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Is it better to turn a light off each time you leave a room, or leave it on if you’ll be coming back to the room shortly?

If you’re into energy conservation, or attempting to cut your home energy bills, you have in all probability asked yourself this question. And prospects are you have accepted the established wisdom, that it is better to leave the light on for short periods, than turn it off, then on again.

In this case, the traditionalisti wisdom is dead wrong.

Here is how the argument goes: When you original power a light on, it will use as much as five (or fifteen) minutes of the regular consumption of the bulb, within the firstborn second. So if a three-year-old flicks the switch without disruption for a minute, on or off each second, they are genuinely burning 5 minutes worth of electricity each 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 reasonably 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 applied 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 in regards to the size of the basi power surge is correct, for the duration of 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 do not forget the formula: Watts = Amps X Volts. In this case, we know both the Watts and the Volts so we may take this equation:

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

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

15,000 / 110 = Amps

In other words, Amps = 136.

Now I don’t recognise when it comes to your house, but mine is surely not going to be capable 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 ranked amperage of 15 or 30 amps. So that toddler turning the light on each other second for a minute, yielding a 136 amp draw, would blow the circuit breaker for the circuit the light is on, and perhaps blow the main circuit breaker for the house.

It is unfeigned 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 littler 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 – in particular if they do it repeatedly for a minute or more, and the bulb was old to begin with.

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

Again, consider the lowly incandescent. You may buy a cheap 100 watt bulb for around 25 cents and it lasts in regards to 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, each 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 remunerated for the bulb, or 0.05 of a cent (that’s $0.0005!)

And each 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 in truth 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 in all likelihood 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 recognise you will be back soon. But you get distracted – a knock at the door, a phone call, you abruptly do not forget an errand you have to run – and half an hour or various 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 do not forget to go back and turn the light off. Days later you discover it is still on. One distraction like that may cost you far more than the cost of one hour of the operating life of the bulb.

So make it your system of belief to turn off lights. Not only will you save electricity when you turn off lights, and save cash 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.