Power Consumption of Fluorescent Tube Lights

A fairly convincing hand-waving argument is:

40W lamp (say) for ¹/₂ hr. How much current would the lamp have to draw to use the same power in the second or two it takes to start up? What is the current rating of the lighting circuit?

I'm convinced!

Thank you both for your answer, I will print this out and blame you guys.....not really.

Much appreciated and take care.

Case491

Should've said energy instead of power - I was a bit pushed when I posted it.

Anyway, the sums go:

40W x 1800s = 72000Ws

Say, 2 seconds start-up time, that's 36000W (i.e. about 156A @ 230V (resistive load)).

There is more to it than just power consumption...switching anything on and off all the time will shorten it's life. Any of you have had a TV (or PC power supply) fail ? I bet it failed at switch on... ?

Failure due to switching on and off depends on the quality of the switch from the manufacturer. We have an old tv (black and white) which has been switched on and off from 1983 (yes 1983...) and it has never failed. Save energy and keep those lights off. That is what we advise our fellow South Africans to do (our reserve margins are constrained)

Nothing ever fails to turn off.

My diesel car did when the fuel cut-off solenoid failed!

You should have just pulled the leads off the spark plugs??????????

I don't know about power consumption but here is a bulb that has 100 years of life on it.

Truly amazing!

It is amazing despite that it might be running at only a fraction of the load that modern equipment is manufactured to. Old fashioned way of manufacturing was way over the top.

If a modern light bulb is run at 25% of its rated load, it will last more than 4 times as long as when you would get if you did not. Still impossible to get 100 years. Good endorsement for the company that made it.

My uncle had one of those bulbs years ago (back in the 1940's). He used it in the closet, so it didnt get much continuous use, but was switched on/off with daily frequency. Being young at the time, I was surprised to see this bulb in use, since it looked odd for the times even then. For some reason, I always wanted to obtain the bulb for my own use, but when he died in the late 1970's the aunt cleaned out the closet, removed the bulb and pitched it out. I dont know why (probably because it didnt give much light). I lose more museum pieces that way.

Actually its much better then that. If it is run at 75% of its rated load it will last 4 times longer, at 25% of its load it will last longer then 20 times its rated load and save 60%. Assuming we are talking about incandescent bulbs here.

Sure it cannot be linear the way I thought. Thanks for pointing it out.

Here's a better question: how long is a short piece of string?

In terms of raw energy, turning a lamp off (fluorescent or not) consumes less energy than leaving it on. There is a surge when first switched, due to the relatively lower resistance of cold filaments, unsaturated inductors and uncharged capacitors. The gases in fluroescent tubes also operate less efficiently when cold, so for a given light output the power consumed is relatively higher during warm-up. So the answer to your question is no, they will cost more to run all the time.

However energy consumption is only one part of the cost of running lamps. The heating/cooling cycle causes thermal shock on the electrodes in the lamp, and to a lesser extent to the wiring in the control gear of a fluorescent tube. (Ordinary lamps, referred to as general lighting service or GLS, have no control gear). Add to that tarnishing of reflectors, yellowing of diffusers and the degradation of seals and connectors, all lead to a maintenance cost over a period of years. Depending on it's location and servicability, the replacement cost of a lamp can exceed it's energy consumption cost for many years. As the switching cycle can drastically reduce the lifetime of (especially) fluroescent lamps (which is why traffic lights use GLS lamps), the maintenance costs can far exceed the energy consumption. In these instances, leaving the lamp burning will consume more energy, but probably save money on maintenance.

For example, it is interesting that traffic light lamps each last a similar amount of time, even though the "green" lamp burns much longer. The amber and red lamps are on much less, but switched more often. Of course, the use of LEDs has turned this on it's head.

Modern technology now exists to dim fluorescent lamps smoothly and efficiently. As such, it is now possible to get the best of both worlds - consume less energy and yet extend lamp life as well.

Makes a lot more sense than building new power stations and wind farms.

A short piece of string is twice as long as half its length.

All good answers, but the real reason is:

Does anyone remember back when an 8' T12 single pin fluorescent lamp cost 6-7 usd , and the 4' bi-pins cost 3-4 bucks each?

also,

Do we all remember what the POWER costs were back then?

We are talking like 1-2 cents per kilowatt hour..

so,

this was actually a good economic decision to leave the fluorescents burning as it was far cheaper to pay the power bill, than to replace those expensive fluorescent lamps.

