My physics teacher regularly said to our class:

Turning these lights [pointing to the ceiling of the classroom] off and on uses more energy than leaving them on for 30 minutes.

Is this true?

I am not sure about the exact kind of lamp used in the classroom. I guess it was some sort of neon light (it had this characteristic flickering when turning it on). But the exact kind of lamp does not matter so much. I am equally interested in an answer to the following question: "Is there any light source that was in popular use 15 years ago that uses more energy to be turned off and on that leaving it on for 30 minutes?"

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    I've heard this claim on a frequent basis over the last few decades. Commented Dec 12, 2018 at 22:51
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    The light he is speaking of is a fluorescent tube. Commented Dec 12, 2018 at 22:56
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    Not an answer because it's "theory", but it'd be interesting to work out the current draw that would be required for this to be true, and compare to the rating of your circuit breaker. Or, to estimate the temperature increase from dissipating this much power, and compare to the bulb's melting point. Commented Dec 13, 2018 at 1:39
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    Your physics teacher is not worth his salary? Just try to calculate the current that would be drawn during that half second that it is switching on (which is a simple school level exercise to do)!
    – user22865
    Commented Dec 13, 2018 at 8:29
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    I assume he wasn't trying to claim that repeatedly turning it on and off for 30 minutes would draw more power? That's a pretty different claim than doing it once every 30 minutes (and would then depend on the rate of switching).
    – JMac
    Commented Dec 13, 2018 at 14:49

1 Answer 1


This page at Cambridge University says its a myth. There is a burst of energy when you turn them on, but its equivalent to 2 seconds of run time. Also the light lifetime is not seriously affected by turning it off and then on again occasionally.

The energy consumed to start a typical lamp is the equivalent of 2 seconds running time, so it is wrong to say it takes a lot of power to start them. It is true there is a current surge but this takes place in less than one-eighth of a second and because it happens so quickly it takes very little energy.

The Mythbusters performed a practical experiment to measure the amount of power required to turn on various lights compared to their steady-state consumption when on. They found the following:

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.

It is true that switching on/off fluorescents reduces lamp life but lamps are designed to be switched on/off up to seven times a day without any effect on their life. How many times a day do your colleagues switch on/off to save energy? Probably not enough times to reduce the lamp life.

Addressing the last part of the question is harder.

Is there any light source that was in popular use 15 years ago that uses more energy to be turned off and on that leaving it on for 30 minutes?

At risk of doing some Own Research, some back of the envelope calculations suggest that this is unlikely. A typical 4 foot flourescent tube consumes 36 watts, and a light fitting will often contain two tubes, making a total steady-state power consumption of 72 watts. If the starting process takes 5 seconds then to take as much energy in those 5 seconds as it takes during 30 minutes of continuous operation the light fitting would have to draw 72x30x60/5 watts, which is about 26kW. At the UK standard of 240 volts that would be 108 amps, or about 8 times the power of an electric kettle. For US 110 volt circuits it would require 236 amps. However flourescent lights have never required any special cabling to deal with such high currents.

  • Off and on, at a relatively slow interval of 1 second, would give you 30 seconds of on time, plus 60 seconds worth of energy for the 30 start up power surges. I'd call that a confirmation.
    – user11643
    Commented Dec 12, 2018 at 22:25
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    @fredsbend The teacher doesn't mean that. They mean (claim) that turning them off and on once uses as much energy as 30 minutes of running. Commented Dec 12, 2018 at 22:29
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    I remember people saying this to me frequently when I was a kid. Commented Dec 12, 2018 at 22:50
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    The "Cambridge University" page isn't a link to experts in Electronic Engineering who have studied fluorescent lights. It is a link to administrators and co-ordinators who are trying to encourage staff to reduce costs. They reference a video, but that doesn't provide references either. So I am left wondering why I should trust these claims over those of a physics teacher?
    – Oddthinking
    Commented Dec 13, 2018 at 8:33
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    You might talk about normal lighting circuit breakers that would trip if starting consumed an elephant of electricity.
    – daniel
    Commented Dec 13, 2018 at 11:20

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