You buy a compact fluorescent lamp. The packaging says it will last for 6000 hours—about five years, if used for three hours a day. A year later, it burns out.
Last year, IEEE Spectrum reported that some Europeans opposed legislation to phase out incandescent lighting. Rather than replace their lights with compact fluorescents, consumers started hoarding traditional bulbs.
From the comments on that article, it seems that some IEEE Spectrum readers aren’t completely sold on CFLs either. We received questions about why the lights don’t always meet their long-lifetime claims, what can cause them to fail, and ultimately, how dead bulbs affect the advertised savings of switching from incandescent.
Tests of compact fluorescent lamps’ lifetime vary among countries. The majority of CFLs sold in the United States adhere to the U.S. Department of Energy and Environmental Protection Agency’s Energy Star approval program, according to the U.S. National Electrical Manufacturers Association. For these bulbs, IEEE Spectrum found some answers.
How is a compact fluorescent lamp’s lifetime calculated in the first place?
"With any given lamp that rolls off a production line, whatever the technology, they’re not all going to have the same exact lifetime," says Alex Baker, lighting program manager for the Energy Star program. In an initial test to determine an average lifetime, he says, manufacturers leave a large sample of lamps lit. The defined average "rated life" is the time it takes for half of the lamps to go out. Baker says that this average life definition is an old lighting industry standard that applies to incandescent and compact fluorescent lamps alike.
In reality, the odds may actually be somewhat greater than 50 percent that your 6000-hour-rated bulb will still be burning bright at 6000 hours. "Currently, qualified CFLs in the market may have longer lifetimes than manufacturers are claiming," says Jen Stutsman, of the Department of Energy’s public affairs office. "More often than not, more than 50 percent of the lamps of a sample set are burning during the final hour of the manufacturer’s chosen rated lifetime," she says, noting that manufacturers often opt to end lifetime evaluations prematurely, to save on testing costs.
Although manufacturers usually conduct this initial rated life test in-house, the Energy Star program requires other lifetime evaluations conducted by accredited third-party laboratories. Jeremy Snyder directed one of those testing facilities, the Program for the Evaluation and Analysis of Residential Lighting (PEARL) in Troy, N.Y., which evaluated Energy Star–qualified bulbs until late 2010, when the Energy Star program started conducting these tests itself. Snyder works at the Rensselaer Polytechnic Institute’s Lighting Research Center, which conducts a variety of tests on lighting products, including CFLs and LEDs. Some Energy Star lifetime tests, he says, require 10 sample lamps for each product—five pointing toward the ceiling and five toward the floor. One "interim life test" entails leaving the lamps lit for 40 percent of their rated life. Three strikes, or burnt-out lamps, and the product risks losing its qualification.
Besides waiting for bulbs to burn out, testers also measure the light output of lamps over time, to ensure that the CFLs do not appreciably dim with use. Using a hollow "integrating sphere," which has a white interior to reflect light in all directions, Lighting Research Center staff can take precise measurements of a lamp’s total light output in lumens. The Energy Star program requires that 10 tested lights maintain an average of 90 percent of their initial lumen output for 1000 hours of life, and 80 percent of their initial lumen output at 40 percent of their rated life.
Is there any way to accelerate these lifetime tests?
"There are techniques for accelerated testing of incandescent lamps, but there’s no accepted accelerated testing for other types," says Michael L. Grather, the primary lighting performance engineer at Luminaire Testing Laboratory and Underwriters’ Laboratories in Allentown, Penn For incandescent bulbs, one common method is to run more electric current through the filament than the lamp might experience in normal use. But Grather says a similar test for CFLs wouldn’t give consumers an accurate prediction of the bulb’s life: "You’re not fairly indicating what’s going to happen as a function of time. You’re just stressing different components—the electronics but not the entire lamp."