Remember, people were used to paying for the cheap, (even back then), incandescent light bulbs, and also used to the cheap Power bills..

I actually made a really good living 40 years ago by recycling the still good, used fluorescent lamps when they started the "Group Re-lamping" of the large buildings.

I would travel

from Hart Michigan down to Chicago, or Windsor/Detroit, and buy those take out lamps from the group relampers for .05 for 4' and .10 for the 8'.

Then bring the big ole U-Haul truck back to Hart, Mi. and unload the boxes in my freezing cold garage..

Then when the weather was a little better, go out and sort out all the bad ones,

clean and re-pack the good ones,

(20 lamps in a case meant for 12, and that worked well until I hit a bump in the road,

and 4-5 cases all exploded at one time)..:o(

Then, I would pedal those to small towns, and small business's, for .50 for 4' and 1.00 each for the 8'.

Given the cost of those new lamps, the people thought they were Hot..

I quickly informed them that they were not, they were cool white lamps. hehe

believe it Or Not,

I could easily make 100 bucks a day pedalling those, and both parties got a good deal.

I often think of that.. and how that helped us survive the winters in Michigan, when we had no other work.

40 years later, I'm still pedalling light bulbs, only now they are CCFL, LED, and electrodeless

Donald

Hi case, Watthour cost is not the sole consideration in fluorescent economics. They will tolerate a finite number of turn ons before requiring replacement and the replacement cost can be a substantial portion of their expense, especially if it takes more than a ladder to reach them. If you are using them a couple of hours a day it is more economical for them to be off when not in use. Conversely, if they are in use over half of the day the economics depends on the cost of replacement.

I feel that all the technical answers have been well put here and that the original poster should be able to hit some middle of the road point that is acceptable to both parties.

If you are using the room very often, say every 10 minutes or so or less, then I would personally leave the lights on till you go home. But if there are many times when nobody is in there and it could go to an hour or more, then switch them off.

What I do like for such rooms, is a motion detector with say a 5 minute on time after the last movement.....then nobody must remember to switch off and if your hands are full, you do not need to switch on either when entering the room.....but I am basically lazy!!!

We once had a room like that at work and we all hated it as it sometimes happened you were in there for some time after you had found the archives you needed, and suddenly the damn lights went off and you had to get up and walk about a bit for them to come on again crap.

That only applies when either the unit was insensitive (often a pot on the unit), wrongly set up (tining) or pointing the wrong way......all are fixable with quality units!

I believe myth busters had this on a while back and they found that the savings of turning the lights off (flouresent) was negligible and not realy worth the effort.

I believe the test ran over a considerable time period if I remember correctly. (a month or more).

oilcan13

I think you got that backwards. The only reason to not turn the lights off is that the amount of time the lights are off, where you are saving electrical energy, is insufficient to compensate for the shortened life expectancy of the tube caused by the start-up surge. The momentary additional surge of electricity during start-up has an insignificant effect on the electric bill as it only lasts for a second or two. This does not significantly affect kWh or kW. (kW charges are not instantaneous but averaged over 15 or 30 minutes) Somewhere between 5 to 15 minutes minimum off time is the break-even point where it is more cost effective to shut the light off than let it run. The varying time parameter takes into account varying costs of the replacement (i.e. a business has to pay someone to do it, a home owner may have to make a special trip to pick up a new tube).

I think you may be talking about a kVA meter here. kW meters are indeed instantaneous, and measure continuously. kVA meters are averaged.

But I'm much more interested in lightbulb jokes. A selection of my favourites are:

How many Irishmen does it take to screw in a lightbulb?

Twentyone. One to hold the bulb, and twenty to drink until the room spins.

How many gay rights activists does it take to change a lightbulb?

Why does the bulb have to change, why can't society accept it as it is?

How many American presidents does it take to change a lightbulb?

They don't, they just keep screwing around.

How many surrealists does it take to change a lightbulb?

To get to the other side.

How many plumbers does it take to change a lightbulb?

Two - one to change the bulb and one to change the boiler, which was just about to go so it was lucky they were called out when they were.

What about the energy expended to turn the switch on/off. I havent seen anyone address that issue.

