# Switching light off or not?

Is it better to switch off a tube light and turn it on again or not switching it off in the first place for a small amount of time? Google searches say "Fluorescent lights do take a small surge of power when turned on, but this is significantly smaller than the amount saved by turning them off" and "The amount of electricity consumed to supply the inrush current is equal to a few seconds or less of normal light operation. Turning off fluorescent lights for more than 5 seconds will save more energy than will be consumed in turning them back on again." So how do we decide... how does it say "5 seconds"? Is there a formula for this? This will obviously depend upon the time for which we turn off a light before switching it on again, and the power consumed because of the inrush current. So are there any other factors? In an objective question, say, how can we know if it is better to switch off a light or not given the time for which light is switched off and the power consumed in switching a light on?

• Another consideration is that if your building is automatically heated to a fixed temperature (warmer than the outside air) by a thermostat, then all the electrical energy you use inside the building that doesn't escape (e.g. as light out the windows or hot water down the drain) turns into useful heat, so it isn't wasted.
– Karl
May 6 at 18:36
• @Karl Then you should also factor in that direct electrical heating is typically much more expensive than other means like gas, oil, or a heat exchanger.
– noah
May 6 at 18:39
• Another consideration is the change in the lifetime of a tube if it is continually switched on and off. May 6 at 19:08
• Yes, switching a fluorescent tube off then on uses more energy than just leaving it on. If the time off is less than about 10 seconds! This used to be much longer, when the lights were more primitive. In terms of pure cost the exchange is a lot different though. Fluorescent lights do not age much, while on. But each off/on cycle does more damage to the light than several hours of "on" time. The balance point for when it is cheaper to switch off your light depends on the cost of the light, the cost of replacing it, and the cost of electricity. Typically, about 15 minutes. May 6 at 19:12
• Thank you, of course, this was just a hypothetical question so I was more concerned about minimizing the energy wastage and didn't take the lifetime of the appliance or other factors into consideration, but yes that will have an effect too May 6 at 19:18

Under the conditions you have given, this is very straightforward to calculate. You measure the energy to switch the light on and call it $$E_\text{switch}$$. Then you measure the power the light consumes during normal operation, which is energy per unit time, call it $$P$$. Then just compare $$E_\text{switch}$$ to $$t_\text{off} \cdot P$$, with $$t_\text{off}$$ the time the light would be off, to see which is larger. If it's the former, leave it on, if it's the latter, turn it off. Given $$P$$ and $$E_\text{switch}$$, we can also calculate $$t_\text{off}$$ by $$t_\text{off} = \frac{E_\text{switch}}{P}$$