Perhaps the closest such evaluation for CFLs is the Energy Star "rapid cycle test." For this evaluation, testers divide the total rated life of the lamp, measured in hours, by two and switch the compact fluorescent on for five minutes and off for five minutes that number of times. For example, a CFL with a 6000-hour rated life must undergo 3000 such rapid cycles. At least five out of a sample of six lamps must survive for the product to keep its Energy Star approval.
In real scenarios, what causes CFLs to fall short of their rated life?
As anyone who frequently replaces CFLs in closets or hallways has likely discovered, rapid cycling can prematurely kill a CFL. Repeatedly starting the lamp shortens its life, Snyder explains, because high voltage at start-up sends the lamp’s mercury ions hurtling toward the starting electrode, which can destroy the electrode’s coating over time. Snyder suggests consumers keep this in mind when deciding where to use a compact fluorescent. The Lighting Research Center has published a worksheet [PDF] for consumers to better understand how frequent switching reduces a lamp’s lifetime. The sheet provides a series of multipliers so that consumers can better predict a bulb’s longevity. The multipliers range from 1.5 (for bulbs left on for at least 12 hours) to 0.4 (for bulbs turned off after 15 minutes). Despite any lifetime reduction, Snyder says consumers should still turn off lights not needed for more than a few minutes.
Another CFL slayer is temperature. "Incandescents thrive on heat," Baker says. "The hotter they get, the more light you get out of them. But a CFL is very temperature sensitive." He notes that "recessed cans"—insulated lighting fixtures—prove a particularly nasty compact fluorescent death trap, especially when attached to dimmers, which can also shorten the electronic ballast’s life. He says consumers often install CFLs meant for table or floor lamps inside these fixtures, instead of lamps specially designed for higher temperatures, as indicated on their packages. Among other things, these high temperatures can destroy the lamps’ electrolytic capacitors—the main reason, he says, that CFLs fail when overheated.
How do shorter-than-expected lifetimes affect the payback equation?
Actually predicting the savings of switching from an incandescent must account for both the cost of the lamp and its energy savings over time. Although the initial price of a compact fluorescent (which can range [PDF] from US $0.50 in a multipack to over $9) is usually more than that of an incandescent (usually less than a U.S. dollar), a CFL can use a fraction of the energy an incandescent requires. Over its lifetime, the compact fluorescent should make up for its higher initial cost in savings—if it lives long enough. It should also offset the estimated 4 milligrams of mercury it contains. You might think of mercury vapor as the CFL’s equivalent of an incandescent’s filament. The electrodes in the CFL excite this vapor, which in turn radiates and excites the lamp’s phosphor coating, giving off light. Given that coal-burning power plants also release mercury into the air, an amount that the Energy Star program estimates [PDF] at around 0.012 milligrams per kilowatt-hour, if the CFL can save enough energy it should offset this environmental cost, too.
Exactly how long a CFL must live to make up for its higher costs depends on the price of the lamp, the price of electric power, and how much energy the compact fluorescent requires to produce the same amount of light as its incandescent counterpart. Many manufacturers claim that consumers can take an incandescent wattage and divide it by four, and sometimes five, to find an equivalent CFL in terms of light output, says Russ Leslie, associate director at the Lighting Research Center. But he believes that’s "a little bit too greedy." Instead, he recommends dividing by three. "You’ll still save a lot of energy, but you’re more likely to be happy with the light output," he says.
To estimate your particular savings, the Energy Star program has published a spreadsheet where you can enter the price you’re paying for electricity, the average number of hours your household uses the lamp each day, the price you paid for the bulb, and its wattage. The sheet also includes the assumptions used to calculate the comparison between compact fluorescent and incandescent bulbs. Playing with the default assumptions given in the sheet, we reduced the CFL’s lifetime by 60 percent to account for frequent switching, doubled the initial price to make up for dead bulbs, deleted the assumed labor costs for changing bulbs, and increased the CFL’s wattage to give us a bit more light. The compact fluorescent won. We invite you to try the same, with your own lighting and energy costs, and let us know your results.