I seem to remember a study that was conducted by The US Navy Naval Facilities Engineering Command around 1979 or 1980 that looked at exactly this question. At that time the answer was a "30 minute rule" If the room was to be emptier for more than 30 minutes then you would turn the lights off. If the room would be empty for less than 30 minutes, leave them on. Of course that was based on the economic conditions at that time. The actual time limits now would be different. Maybe they have an updated study for todays conditions...

"Incandescent bulbs have a continuous path through the filament, and therefore do not require the starting surge"

There is, in fact, a current surge when turning on a filament lamp, due to the lower resistance of the filament when cold.

For example, I have just measured the cold resistance of a 60W 230V filament lamp at about 75Ω. If this resistance was maintained, the lamp would draw about 230/75≈ 3A_rms, the power would be about 230 x 3 ≈ 700W. The actual running current rapidly falls to 60/230 ≈ 260mA.

This coincides exactly with my feelings on the subject.

I will be giving you a "Good answer" rating for your post!!

I think you've hit the nail on the head. To the best of my recollection, it has always been about: is it cheaper to keep the lamps lit and reduce wear and tear on the ballast and tubes and pay the power bill; or turn them off, save power, and pay to replace tubes and ballasts.

That's why we all need to consider the new style CCFL, LED, and the Really New, Low Power electrodeless...

Flashing doesn't affect these in the least, plus they use far less power than Incandescent, or even CFL..

Donald

Did anyone here ever hear that a bulb is brightest the instant before it burns out?

Does this apply to fluorescents as well?

As the tungstem filament in an incandescent bulb thins through evaporation, the current density in the thinned part increases. The increasing current density causes the thinned wire to heat more and more until the wire becomes so thin that the tungsten overheats and "burns out", or instantly evaporates with a surge of heat and light. It is the same thing that happens when an old-fashioned disposable flash-photography bulb flashes. I don't see how it could be applied to a fluorescent bulb.

Bill Morrow

My experiences tend to show that Fluorescent tubes dim with age.....just like people......DUUUHHH.......

Fluorescent lamps dim with age. Right after you retube, the space will never be brighter.

Incandescents (except halogens) dim with age also, due to the buildup of tungsten dust on the inside of the bulb. Although they might fail with a flash.

Also... most HID (high intensity discharge) lights have a minimum off recycle time and take some time after starting to get up to full intensity. Turn the metal halide fixtures off in a gym and it may be 10 to 15 minutes before you can have full light levels back.

No, it does not. The fluorescent lamp gradually loses light due to the emission coating gradually breaking down, and clinging to the inside wall, causing that black spot, and eventual total black section, if left to burn completely out.

On these new Cold Cathode Fluorescent Lamps we are selling thousands and thousands of, they are lasting soooooo long, we have No idea how they will fail.. But maybe after 20, to 30,000 hours of burning, we will tell you..

I have used them for years and they go just like the old ones. A slight flash and a "ping" and that is the end of the bulb, nothing fancy, just darkness.

Oilcan mentioned the Mythbuster episode, and they said that if the fluorescents are going to be off for less than 23 seconds leave them on. Incandescents were only a couple seconds.

Chazl

You are right.--It takes in no energy to Switch OFF,ON. And None during OFF period

He is wrong.On all 3 counts!

There is no doubt that you should switch off the light when you leave the room since every minute you burn the lamp with out necessity is going to consume energy with out any useful purpose. The loss of energy cannot be compared with the very minimal power loss due to switching ' on' and 'off ' the lamp frequently. Considering the gap of electrical energy production and consumption in todays energy management ,saving of each KWH will serve a purpose.The acute shortage of electrical energy production and fast growth of consumption make the situation still grave specially in the developing nations. Even the trickle of energy saved by each individual is considered vital for future generation.Electrical engineers should propagate awareness and importance of energy conservation.In fact large industrial units now install automatic switching 'off'' lights depending on requirement basis and need based luminous intensity at work spot.This step will play a great deal in saving energy. The limited stock of energy we have to preserve for our future generation. So tell your friend that his myth is based on his childhood fear of darkness or his naughty habit of switching ' on ' and switching off lights very often and the scolding he might have received from his irritated parents

If you read the other posts on this subject, you will see the need to factor in the cost of a replacement (switching off and on definitely reduces the life span of all lights), getting the replacement and fitting the replacement and in some cases (CFLs for example) the need to make sure that the defective light is correctly and environmentally disposed of as a lot of CFLs contain heavy metals such as Mercury......not forgetting the inconvenience of the light being out (and maybe the danger of a dark passage!) till someone replaces it! That last point is difficult to factor as a cost!!! But if you break your leg, lose your job or even die......

