# Power consumed by switching on a light **URL:** \ **Category:** Factual Questions\ **Created:** [April 2, 2008, 5:54pm UTC](https://boards.straightdope.com/t/power-consumed-by-switching-on-a-light/443693 "2008-04-02T17:54:14Z")\ **Posts on this page:** 13\ **Page:** 1
**Author:** ![borschevsky](https://avatars.discourse-cdn.com/v4/letter/b/97f17d/32.png) [@borschevsky](https://boards.straightdope.com/u/borschevsky)\ **Post date:** [April 2, 2008, 5:54pm UTC](https://boards.straightdope.com/t/power-consumed-by-switching-on-a-light/443693/1 "2008-04-02T17:54:14Z")
I seem to recall hearing that there is an amount of power used in the process of switching on a light. That is, my lit-up 100W light bulb is using 0.1kWh each hour, but there was some extra power used in order to turn it on in the first place. Am I remembering this correctly? (i.e. is it true?) Is there an easy way to estimate how much power is used in this way? Is there any effect on this based on how recently the light was turned off? That is, if I flick the light off and on fairly quickly, am I wasting a bunch (relatively) of power? Basically, I’d like to know: if I’m going out, and then coming home to cook dinner, should I turn off the kitchen lights? I tried Googling this but was unsuccessful. ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [April 2, 2008, 6:17pm UTC](https://boards.straightdope.com/t/power-consumed-by-switching-on-a-light/443693/2 "2008-04-02T18:17:27Z")
Incandescent lamps have a large in-rush current. That means that they consume much more power when they are heating up than they do when operating. That said, the inrush time is extremely short - something on the order of 1/20 of a second, so the real issue is not power usage, but lamp lifetime. Turning on a lamp shortens it’s life considerably. A rule of thumb is to leave it on if you are going to use it again in 1/2 hour. ---
**Author:** ![Harmonious\_Discord](https://avatars.discourse-cdn.com/v4/letter/h/74df32/32.png) [@Harmonious\_Discord](https://boards.straightdope.com/u/Harmonious_Discord)\ **Post date:** [April 2, 2008, 6:23pm UTC](https://boards.straightdope.com/t/power-consumed-by-switching-on-a-light/443693/3 "2008-04-02T18:23:57Z")
Old florescent lamps use more current when the light starts. Sodium lamps like on street lamps use more power at start up. They use more power until they warm up too. I think that is true for mercury vapor lamps too. In the 70’s they were pushing for energy conservation, one thing being to shut off the lights. This is when the news and such started saying you waste energy by turning off the lights for certain types. Over time it gets repeated and changed until the bit about only certain lights do this is lost. I’m sure others can add to the list. Incandescent bulbs you should just turn on and off. I don’t know how the new compact fluorescents stand, because they use a higher frequency and contain solid state components. The higher frequency and solid state design should use less start up current than the old transformer 60 htz ones. ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [April 2, 2008, 6:36pm UTC](https://boards.straightdope.com/t/power-consumed-by-switching-on-a-light/443693/4 "2008-04-02T18:36:47Z")
[QUOTE=Harmonious Discord] Sodium lamps like on street lamps use more power at start up. They use more power until they warm up too. I think that is true for mercury vapor lamps too. [/QUOTE] Cite? I can guarantee that HID lamps use the least power at startup (ignoring the short inrush current), and slowly reach full power after 10-15 minutes or so. I’m reasonably sure that all discharge lamps work this way. ---
**Author:** ![RedSwinglineOne](https://avatars.discourse-cdn.com/v4/letter/r/a9adbd/32.png) [@RedSwinglineOne](https://boards.straightdope.com/u/RedSwinglineOne)\ **Post date:** [April 2, 2008, 6:40pm UTC](https://boards.straightdope.com/t/power-consumed-by-switching-on-a-light/443693/5 "2008-04-02T18:40:47Z")
Mythbuster did this, and they determined the power used at startup was fairly insignificant. I can’t say anything about their testing procedure. > [@](#): > > Myth: You save on energy bills by leaving lights on. Some people believe that the energy to turn on lights exceeds savings of turning lights off. > > They talked to Mark Reisfelt, manager of the Independent Electric Supply where they purchased their light bulbs. He felt that it was best to turn the lights off. > > To test the myth, they needed to measure energy usage during startup, maintenance (steady state), and shutdown. > > For steady state energy consumption, they turned on several different types of bulbs for 60 minutes and measured their consumption using a Kill A Watt: \* Incandescent 90 Wh \* Compact Fluorescent (CFL): 10 Wh \* Halogen: 70 Wh \* Metal halide 60 Wh \* LED: 1 Wh \* Fluorescent: 10 Wh > > For startup energy consumption, Grant hooked up an inductive current loop to a computer and measured the amount of energy used when the turned on the bulbs. With an inductive current loop, you run a wire through the center, which induces a current in the loop. This current is then measured by a digital sampling oscilloscope. > > Based on the amount of energy consumed turning on the bulb, they were able calculated how long the bulb would have to be turned off in order to make it worth the energy savings, i.e. “It’s best to turn off the bulb if you are leaving the room for”: > > Incandescent: 0.36 seconds > CFL: 0.015 seconds > Halogen: .51 seconds > LED: 1.28 seconds > Fluorescent: 23.3 seconds > In other words, its almost always best to turn the bulb off. Even the 23 seconds for the fluorescent lights isn’t very long, and the rest of the times are pretty much blinks of an eye. ---
