View Full Version : OT: flourescent light power consumption

02-07-2005, 12:07 PM
Hey all,

My question has to do with the merits of leaving the flourescent lights on in my workshop, versus shutting them off when I go upstairs for a short time (like to have dinner, or go online for a bit).

I'd been told that it takes more energy to start the lights each time I turn them on than it does to leave them running.

I don't have any clear info on this so my question is: if I work in my basement pretty much all day....with occasional breaks for lunch, dinner, and other short misc periods, am I really helping to save money by turning off the 3 to 5 flourescent lights that are on....or should I just shut them off at the end of the day?

Thanks for any help,


J Tiers
02-07-2005, 12:36 PM
<font face="Verdana, Arial" size="2">Originally posted by ZINOM:

I'd been told that it takes more energy to start the lights each time I turn them on than it does to leave them running.


I'd say THAT isn't true.

But, it stresses the lights more to be turning them on and off, so for a short time, I tend to leave on. if you do turn on and off, they tend to fail faster.

That's especially true of the compact flourescents, but als for standard tube types, particularly if they are electronic ballast type.

02-07-2005, 12:47 PM
The amount of juice that it takes to start three or four four footers isn't that great.

If you were looking at something like HPS or MH lights, you may want to keep them on.

How long would you say that you are turning them off for? Anything more than about five minutes, and it's a definate savings to turn them off. Anything less than five minutes, and I'd actually have to look up some numbers.

Basically, flourescents pull more power during the warmup period...that is, if it takes five minutes to get to full light output, then it's pulling more power during those five minutes to get everything heated up.

High pressure sodium and metal halide lamps are a perfect example...you turn them on and they flicker and slowly get more bright for five or ten minutes.

All of my flourescents are on full within 30 seconds, even on a cold day.

Keep turnin em off.


02-07-2005, 01:51 PM
I would say it depends on how expensive the lamps are balanced against power cost. I use super high output tubes in my shop for a total of around 30,000 lumens. Those tubes are expensive at around $4.00 apiece. Flourecent lamps are life limited more by starts rather than hours of operation. I have 24 four footers in my computer store that are turned on only once per day. I replaced the tubes when I opened my business 7 years ago and none have failed yet.

How much sense it makes also depends on how much you pay for electricity. Here in BC we have the cheapest electricity in North America at 6.8 Cdn cents per Kwatt hour, nearly all of it hydroelectric so it doesn't make sense to turn off the lights unless they won't be needed for hours. The cost of bulb replacement is much higher than a few thousand watt hours of power.

02-07-2005, 02:04 PM
In Nebraska, electricity is about 3.5 cents a kilowatt. More in the summer because of the seasonally higher rates because of higher demand for power and lower in the winter to encourage more use to balance the load. Thanks--Mike.

02-07-2005, 02:17 PM
Where does Nebraska get its electricity from? That sounds like an artificially low price. Must be some politics involved.

02-07-2005, 02:23 PM
Here's a short article that reinforces the "five minue rule":


Barry Milton

02-07-2005, 04:23 PM
.....See, that's why I ask you guys!

Thank you all for taking the time and thank you Barry for posting that link....that helped a lot!


02-07-2005, 04:28 PM
Hmm. I hear that question often. Most incandescents will draw a surge current of up to ten times the running current for the first second or so. Energy consumption during this period starts off at ten times the running current, tapering off right away to the normal level, so the average increase in consumption for that first second is about three times. If you turn the light off for three seconds, then back on, you've probably saved a few tens of percents of what the normal energy use would have been during that time. Fluorescents behave in a similar fashion, though the characteristics are somewhat different. Suffice to say if you turn off lights for more than a few seconds at a time, you'll be saving on your electric bill. But you won't be saving on replacement bulb costs. Which costs more? A 100 w bulb eats 2.4 kilowatt hours per day if on continuously. Lets use 10 cents/kwh for cost, for easy figuring. Thats a quarter a day. Lets also assume that it will last for a year if left on, and a year and a half if switched on and off a lot. For our assumptions, assume it will be off half the time. I'll say the bulb costs $1 to replace. Over a year, if it's on continuously, it will eat $87.60 worth of juice. Over the year and a half, being on only half the time, it will eat $65.70 worth of juice, and cost an extra dollar to replace. Even if you assume a much shorter life when it is turned on and off a lot, it will still be cheaper to run and replace than if you left it on. Fluorescents are the same in general, they will cost a lot less over time if they are turned off when not needed. Add to the cost of light the ballast, which will fail, sometimes before the tubes wear out even once, and the energy savings still swing in favor of turning them off when not needed. But to switch off for only a few minutes when leaving a room doesn't make sense from a purely human standpoint. That would drive me crazy. Flick, flick, flick, flick, filck, flkik, fklic, flikc, flikk

02-07-2005, 09:27 PM

i have to laugh at your "expensive" $4 light comment. i have 4' VHO flourescent tubes on a fishtank i have, and a "cheap" one of them is $20.

damn fish are livin' better than me!

i do agree that it is probably better to turn them off if you won't be in the room for a while. although i have a compact fluorescent light in a room in my basement where the switch is a pain to get to. it has been on continuously for about 4 years, except for a few power outages. we also used to have a fluorescent fixture with two 4' tubes over the sink in our kitchen. i have no idea how old they were when we moved into the house, but we also left that fixture on 24/7 and the bulbs lasted several years before they finally burned out.

andy b.