The equation is nowhere as simple as you appear to believe....

The original subject matter is weather more energy is consumed by switching off the tube or keeping them on. The simple energy saving solution is switching off the light consumes less energy. The cost of replacement is a price we have pay for energy conservation. The energy conservation is a universal requirement and many Governments and environmental agencies around the world are working hard to educate the public about the importance of energy conservation. May be few countries or some locality may have abundance of energy supply at cheap rates at their disposal.They cannot understand the subject so easily .Hope they will also try to understand the universal shortage of power generation and propagate energy conservation proposals.

Dear Andy Germany Please read this link. This will give some idea how developing countries are working hard to improve the power situation.

http://powermin.nic.in/acts_notification/energy_conservation_act/introduction.htm

thanks and regards

just while we are on that suject...................sort of ..................mercury

I found it interesting. But my point was that you have to factor in all of the cost points and the CO2 footprint of not only the energy to provide the light, but also to go and get the replacement etc!! maybe I did not make it clear enough, sorry for that if true....

This is easily forgotten in the euphoria of "saving money no matter what it costs!" attitude.....(That is an old German saying translated into English for the edification of all who read these pages!!)

One MUST look at the whole picture, not just one corner.......electric cars are a good example of such misguided savings of energy and the carbon footprint, another is cars driven by compressed air!!! Even Lead Acid batteries are only about 70% efficient. All these points are quickly forgotten in this bid to "SAVE ENERGY!"

Andy, just on the subject of CFLs, on one of the TV news programs this evening it was suggested that if one of these light bulbs are accidentally broken inside the house it is to everyones benefit to move out of the house and ventilate for 15 minutes.

Have you heard of this before? .........down under in Australia we tend not to be up to date with too many things.........just curious.

That will not get rid of the Mercury........!!! But at least the Mercury vapor will be gone!!

One should simply be more careful of any Fluorescent lighting and not to break it.....

In some countries it will cost you a lot of money to "clean up" the house again!!!!

I am not an expert on Mercury poisoning or the results of breaking the tubes, sorry, except to say it is a known health hazard.

I believe it was the origin of the name "Blue Bloods" as there was at some point in the past, a treatment for Venereal Diseases using Mercury I believe that colored the blood blue and as it was very expensive, only people with money could afford it!

(That was from a long ago memory, my apologies if the memory was not as accurate as it should have been!!)

It's been all over the news for the last 2 or 3 days in the UK - widespread panic about 1) what to do with dead CFLs, and 2) how soon you need to put on the breathing apparatus if one breaks .

It seems dead ones have to be taken to a Waste Recycling Centre (AKA town dump).

I heard more-or-less the same advice as you re. broken lamps - plus wearing gloves to clean up, and sweeping up (not vacuuming) the debris.

Can't get too worked up about it myself, having spent many happy hours aged 4 (laid up with a broken leg) playing with a dish of mercury my Grandfather had lying about. I've survived 50+ years from then, and still going on OK, DV.

Same here and tell you something else as well.

I went to the down dump, as you so eloquently put it, to dispose of some paint cans that were part empty. I thought, being the responsible citizen I am, that was the right thing to do but NOOOOOOOO, how wrong I was.

They told me they cannot take paint, they take oil and all sorts of other chemicals they cannot pronounce the name of but no paint.

So I asked them what should I do with this paint and the answer is going to knock you of your chair so hold tight, they said that I should pour the tin into newspapers and roll them up in my normal rubbish and put them out for collection. They will take the old tins in the metal recycling so that is ok then???????

I still cannot believe that I heard it right.

In such cases I put a wick in an burn it off......stupidity is catching, I caught it!!

stupidity is catching, I caught it

We already knew that, why tell us

I remember playing mercury in the chemistry lab at school, I even had some at home in my chemistry set, it's a long way back but I think you could buy it at the local pharmacy. When I had finished with it I used to pour it down the sink.

That must still be there then, in the lowest point of the swan neck or stench trap!

Never even crossed my mind.......you're probably right. Mind you nothing much crosses my mind nowadays.

How does this relate to the new "spiral" type florescents - the type to replace incandesents?

When switched on, they are dimmer for a minute or so before becoming completely warmed up and brightest.

