When I leave a room just for five minutes, is it worth it to turn the lights off and then back on again?
This is a more complicated issue with various factors at play, both the type of light bulb and the cost of your electricity. Every light bulb has a certain amount of operating hours that it remains working, and this is in fact related to and affected by the amount of times it is turned on and off. The more times you turn on and off the power of the light bulb, the shorter the lifespan. That being said, given the amount it actually effects the light bulb it could still be cost-effective to turn it on and off.
Regarding incandescent bulbs, they are fairly inexpensive to produce and also are very inefficient in that only 10% of the energy used by the bulbs are actually used to produce light, whereas the rest of the energy is simply given off as heat. Additionally, turning the light bulbs off in the summer will help keep your room cool. So, despite the shortening of the lifespan of the incandescent bulb, because it is so inexpensive to replace and because of the marginal effect of the flipping power, it actually is worth it to turn the light off even when you are just leaving the room for one minute.
Regarding fluorescent bulb it gets more complicated. Their operating life is more affected by frequent power changes, and a general rule of thumb as told by the Department of Energy is that you should only turn off the light if you will leave it off for 5-15 minutes. Here are the factors at play: the cost of replacing the lamp, and the cost of the extra initial rush of energy to get the fluorescent bulb started. The initial rush of energy only lasts about 1/120th of a second, and uses about as much energy as the bulb does normally in 3-5 seconds. Therefore, it is seemingly worth it to turn off your lightbulb even if you are only leaving the room for 5 seconds. However, because of the high cost of fluorescent bulbs and the greatly shortened life span of each lightbulb as a function of frequent switching on/off, the true economic cost of turning it on or off is about the cost of keeping the light on for 5-15 minutes.
With regards to incandescent bulbs it is always worth it to turn off the lights. However, with respect to fluorescent bulbs it depends more on how long you are going to be leaving the room for. If you are indeed gone for 5 minutes, definitely turn it off and then back on. Even if you are not leaving for a whole five minutes, though, from by a green point of view, it may cost more money for you to turn them off for less than 5 minutes, but you will be saving energy even if they are off for only 5 seconds.
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