I have been told time after time that at school if you are not going to leave the room for more than 10 minutes to keep the lights on instead of turning them off because the amount of power it takes to turn them back on is more than just leaving them on. I am also told this is only for fluorescent tube lights?

Is this true and if not where did this myth come from?

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    mythbusters found it was around 30 seconds where the equalize point is located Commented Nov 4, 2013 at 13:53
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    Welcome to Skeptics! This site is about examining the evidence behind individual notable claims. Could you please provide a quote of the claim you want us to examine and provide a reference (a link, for example) to its source? You can read FAQ: Must all questions be notable for more info.
    – user5582
    Commented Nov 5, 2013 at 8:13

1 Answer 1


No, it's not true.

According to this article in Scientific American, there is a slight additional energy required to start up a fluorescent bulb, but it lasts an extremely brief time:

that power surge is so brief that its energy draw doesn't amount to much: the equivalent of a few seconds or so of normal operation, according to U.S. Department of Energy estimates.

The article recognizes that there is some additional wear-and-tear each time the lamp is turned on, which may have an impact on life. It recommends turning them off if you won't need them for more than five minutes.

A simple rule of thumb that balances both concerns is to shut off fluorescents if you’re planning to leave a room for more than five minutes, according to Francis Rubinstein, a staff scientist in the Building Technologies Department at Lawrence Berkeley National Laboratory's Environmental Energy Technologies Division.

The above article is directed primarily at CFL bulbs, but here is a reference from the California Energy Commission based on more conventional tubular fluorescent bulbs. It comes to the same conclusion, stating

The modern electronic ballast and T8/T5 lamp combination do draw a higher level of current during startup, but it only lasts for a fraction of a second, which is negligible compared to normal current requirements of the lamps.

Their recommendation is the same:

You should turn off your fluorescent lamps if the space is not going to be occupied for more than a few minutes (3-5 minutes is a good rule of thumb)

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    While true, this ignores the other side effect that frequent switching lowers the lifetime of CFLs and fluorescent tubes. This certainly lowers their economic benefit even if it doesn't much affect their power consumption.
    – matt_black
    Commented Nov 4, 2013 at 14:07
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    @matt_black it explicitly includes the effect of frequent switching on lifetime: "The article recognizes that there is some additional wear-and-tear each time the lamp is turned on, which may have an impact on life.". That's why the recommendation is for 5 minutes, rather than a few seconds.
    – 410 gone
    Commented Nov 4, 2013 at 16:25
  • @EnergyNumbers Well spotted. I wonder, though, ignoring the energy use, whether the effect on lifetime is fully considered? there is already a skeptics.SE question/answer on this (which I answered) here: skeptics.stackexchange.com/questions/8365/… . I wouldn't mind seeing some new analysis, though.
    – matt_black
    Commented Nov 4, 2013 at 21:09

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