Leaving fluorescent lights switched on at home…

Loch37408 · 13 October 2004 at 7:46PM

Leaving fluorescent lights switched on at home…

Hello,

Now, I know as good Money Savers we should all be leaving lights switched off at home whenever we leave the room.

However, I remember being told a long long time ago that fluorescent lighting uses more energy when it is being switched on that than it does for it to be kept running for a number of hours, is this correct or an urban myth?

If you are going to go back into the room within a hour it is cheaper to leave it switched on, is this correct or a old wives tale?!

TIA

loon_2 · 13 October 2004 at 8:55PM

if you are referring to the low energy lightbulbs... you are correct. because they take a while to warm up, they do work out cheaper if you leave them on; that is to say, do not put them in light sockets that are only turned on for a short while. eg. toilets, cupboards, etc.

mrcow · 13 October 2004 at 9:28PM

The below may help?

I've also read that with "normal" lightbulbs, if you are leaving the room for more than 3 seconds, then it's worth turning them off - this jumps to 5 seconds with energy saving lightbulbs.

Should I Turn Off Fluorescent Lighting When Leaving A Room?

Short Answer: Turn them off if you will be gone for more than about 15-20 minutes (for details keep reading).

There are a few misconceptions about fluorescent lighting that keep too many people from turning lights off to save energy. The first misconception is that it takes more energy to start a fluorescent light than it takes to run it. The second misconception is that turning a fluorescent light off and on will wear it out right away. Like many of our myths about energy, there is a small amount of truth in the belief. (Special thanks to Steve Selkowitz of Lawrence Berkeley National Labs for doing the research that this article is based on.)

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 (correctly called a "lamp"), 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.

Misconception #2: Turning fluorescent lamps off and on wears them out right away.

Reality: Electric lights have a published rating for expected life. This rating is in the hundreds of hours for many incandescent lights, and in the thousands of hours for most fluorescents. Fluorescent lights have a life rating based on how many hours they are left on every time they are turned on. This is usually referred to as "burn time", and for fluorescent lights the burn time is three hours.

Every time a fluorescent light is turned on, a tiny amount of the coating on the electrodes is burned off. Eventually, enough coating is burned off, and the lamp fails to start. Most full-size fluorescent lamps are rated to last 20,000 hours when left on for 3 hours every time they are turned on. This means that the lamp has roughly 6,667 starts available to use up. (20,000/3 = 6,667)

Longer burns extend lamp life. If you "burn" your fluorescent lamps shorter than 3 hours per start, you use up your potential starts faster. If you "burn" them longer than 3 hours per start, you use up your starts more slowly. However, you are paying energy costs for the operating time of the lamps, and the most efficient lamp is the one that is not on when it is not needed. See Table 2 for the effects of longer burn time on lamp life.

But longer burns use more energy. Operating a light when it is not needed is simply spending money for no purpose. Today's rapidly rising electric rates mandate that every building becomes leaner with energy use to control costs. See Table 3 for a comparison of operating costs for a typical fixture.

Find the trade off point.
There is a point where the amount of money you save from turning off the light exceeds the cost of reducing lamp life by more frequent starts. If you use the formula in Table 1 at $0.05 KWh, you come up with a time of about 15 to 20 minutes for that point. As energy rates go higher, that time becomes shorter. If you pay less than a nickel per kilowatt hour, your turning-off point would be longer.

The kind of ballast you use may make a difference if you turn your fluorescent lights off frequently. There are three different kinds of electronic ballasts: instant start; rapid start; and programmed start. Which one you use can influence your choice of how frequently to switch off your fluorescent lights. Check with your ballast supplier, or contact a lighting specialist at the Lighting Design Lab for more information on different types of ballasts.

Table 1:
The "Official Formula" to calculate how often to switch lamps off and on

In "Economics of Switching Fluorescent Lamps" IEEE Transactions on Industry Applications Vol 24, No 3, May/June 1988, Carriere & Rea provide a function f(u), that describes the lamp life in burning hours relative the rated lamp life under that standard burning cycle of 3 hours lamp operation per start.

Actual Lamp Life =
Rated Lamp Life x f(u) where, f(u) =
1.71 (1-exp[-(u/3.89)^0.505]) u =
burning cycle, hours of operation per start

Table 2: Burn hours and Fluorescent Lamp Life for a Typical 4-foot Rapid Start Lamp (32W)
3 hr 6 hr 12 hr 24 hr
20,000 24,000 28,000 34,000

Table 3: Comparative annual operating costs for a 3-lamp T-8 fixture with electronic ballast (94W) (@$0.05/kWh)
Burn Time
per day Annual
Hours Annual
Cost
3 hrs 1095 $5.15
6 hrs 2190 $10.29
12 hrs 4380 $20.59
24 hrs 8760 $41.17

Taken from:
http://www.lightingdesignlab.com/articles/switching/switching_fluorescent.htm

Unfortunately it's a US site, so the calculations are in USD.

Hope this helps?

Loch37408 · 14 October 2004 at 12:32PM

A very comprehensive reply indeed mrcow! Many thanks.

Just for the record I am talking specifically about fluorescent strip lighting - the type commonly found in kitchens.

Tania_2 · 14 October 2004 at 7:26PM

Thanks for that - just nipping to kitchen to turn off light. Ours is left on loads of time. Now when hubbie goes on, ill show him this thread. x

zcaprd7 · 21 October 2004 at 3:13AM

Heh, Heh - this is very similar to the thread on energy saving light bulbs where I argued that as all the 'wasted' heat was diffused into the surrounding area i.e. heating up the building, the (during winter at any rate) energy wasn't being wasted at all, because your heating bills would be that much lower...

So that's 50% one can slash off the quoted energy saving rates - I was suprised that nobody questioned the fact that energy companies promote the darn things - yet more green propaganda.

I guess they are happy for a misguided change because it raises general awareness of energy saving? Or do they really believe it makes a big difference?

Marga_3 · 21 October 2004 at 1:46PM

Errrr......if I can just interrupt a minute? Can anyone tell me the score with those silly halogen lights? I was recently foolish enough to buy a new light-fitting made up of those small halogen lights (do you know the ones I mean?). Anyway, the type of fitting required for them (bayonet) means the cheapest ones I seem able to buy are £2.00 each, and usually you have to buy them in packs of 6! When switched on, I'm lucky if they last a day or two. However, I have now started leaving them on all day and they seem to survive a bit longer now ( I think the first one of this batch lasted about 2 weeks using this system). Can anyone shed any light on this..........errrrr?

aidyonline · 21 October 2004 at 1:49PM

My word,

I thought i was an angry loner til i read this pag ;D ;De!!!

no-one dare call me a nerd again!!

chevalier · 21 October 2004 at 2:14PM

Hi Marga
If your bulbs are failing that quickly then you may have a problem with your circuit not the bulb. I would suggest that you get an electrician to check.

Marga_3 · 21 October 2004 at 4:00PM

8) :P Ps......LIGHTEN UP EVERYONE!!!....... ;D

Insert from Martin
Quite right Marga - there were a few less 'enlightened' posts in this thread, which i have switched off

nationwide_3 · 21 October 2004 at 6:52PM

Marga,

I had the same problem with those small halogen lights when I had my new kitchen fitted in my last house. The electrician talked me into having that type saying it was the in thing and very cost effective, this proved to false economy.

I ended up searching around for the cheapest deal I could find and found that Screwfix Direct was the cheapest place selling them at the time.

Leaving fluorescent lights switched on at home…

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