**Author:** ![Harmonious\_Discord](https://avatars.discourse-cdn.com/v4/letter/h/74df32/32.png) [@Harmonious\_Discord](https://boards.straightdope.com/u/Harmonious_Discord)\ **Post date:** [April 2, 2008, 7:20pm UTC](https://boards.straightdope.com/t/power-consumed-by-switching-on-a-light/443693/6 "2008-04-02T19:20:11Z")
The sodium lamps consume power the whole time as they power up or warm up. Until they warm up they put out much less light. You have to turn them on for many minutes before they light the room enough to actually see where you’re going. [The Encyclopedia of Alternative Energy and Sustainable Living](http://www.daviddarling.info/encyclopedia/L/AE_low-pressure_sodium_lamp.html) > [@](#): > > Low-pressure sodium lamps work somewhat like fluorescent lamps. Like high-intensity discharge lighting, low-pressure sodium lamps require up to ten minutes to start and have to cool before they can restart. Therefore, they are most suitable for applications where they stay on for hours at a time. They are not suitable for use with motion detectors ---
**Author:** ![borschevsky](https://avatars.discourse-cdn.com/v4/letter/b/97f17d/32.png) [@borschevsky](https://boards.straightdope.com/u/borschevsky)\ **Post date:** [April 2, 2008, 7:29pm UTC](https://boards.straightdope.com/t/power-consumed-by-switching-on-a-light/443693/7 "2008-04-02T19:29:02Z")
Thanks! Good old Mythbusters 🙂 ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [April 2, 2008, 7:52pm UTC](https://boards.straightdope.com/t/power-consumed-by-switching-on-a-light/443693/8 "2008-04-02T19:52:53Z")
[QUOTE=Harmonious Discord] The sodium lamps consume power the whole time as they power up or warm up. Until they warm up they put out much less light. You have to turn them on for many minutes before they light the room enough to actually see where you’re going. [The Encyclopedia of Alternative Energy and Sustainable Living](http://www.daviddarling.info/encyclopedia/L/AE_low-pressure_sodium_lamp.html) [/QUOTE] True, but that is not the issue. It’s power consumption we’re talking about. ---
**Author:** ![Harmonious\_Discord](https://avatars.discourse-cdn.com/v4/letter/h/74df32/32.png) [@Harmonious\_Discord](https://boards.straightdope.com/u/Harmonious_Discord)\ **Post date:** [April 2, 2008, 9:02pm UTC](https://boards.straightdope.com/t/power-consumed-by-switching-on-a-light/443693/9 "2008-04-02T21:02:51Z")
[QUOTE=beowulff] True, but that is not the issue. It’s power consumption we’re talking about. [/QUOTE] The power consumption per lumen of light is higher, is what I’m saying. If you don’t think the warm up time influences if you should turn it on or not fine, but I think it does. There were multiple questions asked. ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [April 2, 2008, 9:26pm UTC](https://boards.straightdope.com/t/power-consumed-by-switching-on-a-light/443693/10 "2008-04-02T21:26:22Z")
[QUOTE=Harmonious Discord] The power consumption per lumen of light is higher, is what I’m saying. If you don’t think the warm up time influences if you should turn it on or not fine, but I think it does. There were multiple questions asked. [/QUOTE] Go back and look at your statement in my post #4. ---
**Author:** ![Napier](https://avatars.discourse-cdn.com/v4/letter/n/ce73a5/32.png) [@Napier](https://boards.straightdope.com/u/Napier)\ **Post date:** [April 3, 2008, 1:53am UTC](https://boards.straightdope.com/t/power-consumed-by-switching-on-a-light/443693/11 "2008-04-03T01:53:39Z")
The cutoff times for different lamps looks one way when you consider power consumption, and it looks another way when you consider out of pocket costs that include power and lamp replacement. The fluorescents I just installed downstairs say their components are balanced so that if they are run in sessions of a half hour the two most perishable things in the tube should run out at about the same time. ---
**Author:** ![Pasta](https://avatars.discourse-cdn.com/v4/letter/p/ecccb3/32.png) [@Pasta](https://boards.straightdope.com/u/Pasta)\ **Post date:** [April 3, 2008, 4:35am UTC](https://boards.straightdope.com/t/power-consumed-by-switching-on-a-light/443693/12 "2008-04-03T04:35:37Z")
Coincidentally, I calculated this very thing sometime last year. I don’t have time to reproduce the calculation at the moment, but I do remember that the break-even point for incandescent bulbs was a fairly small fraction of second (maybe of order one tenth?). That is, if you were leaving for, say, thirty milliseconds, then you might leave the lights on. ---
**Author:** ![Number43](https://avatars.discourse-cdn.com/v4/letter/n/58956e/32.png) [@Number43](https://boards.straightdope.com/u/Number43)\ **Post date:** [April 3, 2008, 6:00am UTC](https://boards.straightdope.com/t/power-consumed-by-switching-on-a-light/443693/13 "2008-04-03T06:00:49Z")
For what it’s worth, I have met multiple people who were under the mistaken impression that tuning a normal lightbulb off and then on would use up electricity equivalent to leaving it on for an extended period of time.