# Efficiency of Fluorescent Lights

## Main Question or Discussion Point

I read in a science question/answer book that it is takes as much energy to turn on a fluorescent light as it doesn to run it for 1 hour. So if this is true, it would mean that if you plan on using the light again within an hour, it is more efficient to leave it on, rather than turning it off and then back on again.
Fluorescent lights are aleady much more efficient than incandecent light bulbs, but why not take the time to save an extra 10 of a cent.

So has anyone else heard this before, or seen it somewhere else? I have only seen it in that one source, so I dont know if it is true, or if it is by how much.

## Answers and Replies

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brewnog
Gold Member
I heard this on the grape vine, but thought it only applied to 'strip-light' style fluorescent tubes, rather than the 'energy saving' replacement light bulbs for domestic use. Will look it up properly when I get a spare minute...

brewnog
Gold Member
Ok, here we go:

http://lightingdesignlab.com/articles/switching/switching_fluorescent.htm

Misconception #1: It takes more energy to start a fluorescent that it does to run it, so leave the lights on all the time to save money on your electric bill.

Reality: When you turn on a fluorescent light bulb, there is a very brief jump in current when the ballast charges the cathodes and causes the lamp to start. This inrush of current can be many times greater than the normal operating current of the lamp. However, the spike of current draw normally lasts no longer than 1/10th of a second, and draws the equivalent of about 5 seconds of normal operation. So, if you turn your fluorescent lamp off and on more frequently than every 5 seconds, you will use more power than normal. So, normal switching of fluorescent lamps has very, very, very little effect on a power bill.

The article also explains how long burns extend the lamp life, but since they use more energy, there's a trade-off point. The calculation needed is shown.

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brewnog said:
but thought it only applied to 'strip-light' style fluorescent tubes, rather than the 'energy saving' replacement light bulbs for domestic use.
Yes, those were the ones I was refering to.

So really, that book was wrong, or atleast very inaccurate.
So, if you turn your fluorescent lamp off and on more frequently than every 5 seconds, you will use more power than normal
5 seconds, not even close to 1 hour.

OK, thanks, I thought it sounded a little hokey.

russ_watters
Mentor
This misconception may come from people thinking HID lamps (those big lamps at stadiums or in parking lots) work the same as fluorescents. An HID takes several minutes to heat up before operating and likely does use as much power in those few minutes as in the rest of the hour combined.

russ_watters said:
This misconception may come from people thinking HID lamps (those big lamps at stadiums or in parking lots) work the same as fluorescents. An HID takes several minutes to heat up before operating and likely does use as much power in those few minutes as in the rest of the hour combined.
I work on HID ballasts. A 100 Watt arc tube would use 200 Watts for
maybe one minute. This is only twice, not 59 times the running power.

If it used as much power to start (in one minute) as it did to run for the
rest of the hour, then it would have to use 5900 Watts for the one minute.
I assure you, it does not.

What does take a couple of minutes is for the lamp to reach peak temperature
and emit maximum light. The power by then is usually at the constant burn
power even though the light is getting brighter.

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