Meeting People: Leaving the Light On
The energy conservationists have a bone to pick with Tom Bodett and his Motel 6.

We've all been taught to turn off lights we don't need in order to save electricity. But what about the case when "I'm coming right back?" Is it better to leave the light on or turn it off? If the bulb is an incandescent bulb (what we normally think of as a light bulb -- the bulb-shaped lights that get hot quickly and give off a yellowish color), you should turn it off while you're gone, even if it's just for a minute. If it's a fluorescent bulb (typically, the long thin tubes that we see in buildings and basements and that give off a bluer light), it's common wisdom to leave it on if you really are coming back within ten to fifteen minutes.

Why do we treat the fluorescent bulb differently? Basically, because it works differently.

An incandescent bulb works by running electricity through a small metal wire (called the filament) inside a sealed bulb that has had most of the oxygen removed (oxygen is a requirement for fire and would cause the very hot filament to ignite instantly and ruin the bulb). Although there is no oxygen in the bulb, it is not a vacuum either: nitrogen and argon, two inert gases, are pumped in in place of the oxygen.

A fluorescent bulb is more complicated. First, the bulb is filled with mercury vapor (hence, it isn't a good idea to stand around sniffing after a fluorescent bulb has broken open) instead of the inert gases found in incandescent bulbs. Secondly, it contains two filaments, one at each end of the bulb, with one being positively and the other negatively charged. Thirdly the flow of electricity through the filaments does not directly generate the light; instead, the energized filaments excite the atoms in the mercury vapor that fills the tube to get the mercury vapor to create an arc between the two ends of the bulb. That arc emits light for the full length of the bulb. However, the light that the mercury vapor emits is ultra-violet and thus invisible to our eyes. To convert it to visible light, the inside of the bulb is coated with phosphor, whose white glow gives off the bluish light we associate with fluorescent bulbs; consequently, although you can buy a clear (unfrosted) incandescent bulb, you can't buy an uncoated fluorescent bulb -- much like you can't buy a television with a clear glass picture tube. Finally, to make the lamp work, there is some additional electronics, including a device called a "ballast" that gives the filaments a jolt of power to initially excite the mercury when the bulb is first turned on.

The jolt to start fluorescent bulbs has given rise to the myth that it's wasteful to repeatedly turn them off for short periods. A similar issue arises in the automobile, which takes more gas to start the engine than it does when it is idling -- so there is a point at which it is cheaper to leave the car running rather than to shut it off and restart. That is no longer the issue with fluorescent bulbs. In the last 25 years, fluorescent lights have become much more efficient, and so the size of the jolt of electricity needed to start the light relative to the ongoing use of electricity isn't as significant as it once was.

Then why did you say to leave the fluorescent light on if I'm only going be gone for a minute? The real cost of turning the light on and off comes from the wear and tear on the light bulb itself. Both incandescent and fluorescent bulbs undergo stress every time you turn them on (think of it as a miniature version of the damage that a lightning strike would do to your computer). That process of going from full off to full on, from cool to hot, takes a little life out of both kinds of bulbs. And while the argument for fluorescent bulbs is that they are more efficient producers of light, they are also more expensive. Consequently, the value you chip away every time you turn the light on is more for the fluorescent bulb than the incandescent bulb, and so it is wiser not to flip the fluorescent bulb on and off as often because the savings in electricity will be overshadowed by the cost to replace the bulb sooner.

So Tom, leave the light on, but only if it's fluorescent and only if you really are expecting someone in the next ten to fifteen minutes.

Source: Encyclopedia Britannica

Updated September 30, 2003 - go to our home or life advice page