Science Questions

How much energy to flourescents take to turn on?

Fluorescent lights are already efficient when they’re running but I’ve heard that it takes a lot of energy to turn a fluorescent light tube on. So is it more efficient to turn off a fluorescent tube immediately when you’ve finished using it or is it better to leave it on and then wait until you’re more likely to not use it again for a while? This question was first asked in last week's show. Derek, Belgium.

Last week Derek in Belgium asked a question about fluorescent lights, but we didn’t know what that period of time was to make switching off a light more efficient than leaving it on.

I have to say a very big thank you to Shia Frederick and also to Kurt Challetts who both found me the answer and they’ve sent me some references.  Based on experimentation they have concluded that – these references they’ve sent me – a fluorescent light uses as much energy getting started as it des to run for 23.3 seconds.  In other words, that is a bit of a myth about leaving it on for very long.  If you’re going to be in the room and gone for less than 23.3 seconds there’s no point it on, basically.  You might as well turn it off because it uses not very much more energy.  To put that in comparison, an LED has to run for 1.28 seconds before it uses as much energy.  The good old-fashioned incandescent lamp: 0.36 seconds before it’s used as much energy it does running as it does to turn it on.

January 2008