only lasted 15 percent as long as those that were on for three hours before being switched off—that means just 1,500 hours of active life, rather than the advertised 10,000! Here are the results across different duty cycles:
| Duty Cycles | Lifespan versus 3-hour On Cycle |
|---|---|
| 5 minutes on, 5 minutes off | 15% |
| 15 minutes on, 5 minutes off | 30% |
| 1 hour on, 5 minutes off | 80% |
| 3 hours on, 5 minutes off | 100% |
Thus, by turning CFLs on and off more often, you are shortening their useful life.
Embodied Energy in CFLs
CFLs that are turned on and off instead of being left on save energy when they are off, which seems good. But because of the shorter lifespan that results, more energy must be used to manufacture and distribute replacement bulbs. The total energy used to make and transport a CFL—or any other product—is known as its “embedded” or “embodied” energy. According to the Rocky Mountain Institute, a nonprofit organization that advocates energy efficiency, the embodied energy in a CFL results in about 28 pounds of total carbon dioxide emissions. Based on Environmental Protection Agency data for Naperville, Ill., that’s about the same amount produced by leaving an 18-Watt CFL (equivalent to a 75-watt incandescent bulb) on for 43 days straight!
Mercury Content of CFLs
The environmental impacts of CFL use go beyond energy and carbon emissions. CFLs contain small amounts of elemental mercury, exposure to which can cause tremors, emotional changes, insomnia, kidney damage, or even death to humans. Thus, CFLs must be disposed of properly when their useful life runs out. Because mercury vapor can escape from a broken CFL, extra precautions, including airing out the room, should be taken if a CFL breaks. Nevertheless, the Rocky Mountain Institute reports that because of the prevalence of mercury-emitting coal-generated electricity in the United States, even if a CFL is disposed of in a landfill, its use results in 40% less total mercury entering the environment than would have from using an equivalent incandescent bulb.
Additionally, concerns about mercury exposure from CFLs may be overblown. A study by Lawrence Berkeley National Lab researchers found that "if simple common sense is used in disposing of the broken CFL, the resulting exposure to mercury is equivalent to about 1⁄50 of an ounce—a single nibble—of Albacore tuna.” That is, if treated properly, broken CFLs are still a lot safer than your average tuna salad sandwich . . . but you still shouldn’t eat a broken CFL.
Putting It All Together
In the end, the question of how long you need to leave the room before it’s worth turning your CFLs off comes down to a series of economic and environmental tradeoffs.
| Leave CFLs On | Turn CFLs Off | |
| Economic Benefits | Buy fewer new CFLs | Spend less on electricity |
|---|---|---|
| Environmental Benefits | Less energy needed to make and distribute CFLs Less risk of mercury exposure |
Lower emissions from power plants |
While no single answer will be right for everyone, the Department of Energy’s recommendation that most people leave their CFLs on when leaving the room for up to 15 minutes is certainly a good rule of thumb.
Andy Martin, ’02, plans to receive his joint MBA/MS from the Graduate School of Business and the Emmett Interdisciplinary Program in Environment and Resources in January 2010.
