# Turn lights off or leave them on? **URL:** \ **Category:** Factual Questions\ **Created:** [October 25, 2009, 11:55pm UTC](https://boards.straightdope.com/t/turn-lights-off-or-leave-them-on/515114 "2009-10-25T23:55:44Z")\ **Posts on this page:** 6\ **Page:** 1
**Author:** ![Yodalicious](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/yodalicious/32/8195_2.png) [@Yodalicious](https://boards.straightdope.com/u/Yodalicious)\ **Post date:** [October 25, 2009, 11:55pm UTC](https://boards.straightdope.com/t/turn-lights-off-or-leave-them-on/515114/1 "2009-10-25T23:55:44Z")
Years ago I was to told that more energy was used turning lights on (going form power level 0 to 100) than leaving them on (constant power level 100). This person said if you turn a light on, it was more efficient to leave the light on than to keep turning it off and back on, when you leave and go back into a room. Any truth to this? If it is more efficient, would it also cost less? Or, did this guy just not know what he was talking about? FWIW, I follow people around in my house and turn off lights behind them. ---
**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:** [October 25, 2009, 11:57pm UTC](https://boards.straightdope.com/t/turn-lights-off-or-leave-them-on/515114/2 "2009-10-25T23:57:44Z")
This has been done to death. Unless you are going to be returning to the room in under a minute, you save energy by turning the light off. The are tradeoffs between lamp life and the number of cycles, but it’s generally a good bet that you are better off turning the lights off than leaving them on. ---
**Author:** ![Floater](https://avatars.discourse-cdn.com/v4/letter/f/dc4da7/32.png) [@Floater](https://boards.straightdope.com/u/Floater)\ **Post date:** [October 26, 2009, 10:18am UTC](https://boards.straightdope.com/t/turn-lights-off-or-leave-them-on/515114/3 "2009-10-26T10:18:54Z")
> [@Daddypants](#): > > Years ago I was to told that more energy was used turning lights on (going form power level 0 to 100) than leaving them on (constant power level 100). This person said if you turn a light on, it was more efficient to leave the light on than to keep turning it off and back on, when you leave and go back into a room. Any truth to this? If it is more efficient, would it also cost less? Or, did this guy just not know what he was talking about? > > FWIW, I follow people around in my house and turn off lights behind them. So was I taught. However, it’s about the difference between ordinary light bulbs and fluorescent lights. A light bulb uses much more electricity than a fluorescent light when lit, but the fluorescent light uses a lot more energy during the switch on phase. The moral of this is always switch off a light bulb, but not a fluorescent light (unless nobody will be around for a longer period of time). Of course technology has most probably advanced during the last forty years so I have no idea if this is still true. ---
**Author:** ![racer72](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/racer72/32/3075_2.png) [@racer72](https://boards.straightdope.com/u/racer72)\ **Post date:** [October 26, 2009, 11:42am UTC](https://boards.straightdope.com/t/turn-lights-off-or-leave-them-on/515114/4 "2009-10-26T11:42:19Z")
This was addressed on Mythbusters. They compared leaving a light on for 5 minutes or so and turning it off. Even the light that used the most power to start, you lost any benefit of leaving the light on in less than one second. For compact flourescents it was about a tenth of a second and for an incandescent it was almost impossible to measure, it was something on the range of .001 seconds. They also visited a light bulb that has been burning continuously since 1904. ---
**Author:** ![Yodalicious](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/yodalicious/32/8195_2.png) [@Yodalicious](https://boards.straightdope.com/u/Yodalicious)\ **Post date:** [October 26, 2009, 12:47pm UTC](https://boards.straightdope.com/t/turn-lights-off-or-leave-them-on/515114/5 "2009-10-26T12:47:22Z")
Thanks everyone. Now, I just have to convince my “extended stay” visitors to turn off the lights when they leave a room… and the televisions. Christ, my electric bill is gonna be outrageous. ---
**Author:** ![Floater](https://avatars.discourse-cdn.com/v4/letter/f/dc4da7/32.png) [@Floater](https://boards.straightdope.com/u/Floater)\ **Post date:** [October 26, 2009, 12:59pm UTC](https://boards.straightdope.com/t/turn-lights-off-or-leave-them-on/515114/6 "2009-10-26T12:59:57Z")
> [@Daddypants](#): > > Now, I just have to convince my “extended stay” visitors to turn off the lights when they leave a room… and the televisions. Televisions (and other electric appliances) should always be switched off when you don’t use them, not so much for the electricity consumption aspect but for fire prevention purposes. Heat inside an apparatus and ditto dust is not a terrifically good combination.