This used to be a valid question with older fluorescent fixtures, but not anymore. A light that is off doesn't use power and it doesn't use extra power when turning on, so turn it off to save power. It depends what you want to consider. If you throw bulb life into the mix (most bulbs fail due to thermal cycle fatigue) then you might be better off leaving the light on. Incandescent lamps have substantially higher inrush current before the filament heats up. This probably does not amount to enough to significantly increase the operating cost, however it reduces the life of the bulb. Incandescent light failure is almost always when cycling the lamp on.
It's always best to turn the lamp off when it isn't needed. The extra power from the inrush occurs over an extremely short time and the energy used is minimal.
On 240 volts suppose the inrush is 10 amps for 1/20 second (it is probably much less), the energy of the inrush is 2400/20 watt-seconds, 120 watt-seconds, which is the same as leaving it switched on for 1.2 seconds.
On that basis you should turn it off if you plan to be outside that room for 2 seconds or more.
Bequerel left his crystals in the sunlight for several hours as part of his experiment.
Is it cheaper to leave your gas furnace setting at 68 degrees in the winter
Leave it out for several hours or overnight in a bowl
Sure. If more than a few hours, refigerating would be best.
I personally think about 4 hours would be the most safe, that is what all the safety tips say. But if you want to go on the line, the absolute longest would be 7 hours.
Always cheaper to switch it off and on rather than leaving it on.
No, because the germs will attack it if it is allowed to heat twice. This might work, however, if you leave it for several hours.
How many hours do you need to get maternity leave?