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When should you turn off lights...how long

Number Six

JREF Kid
Number Six
Joined
Sep 5, 2001
Messages
5,016
What I'm trying to ask is this: If a light is on in a room and you're leaving the room but coming back to it later, how long is it that you'll be out of the room before it's worth it to turn the light off to save energy? Should you leave it on if you're coming back in 30 seconds? 1 minute? 5 minutes? What? I've heard many different answers to this question over the years. I don't see why it should be so complicated.
 
Depends on the bulb wattage. But there's also the matter of wear on the switch. Then there's the metaphysical question- what do you mean by "worth it"? To you? To society as a whole? To the lightbulb manufacturer?

I try to see it this way. Lights give off heat. It's winter. I use lights as heaters and view the visible EM output as a free bonus.

Of course it's a daft answer. But it hasn't been keeping ME awake for years.;)
 
Normally you're billed for electricity at X cents per kilowatt hour. If you run a 100 watt bulb for 10 hours it'll cost you X cents. The "worth" of energy is presumably related to what you pay for it..

With fluorescent lights I have been told that more energy is used in turning it on than in running it for a considerable time thereafter. However I cannot quantify "considerable"...
 
Number Six said:
What I'm trying to ask is this: If a light is on in a room and you're leaving the room but coming back to it later, how long is it that you'll be out of the room before it's worth it to turn the light off to save energy? Should you leave it on if you're coming back in 30 seconds? 1 minute? 5 minutes? What? I've heard many different answers to this question over the years. I don't see why it should be so complicated.
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I don't have The One True Right Answer, but you could always do a Web search on something like turning+off+fluorescent+lights and see if any of the links give you answers that suit you.

This link gives a short answer of, "Turn them off if you will be gone for more than about 15-20 minutes", and also gives some of the reasoning behind the answer.

_Q_
 
Number Six said:
What I'm trying to ask is this: If a light is on in a room and you're leaving the room but coming back to it later, how long is it that you'll be out of the room before it's worth it to turn the light off to save energy? Should you leave it on if you're coming back in 30 seconds? 1 minute? 5 minutes? What? I've heard many different answers to this question over the years. I don't see why it should be so complicated.
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If you're talking about an incandescent lamp, then you're using a constant amount of power while it's on and none while it's off, so in theory it's always better to switch off the lamp when you leave the room for a time, no matter how short.

The inrush current when you switch the lamp on will be several (perhaps more) times higher than the steady state current since the impedance of the filament increases rapidly as it heats up, but this transient condition will only last for several hundred milliseconds at the most so from a power cost perspective it can be ignored.

However, I've been thinking about this same question for a while now because I live with a person who insists I turn the lights off when I'm not in the room, and it seems to me that all is not as simple as I implied above. Whenever you switch on an incandescent lamp, the high inrush current will cause the filament to twist violently due to the sudden heating, so I'm guessing that the NUMBER of times you switch on a light has a direct impact on how long it will live. I don't think I've ever been sitting in a room with the light on and it suddenly burnt out, they always seem to burn out as you switch them on, a single flash and then they're dead.

So, when it comes to the question of how much you'll save on your power bill from switching the light off when you're out of the room, I think you have to take into account the extra wear and tear on the bulbs from all the extra on/off cycles.
 
Iconoclast said-"I'm guessing that the NUMBER of times you switch on a light has a direct impact on how long it will live. I don't think I've ever been sitting in a room with the light on and it suddenly burnt out, they always seem to burn out as you switch them on, a single flash and then they're dead."


-absolutely. You are also wearing out the switch contacts which are way more expensive than the bulb.

The whole question gets more complex still if you vector in newer, "low power" high illumination bulbs and LEDs, which, I'm told, will replace all light bulbs over the next twenty years.

For peace of mind maybe you should just agree to decide on a time and stick to it.
 
Soapy Sam said:
So...how many sceptics does it take...?:)
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My favourite is:

Q: How many social workers does it take to change a light bulb?
A: Just one, but the light bulb's got to want to change.
 
Soapy Sam said:
So...how many sceptics does it take...?:)
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Flourescent or incandescent?

If we are doing the problem in this forum, can we assume incandescent bulb is spherical with constant desnsity, and is in a vacuum? (Gets out integration tables)
 
Walt , take it slowly.
You can do it.
One letter at a time.
F-L-U-O-R.......
 
Q: How many PC helpdesk people does it take to change a lightbulb?

A: Look, we've got a bulb identical to yours here and it's working fine, so what did you change?
 
Soapy Sam said:

-absolutely. You are also wearing out the switch contacts which are way more expensive than the bulb.

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How much do you pay for switches and light bulbs ?

I priced the cheap switches at $0.58 when purchased in 10 packs.

Not what I would call way more expensive, unless you are buying FEIT bulbs by the gross.
 
Soapy Sam said:
-absolutely. You are also wearing out the switch contacts which are way more expensive than the bulb.
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Switches are $0.26 at the Menard's down the street from me. But the really good ones can cost up to $5. But even the cheap ones will last for years.
 
Zep said:
Q: How many PC helpdesk people does it take to change a lightbulb?

A: Look, we've got a bulb identical to yours here and it's working fine, so what did you change?
Click to expand...


User: Well, I didn't change ANYTHING!

HD: Are you sure?

User: Well, it was kind of dim, so I put in a higher wattage bulb. Then my wife complained it was too bright, so I put in a dimmer switch. But other than that, I didn't change a thing.


I work a computer help desk, so I may seem a little resentful of the joke. In reality, whenever a user says "I didn't change anything" they are lying to cover up something stupid they did. Help desk people don't care -- they assume that ALL users do soemthing stupid all the time. We don't care what stupidity you have done -- only about getting your problem fixed. Lying to us only makes it harder to fix your problem. *end of rant*
 
How many Teamsters does it take to change a light bulb?
14, You got a problem with that?
 
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