I am a student teacher and tried to switch the classroom lights off/on as a signal for my students to quiet down. They protested, telling me that my doing so wastes much energy. I remember hearing this as a kid, but I figured florescent lights have gotten more efficient over the years. Can you give me a figure on how much does the energy cost to turn off/on the lights once in an average sized elementary classroom, so I can tell these worriers to relax?
General Electric has this to say on the subject:
http://www.gelighting.com/na/faq/faq_eneffic.html
The main issue with turning lights off and on is that it shortens bulb life–although I doubt that two flicks a day is going to halve the life of a bulb.
Each fluorescents uses 40 watts per bulb, so you can count the number of lights in your room (they may be mounted either as pairs or as quadruples) and multiply by 40 to get the wattage. Divide that number by your utility’s pricing (that will be in kilowatts, or thousands of watts per hour) to find out how much it cost to light your room for an hour. This should give a reasonable idea of how little is being spent on the electricity, to begin with.
I would think that a school should be able to find bulbs at around $3.00 each, so IF you shortened each bulb’s life by half (not likely), you can figure that you are wasting $3.00 times the number of bulbs every 7.9 years (i.e. if the school spends $3.00 on a 10,000 hour bulb that runs 7 hours each day for a 180 day school year, it will have to replace each bulb every 7.9 years and you would be forcing them to spend/waste that much again).
General Electric also has an interactive lighting “auditor” that you or your students can use to estimate costs of powering the room at
http://www.gelighting.com/gelauditor/school/
It is almost certain that you are using (for the purpose of the GE audit) T12 lambs of the 48" 40w variety.
oooof
They “would normally have to replace each bulb every 7.9 years and you would be forcing them to spend/waste that much again”.
GE also has a “Learning Unit” on lighting and power that teachers can download, here:
http://www.gelighting.com/na/home/gela/gelaform.html
Tom~
amazing. also a mouthpiece for GE. simply amazing.
Well, go look up Sylvania and see whether they off the same info.
Here you go, Sylvania’s response to the “leave it on–turn it off” question:
http://www.sylvania.com/forum/#8
and their calculation for computing cost:
The GE site glosses the “technical details” for the man-in-the-street.
A florescent bulb uses ballast (they used to tell us 25 years ago) which has to build up a big charge to start the bulb. Once it’s started, that charge is lost – wasting energy. It was said the cost of building up the charge was equal to 20 minutes of normal operation. Whereas the damage to an incandescent bulb filament (there’s very little lost charge involved) is only about 5 minutes worth of normal operation.
Whether any of that’s still true – after all there are so many different kinds of bulbs now – is open to debate. Figuring out which kind of bulbs you have is probably a … uh … waste of energy. Figuring out what the tradeoff is, given the cost of electricity in your area, the current cost of oil, etc., seems like … well … a good SDMB topic, but an indefensible position to take with kids (and of course, by extrapolation, their parents).
In your shoes, confronted with dozens of kids with different opinions, I’d roll with the punch, and not flick the lights.
Pardon for cross-posting tomndebb.
Sylvania is also glossing, and of course, in the process making misstatements:
“Since the cost of the electricity (even in the cheapest markets) is so much more that the cost of replacing lamps a little sooner, you would maximize your savings by turning off the lamps when you don’t need the light.”
Replacing bulbs in certain situations is quite expensive. Sylvania is talking about the cost of an individual walking over to a drawer, getting a light bulb, unscrewing one, and screwing the other back in. In a school or business, bulbs are so expensive to replace that they often change all bulbs at once, rather than pay the cost of calling out the maintenance man for each bulb.
Taking the source in consideration, there’s every reason for Sylvania to stress the cost of electricity, while minimizing the life-shortening damage done to their bulbs.
Even accepting the 20 minutes figure, (and I’m not sure you expend that much energy for a quick “off-on” situation), that means you use the equivalent of 13 watts per light per “off-on”–on a light that I conservatively rated at 10,000 hours but is more likely to have 15,000 to 20,000 hours of life.
13 watts per light bulb times (say) 32 bulbs is 416 watts times (an arbitrarily high) $0.15/kwh gives 6.24 cents per flick plus the added depreciation on the bulbs, themselves. Only New York comes close to 15 cents per kilowatt hour (the average across the U.S. is about half that) so we’re still probably talking 3 1/2 cents a flick provided Ms. Reid is not in New York or some similarly expensive location.
(Of course, if Ms. Reid is in Hawaii, with the higher costs of electricity and the cost of shipping the bulbs to the islands, it could come to 6 cents per flick.)
Ballast units are inductors, when you ‘charge’ one up there is a buld up of magnetic fields which oppose the incoming supply, and when you trun off where do you imagine that energy goes ?
It does not dissappear to be lost in some black hole, the magnetic fields collapse and try to maintain the supply.
It does not take twenty minutes to charge one up, and it does not take the same time to turn them off, we are talking milliseconds here, I seriously doubt that the average meter could accurately monitor such a small current blip.
This is one of those urban legends that is sometimes repeated by EE’s who probably are being mischievous.
They’re playing you. Tell them THEY’RE the ones who are wasting energy by not quieting down.
In fairness to the UL (which I neither support nor deny, since I really have no information on it), the claim was not that it took twenty minutes to charge, but that the surge of power was the equivalent of running the light for 20 minutes. I suspect that even if there was a measurable penalty years ago, that it has since been greatly reduced.
It also would depend on what type of bulbs are being used. There are some ultra-high-effeciency flouresent bulbs that will lose many years off their lives by turning them on and off. The bank I work at tried them out as a cost saving measure since almost all lights in the building are on 24 hours a day. But they tested them by putting them in the bathrooms were people keep turning the lights off when they leave and the bulbs only lasted a couple weeks. When they put the old flouresents back in the bathrooms and put the more expensive high-efficiency ones in the rest of the building everything worked out fine.
Get an air horn instead. Or institute spanking. Or grades for being quiet. Or giving candy to the quiet kids, and watching to nmake sure tohe loud ones don’t take it.
The more one thinks, this seems like a good topic for a Ph.D. thesis. Which goal is most important? Or are they all important?
- Reducing the overall electricity expended.
- Reducing the overall cost.
- Reducing the (paid) human labor.
- Reducing the (unpaid) human labor.
- Reducing the overall pollution.
- Setting a good example for children and the general public, who won’t read the thesis, but will reduce the situation to a one sentence motto.
casdave, the answer about where the ballast charge is presumably that the electricity is dissipated in heat. If you touch them they’re warm. (If you lick one it would be like kissing J . . . oh-oh . . . I’m not going there . . . )
And to give an example that fits in nicely with Swede Hollow’s, not only are there lights that should not be turned off at all, there are lights that should be turned off almost right away because of the short life span: movie lights. (At least, that’s what the video folks told me.)