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Articles » Home & Family » Leave lights on or turn them off - which is more energy efficient?
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- Article Views: 3054
- Word Count: 1122
- Date Contributed: Jan 15, 2009
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| Leave lights on or turn them off - which is more energy efficient? |
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.
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)
(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 / 110 = Amps
Which means that the light was drawing 136 amps of power.
Now I'm not sure about your electrical service, but mine is a 100 amp service, which means that light is going to blow not only the 15 or 30 amp circuit breaker it's on, but the main breaker for the whole house. You can't, after all, send 136 amps of power through a 100 amp power supply for longer than a fraction of a second. So that kid switching the bulb on and off sxity times in a minute would either blow the breaker for the room where the light is, or for the whole house, in fairly short order.
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.
Fine, but what about burning out the bulb? Doesn't turning it on and off too many times wear it out?
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 that bulb the kid burns out was probably a bit old to begin with, and there have probably been many times the kid played the flick switch without burning out the bulb. And even if you assume that each flick of the switch reduces the bulb's life by one hour (it's probably nowhere near that much), you'll still cut your energy consumption if you switch the lights off whenever you step out of the room.
Let's consider that 100-watt bulb again, which costs as little as a quarter these days, and lasts 900 to a thousand hours. They burn 0.1 kwh for every hour you leave them on. Assuming that a thousand on-off cycles destroys the bulb, and assuming we pay ten cents per kwh for our electricity, it's going to cost us one cent to run the bulb for an hour (100 watts for an hour is 0.1 kwh, at ten cents per kwh means one 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 turning off that same the light for 5 minutes cuts 5/60 of the 1 cent it costs to run the light for an hour, or 1/12 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's one other problem with the argument that it's better to leave the light on for a short time. It doesn't consider what happens when we forget we've left the light on.
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.
For more information on saving electricity see the author'w website, http://www.green-energy-efficient-homes.com, and specifically "Energy efficient lighting " (http://www.green-energy-efficient-homes.com/energy-efficient-lighting.html), "Turn off lights" (http://www.green-energy-efficient-homes.com/turn-off-lights.html) and the "CFL savings calculator" (http://www.green-energy-efficient-homes.com/cfl-savings-calculator.html) which you can use to figure out whether it's cost effective to install a compact fluorescent bulb in a particular location.
Article Source: UnArchived Articles
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