Do lights use more energy to warm up?

Written by pete


This week’s question comes from Susan Fiore:

> I was brought up to turn the light off when I leave a room, even for a few minutes. My husband says it costs more to turn it off and back on again than leaving it on if you’re coming back soon.

> So which is right? And is it different for florescent, incandescent and halogen?

Can you help? Leave your answers to Susan’s question in the comments below

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  1. Karsten 'quaid' Wade

    Pardon the lack of references, but I do know I have read this information within the last year.
    There is a negligible amount of extra electricity involved in the turning on and off of a light. However, the real issue is lifespan of the bulb itself. A lightbulb that is turned on and off more often is going to have a shorter life because of the extra wear on the materials.
    The article I read referred to research that the energy cost to replace the shorter-lifespan lifebulb is more than the energy of leaving it on for a few minutes.
    The key factor is if you are going to be going back to the room soon or not. If there is a chance you are not going to be using the light in the next five to ten minutes, then turning it off probably saves the same or more energy than the reduction in lifespan of the bulb for the extra power cycle event.

  2. Bob Moore

    This is pretty easy to test out with a Kill-A-Watt. I hooked up a lamp, screwed in a compact fluorescent, plugged it into my Kill-A-Watt, and turned on the lamp. Voila… no difference between the first few seconds of energy used, and the time running afterward.
    As the poster said above, the only likely difference is in the life of the bulb.

  3. Eric

    There is a Mythbusters episode on this.

  4. veek

    Yet another consideration for the pie chart is habit development. If you get into the habit of mindfully turning off things you don’t need (or not buying them in the first place) this may save considerably more energy (or, better yet, not using that energy in the first place). It’s a mindset thing.
    The question of light bulb life is important too, because right knowledge is also a factor in efficient energy use. Hope this helps.

  5. Craig

    I do believe that the lifespan of the bulb is what is affected most by turning them off and on. This is especially important when using CFL bulbs because they contain mercury. What I try to do in order to compensate for this is my base routine is to turn off lights as I leave the room, then I look for specific examples in my routine where I am coming in and out of the room. A couple of examples:
    * When I get ready in the morning I constantly go from the bathroom to my bedroom. I don’t need the light in my bedroom, but for my bathroom routine I like the light so I leave the bathroom lights on when I get ready.
    * The other reason I don’t need the bedroom light on is that I constantly go in my walk in closet when getting ready (which is in my bedroom). So I leave my closet light on when getting ready.
    So I work off the premise that lights should be turned off, then look for the exceptions. I think this is a safe bet as the above poster pointed out that the habit of turning the light off is more important than the few extra volts it might take in the first 10 minutes of turning on a light.

  6. Tom Harrison

    According to this US Department of Energy Guide everyone above got it right.
    I did want to reinforce veek’s comment (#4) and others relating to getting into the habit. Only bad habits come to me without a little effort :-). So the best way to get into a good habit is to start simple.
    It is a lot easier to say to yourself, and especially to kids, “turn off the lights when you leave the room” than it might be to say “turn off the lights if you are going to be gone for more than 10 minutes (for incandescent) and 15 minutes (for fluorescent)”. Adding this calculus, then overloading it with lifespan of the bulb would require a calculator … which of course uses resources and energy itself!
    Keep it simple: turn out the lights. It works.

  7. Gary Atcheson

    Incandescent and halogen are both going to be similar (tungsten filament with halogen gas instead of argon), with the lifespan being more important than the warm-up time as people mentioned above. Halogens last longer in general but the lifetime is reduced by turning them on and off. The warm-up time is insignificant in energy usage.
    Fluorescent (particularly the CFL screw-in substitutes for incandescent) have a ballast which makes them different. The ballast requires a small surge of power to start the arc. The lifetime of the ballast is also reduced by turning it on. All this varies by manufacturer and model, so general numbers are tough. But as a rule of thumb, I like the 10 minute rule: turn the fluorescent light out when you leave the room unless you will be back in less than 10 minutes.
    Incidentally, incandescent bulbs do have a quick warm-up time while the filament heats up. The time is so short we do not notice it when flipping a light switch. But you can observe the difference between ordinary car turn-signals and the newer LEDs which have such a sharp on/off transition.

  8. Oliver McDonald

    Mythbusters recently tested this question. In terms of energy use, if you are leaving a room for more that about 1/2 second, turn off the incandescent, CF or LED light. For the long florescent tubes, if you are leaving for more than 23 seconds, turn it off.
    On lifespan of the light, the average bulb or tube is good for ~ 5000 on off cycles, so even if you do turn it on/off every time you enter/exit the room, it will not last a significantly shorter time, than if you leave it on lighting an empty room

  9. Paula Thompson

    I’ve been spending a fortune for eco light bulbs indoor & out. The claims are fantastic 25 years, 50 years — not at my home –often not even a year. I do mark them so I know when I put them in.
    I’ve looked for solar lighting but not too much that I could find.
    What do you know about these claims & is there anything to do about it.
    Thanks for info, Paula

  10. Tom Harrison

    Paula – unfortunately, my research has shown that many of the compact fluorescent (CFL) bulbs available are very poorly manufactured. This is a travesty, because good CFLs are available. The good ones work pretty much the same as the incandescent bulbs they replace, they use 1/4 to 1/5 as much electricity, and although I haven’t had the five years to verify their claims of longevity, it’s clear from a visual comparison that well made bulbs are better than cheapo ones.
    I wrote up a review this fall on my blog, at, but to save you the work, the short answer is that GE Energy Smart CFLs, at least the ones I have tested, are the ones you want. They are readily available online and also even at most pharmacies and many markets, at least here in the Northeast, where I live. They are not terribly expensive, either.
    Dimmable CFLs still don’t work well, and specialty bulbs (candle bulbs, ceiling spotlight bulbs, etc.) may not fit, or look nice. But standard bulbs, and some others work great, last, have very nice light, and are made well. See my blog or other review sites for more details if you like.