Is it Better to Leave the Lights On, or Turn Them Off, When You Leave a Room?
Robin Green asked:
Is it better to turn a light off every 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 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’s not hard to demonstrate that this is nonsense. Suppose the kid is toggling a 100 watt light. Over the course of sixty seconds, if we accept that switching on the light on uses the equivalent of what the light normally uses in five minutes, we have used 100 watts times 150 minutes.
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.
Remember your high school physics class, where you learned the rule: Watts = Amps X Volts? In this case, we know both the Volts and the Watts so we can fill in:
15,000 watts = Amps X 110 volts
(Let’s suppose the mischievous kid lives in Canada, where power in homes is normally 110v). To solve for Amps, we divide both sides by 110v, which yields:
15,000 watts / 110 volts = Amps
Which means that the light was drawing 136 amps of power.
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.
So what’s the scoop? Yes, there is a power surge when a light is turned on. But that surge lasts only a tiny fraction of a second, and it works out to far less energy than the usually quoted five or fifteen minutes of leaving the light on.
All right, you say, but won’t the light burn out if I keep flicking it on and off?
You just have to watch that toddler in action for a while to know the answer: I’ve seen kids wreck a light bulb in a matter of minutes with the on-off trick. The more times you turn a bulb on or off, the sooner it burns out.
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, each time the light gets switched off (which entails switching it back on later) you are spending a thousandth of the 25 cents you spent on the bulb, or one twentieth of a cent (a mere $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 0.08 of a cent.
So switching the light off for five minutes cuts your electricity costs by more than three times the extra you’ll be spending on shortened bulb life. And remember, we assumed that each flick of the switch uses an hour of the bulb’s life, but it’s probably far less than that – we just chose an hour to prove the 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 leave the room for a few minutes to put something away, but you leave the light on as you plan to return shortly. But a neighbor at the door, a friend on the phone, or some other distraction, keeps you away from the room where you left the light on – and half an hour or more, you remember that light left on. Even worse, if the light was in a room you don’t visit often – the basement work room or that empty third bedroom, you might not discover the light has been left on until several days later. Forgetting to turn a light off in one case like that can eat up way more money and energy than shortening the bulb’s life by an hour.
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.
Is it better to turn a light off every 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 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’s not hard to demonstrate that this is nonsense. Suppose the kid is toggling a 100 watt light. Over the course of sixty seconds, if we accept that switching on the light on uses the equivalent of what the light normally uses in five minutes, we have used 100 watts times 150 minutes.
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.
Remember your high school physics class, where you learned the rule: Watts = Amps X Volts? In this case, we know both the Volts and the Watts so we can fill in:
15,000 watts = Amps X 110 volts
(Let’s suppose the mischievous kid lives in Canada, where power in homes is normally 110v). To solve for Amps, we divide both sides by 110v, which yields:
15,000 watts / 110 volts = Amps
Which means that the light was drawing 136 amps of power.
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.
So what’s the scoop? Yes, there is a power surge when a light is turned on. But that surge lasts only a tiny fraction of a second, and it works out to far less energy than the usually quoted five or fifteen minutes of leaving the light on.
All right, you say, but won’t the light burn out if I keep flicking it on and off?
You just have to watch that toddler in action for a while to know the answer: I’ve seen kids wreck a light bulb in a matter of minutes with the on-off trick. The more times you turn a bulb on or off, the sooner it burns out.
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, each time the light gets switched off (which entails switching it back on later) you are spending a thousandth of the 25 cents you spent on the bulb, or one twentieth of a cent (a mere $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 0.08 of a cent.
So switching the light off for five minutes cuts your electricity costs by more than three times the extra you’ll be spending on shortened bulb life. And remember, we assumed that each flick of the switch uses an hour of the bulb’s life, but it’s probably far less than that – we just chose an hour to prove the 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 leave the room for a few minutes to put something away, but you leave the light on as you plan to return shortly. But a neighbor at the door, a friend on the phone, or some other distraction, keeps you away from the room where you left the light on – and half an hour or more, you remember that light left on. Even worse, if the light was in a room you don’t visit often – the basement work room or that empty third bedroom, you might not discover the light has been left on until several days later. Forgetting to turn a light off in one case like that can eat up way more money and energy than shortening the bulb’s life by an hour.
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.
Posted November 28, 2009 - Filed In Environment
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