Dear Cecil: Living with a future ecologist is driving me crazy. She'll shut off the water while I'm brushing my teeth or turn off the lights while I'm in a five-minute shower. Isn't it harder on the generator to shut off a light for five minutes and then turn it back than just leaving it on? Also, isn't it true that it takes more electricity to turn the lights on and off frequently than just to leave them on? Any info is welcome, as I am... Infuriated, Chicago

Cecil replies:

Junior eco-buffs are a curse. I had a friend once who used to make me feel like I was squandering the nation’s resources every time I spent more than 20 minutes in the shower. My feeling was, some people squandered more resources than me just by breathing.

Now then. We won’t discuss the water situation here, since it varies too much from time to time and from region to region to make generalizations possible. Electricity, however, is another matter. Humbling as it may seem, it doesn’t make any appreciable difference to the generator what you personally do with your lights, since several hundred thousand consumers are on the same line as you are. Where it does make a difference is in (a) your electric bill and (b) your light bulb bill, particularly for fluorescent lights.

At one time, manufacturers strongly advised against switching fluorescent fixtures on and off frequently because you could reduce tube life as much as 20 percent. Downtown office buildings once left their lights on all night on the theory that it was cheaper to burn the extra juice than send a maintenance worker around every few months to change the tubes. However, the 70s saw the introduction of longer-lasting, rapid-start tubes that last for 20,000 hours, as opposed to 10-12,000 with the old ones. These you can flick on and off a little more casually, since you’ll only be reducing tube life from 5 to 10 percent. But don’t get nutty about it. The rule of thumb is that if you’re going to be out of the room for more than 15 minutes, you’ll save money by turning out the lights. Thus you can cheerfully leave the lights going for the five minutes it takes you to get through with your shower. Frankly, taking a shower in the dark seems like a pretty daredevil proposition to me. You sure your girlfriend’s motive is strictly conservation?

If you’ve got incandescent bulbs, you needn’t sweat the possibility of burning them out or wasting electricity by diddling the switch too much. It does take a slight extra shot of electricity — called the “inrush current” — to boost the tungsten filament of a bulb up to operating temperature. But the amount involved is less than it would take to burn the bulb continuously for even a couple seconds. On the other hand, if you do go to all the trouble of shutting off your 100-watt bathroom light for an extra five minutes every morning, you’ll save all of about three kilowatt hours per year — less than 50 cents’ worth. Whoopee.

The effective way to conserve power is by replacing wasteful incandescent bulbs with fluorescents, which use less wattage to produce the same amount of light. More importantly, buy more energy-efficient appliances, since it’s the big items that consume 90 percent of your electricity.

Cecil Adams

Send questions to Cecil via cecil@straightdope.com.