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I'm not sure if this qualifies as electrical engineering, but I remember my old physics teacher at school saying that if you switch a light on in a room and you're planning to return relatively quickly, it uses less electricity if you leave the light on until you return as switching the light on uses more eletricity than leaving it on for a relatively short period, but how long before leaving it on becomes more energy consuming than switching it off until you return and switching it on again?

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    \$\begingroup\$ Did you check this before? energy.gov/energysaver/articles/when-turn-your-lights \$\endgroup\$
    – Shabab
    Oct 11 '13 at 20:24
  • \$\begingroup\$ No. That answers the question though. The answer is switch the light off unless it's an energy saving bulb, in which case don't switch it off unless your not planning to come back for more than 15 minutes. \$\endgroup\$ Oct 11 '13 at 20:27
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    \$\begingroup\$ it's quite possible that simply moving the hard drive heads to save the replies to this question have negated any positive energy savings for the next thousand years. \$\endgroup\$ Oct 11 '13 at 21:51
  • \$\begingroup\$ lol- good point. I had a feeling it was a bad idea asking the question too. \$\endgroup\$ Oct 11 '13 at 21:59
  • \$\begingroup\$ Duplicate of electronics.stackexchange.com/q/55831/2028 \$\endgroup\$
    – JYelton
    Oct 11 '13 at 22:18
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The answer to your question can be found at http://energy.gov/energysaver/articles/when-turn-your-lights.

In short, it depends on what type of bulb you are using.

  1. If you have an incadescent or halogen bulb, you save more energy if you turn the light off.

  2. If you have a CFL

    -If you leave the light unattended for less than 15 minutes, leave it on

    -If you leave the light unattended for more than 15 minutes, turn it off

  3. LED lights are more efficient in that they don't require as much of a "start up" current. They should be turned off in orcer to conserve the energy that would normally be dissipated during normal operation.

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    \$\begingroup\$ Op was asking about energy saving - from the linked site: ... the relatively higher "inrush" current required lasts for half a cycle, or 1/120th of a second. The amount of electricity consumed to supply the inrush current is equal to a few seconds or less of normal light operation.... Therefore, the real issue is the value of the electricity saved by turning the light off relative to the cost of changing a lightbulb. \$\endgroup\$
    – mattnz
    Oct 11 '13 at 20:57
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    \$\begingroup\$ Good answer over all, but saying LED lights can be left on because the energy savings is negligible makes no sense. If anything, an LED should always be turned off, even if you plan to return within 15 seconds. It has zero "start up" current, and switching has no effect on an LED's lifespan. However, an array of high powered LEDs will use a significant amount of power and should be shut off when not in use. This will greatly increase the LED's lifespan as its working hours and heat dissipation are reduced. \$\endgroup\$ Oct 11 '13 at 21:00
  • \$\begingroup\$ @KurtE.Clothier Thanks for the catch! I edited the answer above. \$\endgroup\$
    – Shabab
    Oct 11 '13 at 21:05
  • \$\begingroup\$ I suppose you are saying the real issue is the value of the electricity saved relative to the effect of switching the light on and off on the lifespan of the bulb @mattnz \$\endgroup\$ Oct 11 '13 at 21:24
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Its far more complicated than indicated by the "leave it on for 15 minutes". But for practical purposes, its about as good as it gets.

The physics professor was wrong- turning the light off will save energy (unless its less than a few seconds for a CFL). However, CFLs have a limited cycle life and cost a lot to replace, so its more cost effective to leave them on and use a bit more energy. One thing to consider is that it takes energy to replace a bulb - manufacture a new one, dispose of the old one, transport etc. This should be factored into any calculation as to when the ideal time to turn it off is. If the light is in a space that requires space heating, the heat it generates is not wasted, so leaving it on is not as wasteful as it might be, however, if the room is cooled, the heat needs to be removed, requiring more energy and turning off sooner would be more energy efficent

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