When to Leave the Lights On: Essential Answer

January/February 2010

Reading time min

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.

I faced a similar dilemma when I recently installed CFLs in my bathroom. Since I had heard that they are inefficient when used for short periods, I have been relying on ambient light whenever possible, and relieving myself with the lights off. A courageous decision, perhaps, given that I was still “in the dark” about whether I was doing the right thing. Your question led me to do some research, and I’m happy to report that both you and I are in the right range. If your kitchen is going to be vacant for longer than 5-15 minutes, you should definitely turn the lights off. For anything shorter than that though, you’re better off leaving them on.

As I suspected, CFLs need some extra energy to get going. To understand why, I asked Will Bishop, ’07, who designs battery packs for the electric car company Tesla Motors. “Turning on a CFL, you’re kind of doing magical stuff,” Bishop says, referring to the complex chemical and physical processes occurring within the corkscrew bulb. That magical stuff requires a jolt of energy. Each time the bulb is turned on, “you have to have an inrush of electricity to get the [proverbial] motor running.”

That first pulse of electricity is extremely brief, though, so  turning the lights off for even five seconds and flicking them back on will use less energy than if you ) had left them on the whole time. However, there’s a downside—your bulbs won’t last as long.

Magical or not, CFLs can only start up so many times before they break down. A Rensselaer Polytechnic Institute study found that, to work for the same total amount of time, CFLs turned on and off every five minutes would need replacement almost seven times as often as CFLs left on for three hours at a time. Thus, the more often you turn your CFLs on and off, the less on-time you’ll get overall, and the more frequently you’ll need to replace them. Buying new bulbs costs money, and each discarded CFL increases the risk of harmful mercury exposure (see Nitty Gritty).

So, how long should you be gone from a room before you turn off the lights? The precise answer depends on the tradeoffs in greenhouse gas emissions (generating more electricity versus making new CFLs) and economics (electricity cost versus the cost of new CFLs). To calculate it down to the minute, we’d need to know exactly how clean and expensive electricity and CFLs are where you live.

As a general guideline, the Department of Energy recommends that most people leave their CFLs on when  leaving the room for up to 15 minutes, but (in strictly economic terms) that interval can be as short as five minutes if your electricity is very expensive. Thankfully, the Rocky Mountain Institute, a nonprofit sustainability think tank, reports that even if you’re turning a CFL on and off every five minutes, its lifetime carbon emissions (from production, distribution and use) will be about one third of those of an incandescent bulb.

That means you can leave the light on if you’re gone for up to 15 minutes, Paula, and I can flip them on if I’m in the bathroom for at least five minutes. Which frankly strikes me as a long time. As long as it’s not the middle of the night, I plan to continue keeping the lights off.

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.

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