Don Clement
02-07-2005, 10:16 PM
Those tubes are expensive at around $4.00 apiece.

Would that be two toonies or four greenbacks?

02-07-2005, 10:31 PM
Andy, I'm in kinda the same boat, want to start using the "square mount" compact flourescents, but was shocked at the price - $75 dollars for a lightbulb?! Managed to pull the right size light strip outta somebodies trash, but HomeDepot only sells 2 foot and four foot Aquarium bulbs. http://bbs.homeshopmachinist.net//mad.gif I don't want to pay twenty bucks for a plain jane regular output 3 foot aquarium bulb at the Pet store so I temporarily pirated a bulb out of the big tank which sits unoccupied.


This Old Shed (http://http:thisoldshed.tripod.com)

Rich Carlstedt
02-07-2005, 11:43 PM
For what its worth, I posed this same question years ago (had starters then !) to a Lighting Engineer (degreed) who worked for our local power company.
he said to turn off if more than 20 minites were involved...so for the past 30 years I followed this advice
He based it on fixture life, parts and KWH costs.
But bulbs failed sooner back then I think ??

02-08-2005, 12:21 AM
One reason that our electricity is so cheap is that our power utility is publicly owned. All profits are returned to the customers.

Our electricity is mostly made at coal fired plants and one nuclear plant. We do have a small hydro plant for shaving of peak loads. The larger plants produce cheaper electricity than the hydro plant, but we keep the hydro running anyway because you must have capacity to be able to meet your peak load without brownout. Buying peak- shaving power in the middle of summer in the big demand season is very expensive.

bernie l
02-08-2005, 08:12 AM
Way more than you ever wanted to know about electricity production in Nebraska.

Nebraska is a public power state. There are several power companies but they are regulated by the state and in effect owned by the users. It used to be most towns had there own electric plants (old timers called them the light plant). And quite a few of them were small hydro plants, not many of those left, one at Kingsley Dam, one at Columbus that I know of probably a few more. Most of the smaller towns have since shut there plants down and gone on line with Nebraska Public Power (NPPD, but there is still a lot of local utilities. Omaha has OPPD, Lincoln has LES, Fremont and Grand Island have there own power plants even towns a small as Crete (pop 4,000, just a guess) have there own plant. There's a couple of nucs one at Fort Calhoun and one at Brownsville, and big coal fired plants at Nebraska City, Grand Island, Sutherland and other places I don't know about. Outside of Lincoln LES finished a small peaking plant that uses two gas tubine engines. Several of the utilities also own major shares of power plants in other states, notably Wyoming. I just shake my head anytime suggests how much money the state could make by selling these assests to a private company. Especially when I see how well private companies have served their customers in other markets.

keep your stick on the ice


02-08-2005, 11:45 AM
We have the cheapest electricity in the nation here. TVA power. When the Wolves that want to purchase power for resale finally break it up look out for the prices to go up.

One note, Thou I won't step into the Off/on to save electricity, I would like to comment on something.

Mounting flourescent lights hard against the wooden structure in your home or shop is asking for a fire. Even thou they have a bi-metal heat switch in the ballast any small failure of the "poorly" made cheap transformer ballast can start a fire with the ballast overheating being transferred to the wood.
Seen it a hundred times. Short chain, thou aggravating to wire in is the best method. Isolates the metal case/ballast from direct contact with the wooden structure most our houses and shops are made from. All chain mounted lights need to be grounded. Seems they develop a static charge.

A ballast usually runs at 600 volts, any cute plans for one other than a "dust collector"? you rectify it and charge near strips to cling the dust from the air.

02-08-2005, 04:44 PM
Thanks for that David - I recently wired in a heap of fluoros into my shed hard up on the timber battens, will change these over to chain straight away.

As for the power consumption of the tubes, mine are all rated at 18w each, is that rating at 600v or at line voltage? (240v here in Oz) Either way they don't use a great deal of electricity, you would need to turn them off for a lot of 5 minute periods before you saw any real savings of cash...

02-08-2005, 07:52 PM
I would shut off filament lights as they are far less effecient than modern Flourescent Fixtures - check their Energy Star rating to learn more about them.

Real old flourescent fixtures are not as efffecient - change out the old oil based ballasts for electronic units - the are quiter and start better in cold shops.

J Tiers
02-08-2005, 09:28 PM
<font face="Verdana, Arial" size="2">Originally posted by Thrud:

Real old flourescent fixtures are not as efffecient - change out the old oil based ballasts for electronic units - the are quiter and start better in cold shops.</font>

Those crap faster if turned on and off a lot too.

With the little compact flourescents, at 9W instead of 75, I tend to leave them on more. Takes a lot of them to equal the draw of one hot filament bulb. My whole basement is lit for the price of one 100W bulb that wouldn't light much of it.

I found that if turned on and off a lot, they last a much shorter time though. And they aren't cheap if good.

But, the cheaper they are, the more on and off action shortens their life..........