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Efficient operation of fluorescent lights

The coupons the utility gave out have been very helpful in retrofitting my home with fluorescent light bulbs. Someone recently told me that these light bulbs are more efficient when just left on for whatever period of time you are planning to use the room or area. I have been turning them on and off just like incandescent bulbs when I leave the room, then on when I go back within a short period of time. What is the most efficient way to operate them? —K. Lowrie, Vancouver

I have always heard that if you turn on a fluorescent light you should just leave it on because it uses more power to turn it on than it does to run it for long periods of time. Is there any rule of thumb to go by when it comes to turning a fluorescent light on and off? —P. Buchanan via e-mail

These questions bring up an old belief about fluorescent lights. Many folks have been led to believe that fluorescent lamps take a tremendous amount of energy just to get started, even though they are quite efficient once they are going. This opinion leads to the advice that fluorescent lamps should be left on in unoccupied rooms to avoid paying an energy penalty for the in-rush of electricity needed to start the lamps.

In your home, you pay for electricity consumed over a period of time. It’s a little like fuel economy in your car—you pay for the gallons you use as a function of the relationship between those gallons and miles traveled. Even if you went up a few steep hills and got single-digit mileage for a brief portion of your overall driving, your fuel cost is still related to total mileage.

With electricity in your home, you pay for the number of watts your various lights and appliances use multiplied by the number of hours the equipment ran. If a 1,000-watt appliance runs for an hour, it is measured as one kilowatt-hour (kWh)—a little less than six cents at today’s local prices.

A typical compact fluorescent bulb is 15 to 23 watts. Left on for 24 hours, it will use between one-third and one-half a kWh—two to three cents worth. When starting, the bulb needs a brief surge to warm up the electronic components that make a tiny electric arc to light the gasses inside the fluorescent tube. According to the Environmental Protection Agency’s Watt Watchers Web site, the electricity consumed during this moment is equivalent to about five seconds of fully-on operation. According to this formula, even if you switch a 23-watt light off and on 10,000 times a month, it will only use about two kWh of start-up energy.

The inverse of the start-up energy calculation is determining how much energy the light uses if it is left on. The 23-watt example lamp uses about a quarter of a kWh for every 12 hours it is on. Leave a lamp on all day while you are at work during the week, and it will use about 5 kWh per month.

Simply put, the lamp uses more energy when it is on than when it is off. Want savings? Turn off unneeded lights and don’t worry about the miniscule amount of energy it takes to restart the light. A rule of thumb from the Illuminating Engineering Society of America’s Lighting Handbook: Turn off a fluorescent light if the room will be unused for more than five minutes.

Another popular argument is that switching lights and other appliances off will shorten their longevity. This is true, sort of. The total number of hours the lamp will last is shortened somewhat by frequent switching. But the calendar life—the number of months or years a light will be in service—is extended by switching the light off when it is not needed. In other words, do you really care if the bulb lasts more hours if those hours are spent illuminating an unoccupied space? Leave a compact fluorescent light on 24 hours a day and it will last about a year. On only four hours a day, the same bulb will last about six years.

It’s good to question energy savings claims—some are too good to be true. But in the case of the compact fluorescent bulb, the savings start the moment you first turn it on, and for years thereafter. Every bulb saves a little. Every household, neighborhood and community that saves contributes significantly to regional conservation efforts.