My question is this:

If I enter a room and switch on the light to be able to see just long enough to retreive something and then shut off the light as I exit the room, will this (over a period of time with the same practice) shorten the life of what they claim to be long lasting bulbs?

In other words - I'm not sure if these spiral bulbs would be the correct bulb to use in a closet...

My wife wants to switch all bulbs over to the new fangled ones and I have tried to convince her that the closets won't matter.

And in hindsite, could actually harm the bulb since it is switched off before fully illuminated. (I'm cheap and the bulbs are expensive).

Hi case491,

I have a fluorescent fixture in a closet with a power cord and two 40 watt lamps. I recently measured it with an ammeter after a breaker blew. It drew the same amperage as a 100 watt incandescent, and I saw no noticeable surge when it was turned on. Conclusion: turn it off when not in use.

S

You would not see the surge on a meter, probably even difficult to see it on anything other than a storage oscilloscope!!! The meter would be far too slow to react!

If the power usage was the same as a 100w incandescent light bulb, I suspect that it was an old fashioned design that is seriously ineffecient.....a modern design will generally use only about 20% of the power for the same light output.....

There again, what is the comparison between 2 x 40 watt tubes and a 100w incandescent with regards to light output? I personally cannot answer that exactly.....maybe you have more light output and a similar energy usage?

It drew the same amperage as a 100 watt incandescent

Let me get this straight? You now have 80 Watt but it draws current like the old 100Watt?

If I were you I would go back the the old incandescent but change the bulb to an 80 Watt so you save yourself the cost of 20 Watt

Just my stupid way of looking at it I suppose but you have been done my friend.

"Let me get this straight? You now have 80 Watt but it draws current like the old 100Watt?"

No, it is supposed to be 80, but it's 100. Why, I don't know. There is no apparent heat in the ballast, and no buzzing. The question, as someone said, is how the light output compares. I have no light meter, but I think the fluorescents have more output. I like the color (they are Philips cool white), and the fact that they are not the pinpoint light sources the incandescents are.

S

(they are Philips cool white),

I wouldn't know for sure, but one would suspect that the fluors were made in China, maybe this could be the problem. I noticed this afternoon while shopping that AWA and Philips transistor radios, DVD players, sound systems, etc., are all made in China.

I thought brand name electrical/electronic gear was something you looked for when purchasing these products?????? Maybe I am wrong. One can but hope.

Are you saying that Philips is not a brand name? What about GE and Sylvania? I look for the most light output at the lowest price. EVERYTHING is made in China.

Am I?............Who knows nowadays with, as you said "EVERYTHING" being made in China..................do brand names still have that connotation OR has it been lost?????

You do have a point I have to agree, which is why I do not bother so much with the brand name anymore, it all comes from the same place, only the price is higher with a "Brand Name" on it!!!!

Don't forget that the ballast uses power too. This could be where your extra 20 watts is coming from. That would only be 0.17 amps at 120 volts.

Case, another factor to consider is in industrial and commercial settings the heat from the fixtures become a significant load on the HVAC systems. Except in the winter of course ;)

So does anybody know -

if I continuously turn on / off the florescents before fully illuminated, will that prematurely ruin the bulb?

For clarification (I guess), See previous post...

Yes.

Thanx!

yes frequent on/off will adversely affect ALL the light sources mentioned- but why not go for white LED lights- the future is here NOW!.

I agree...

and when they get those 4' LED tubes that will work in the fluorescent fixtures, price down enough, I'm sure Wal Mart will use them instead of the t-8s

donald

I will depend on duration of time the flourescent light will be "on" and not needed than instead of "off". My answer is case to case basis. I think that if you need to turn off the light and not use it for a period of time, i suggest it would be better to be "off". and if you think that it will only take a little time to turn it "on " again, I suggest to let it still "on". Dont waste Energy, conserve it.

Just a thought... Obviously, whether you turn your lights on, and leave them on, would depend on the wattage you are consuming, in relation to the cost of the replacement lamp or bulb..

Now if you are using say 150 watt incandescent bulbs, your big cost is your power bill, with the bulbs being minimal by comparison..

If you are using an energy saving alternative, like ccfl or CFL, then the energy, plus the lamp replacement cost should both be considered...

Just a thought, and I cannot wait until I get all the feedback from my major Lamp manufacturers, so that I can share the latest technology with all you brilliant people..

Don't you just love this site/, I know I do..

donald