If you're into energy conservation, or trying to cut your home energy bills, you have probably asked yourself this question. And chances are you have accepted the conventional wisdom, that it is better to leave the light on for short periods, than turn it off, then on again.



In this case, the conventional wisdom is dead wrong.

Here is how the argument goes: When you first power a light on, it will use as much as five (or fifteen) minutes of the regular consumption of the bulb, within the first second. So if a three-year-old flicks the switch continuously for a minute, on or off every second, they are actually burning 5 minutes worth of electricity every other second (30 times in one minute). That works out to 30 x 5 minutes, or 150 minutes, worth of electricity in that one minute.

It is fairly easy to prove that this is impossible. Let's assume the toddler is turning on and off a 100 watt bulb. Over the course of that one minute, if we assume that turning the bulb on uses 5 minutes worth of the typical consumption of the bulb, we have used 150 minutes worth of electricity at 100 watts.

Now, 150 minutes worth of electricity at 100 watts is the same amount of power as 1 minute of electricity at 15,000 watts. And since the light was turned on and off over the course of one minute, it means that if our assumption about the size of the initial power surge is correct, during that one minute the light bulb behaved as if it were burning 15,000 watts continuously.

If you studied electricity at all in high school, you probably remember the formula: Watts = Amps X Volts. In this case, we know both the Watts and the Volts so we can take this equation:

15,000 (Watts) = Amps X 110 (Volts)

(I am assuming the toddler lives in the Americas, where voltage is typically 110). To resolve Amps, we can divide both sides by 110 so we get:

15,000 / 110 = Amps

In other words, Amps = 136.

Now I don't know about your house, but mine is certainly not going to be able to handle a 136 amp current on one light for a whole minute, since the whole house has a power supply of just 100 amps. And my circuit breakers are all 15 or 30 amp breakers - which means they trip off when the power surges to much more than their rated amperage of 15 or 30 amps. So that toddler turning the light on every other second for a minute, yielding a 136 amp draw, would blow the circuit breaker for the circuit the light is on, and possibly blow the main circuit breaker for the house.

It is true that there is a power surge when you power a light bulb on. But the surge is for less than a second - it is actually a tiny fraction of a second. And the amount of the surge is far smaller than the touted 5 minutes of normal use of the light.

All right, you say, but won't the light burn out if I keep flicking it on and off?

Yes, it will burn out faster. I've seen my own kids blow a light bulb with the on-off trick - especially if they do it repeatedly for a minute or more, and the bulb was old to start with.

But even if each time you turn a light on you shorten its life by an hour - and the figure is probably far lower than that - you will still save energy and money if you turn off lights whenever you leave a room.

Again, consider the lowly incandescent. You can buy a cheap 100 watt bulb for around 25 cents and it lasts about 1,000 hours. They burn 0.1 kilowatt hours each hour they are on. If we assume we burn a bulb out in 1,000 on-off cycles, and electricity costs us 10 cents a kilowatt hour, that means it costs us 1 cent to run the bulb for one hour (100 watts = 0.1 kilowatt, X 10 cents = 1 cent).

So, every time you turn a bulb off (which means you will later have to turn it on) you are using 1/1000 of the $0.25 you paid for the bulb, or 0.05 of a cent (that's $0.0005!)

And every time you turn a bulb off for five minutes you are saving 5/60 of the $0.01 it costs to run the bulb for an hour, or 0.16 of a cent.

So you actually save over three times as much by turning the light off for five minutes, as you would by extending the bulb life by leaving it on. And my assumption that it takes an hour of the life of the bulb each time you turn it on is probably a big over-estimate. It was just to prove a point.

There is one other flaw with the leave-the-light-on conventional wisdom: it fails to take into account what happens when we get distracted.

You step out of a room for a couple of minutes to do something else, and you leave the light on because you know you will be back soon. But you get distracted - a knock at the door, a phone call, you suddenly remember an errand you have to run - and half an hour or several hours later, you discover the light you had left on. The worst is when the light is in a seldom-used room - furnace room or a guest bedroom - and you don't remember to go back and turn the light off. Days later you discover it is still on. One distraction like that canl cost you far more than the cost of one hour of the operating life of the bulb.

So make it your philosophy to turn off lights. Not only will you save electricity when you turn off lights, and save money overall, but it will remind you to be an energy saver in other ways. And you will be setting a visible example to others, who will become more conservation conscious as well.