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Name: Mary Pat R.
Status: educator
Grade: K-3
Location: MN 
Country: N/A
Date: 1/26/2005


Question:
We are trying to save energy with our fluorescent lighting in class. In the past we were told that it is not energy efficient to flip them on and off, but to leave them on if they will be turned back on in less than 30 minutes. Now we are told to turn them off every time we leave the room, no matter how long it will be before we turn them on again.

So... How do we best save energy with fluorescent lights?


Replies:
Mary,

Without a doubt fluorescent bulbs USE A LOT OF POWER DURING STARTUP (1-2 seconds). But that is about it. After they have been "activated" or fully energized than the electrical power required to maintain the light is VERY VERY LOW. This is something you probably already knew about FLUORESCENT vs. INCANDESCENT BULBS.

To save energy just turn them off while not in the room. I do not know what the magical number is as far as wait time. But I would say that 30 minutes seems a little bit too long. I will tell you that the worst thing you can do to BOTH lower the life of the bulb and WASTE the MOST electricity is to RAPIDLY FLIP THEM OFF AND ON.

Hope this helps.

Darin Wagner


The reason for the 30 minutes specification was never energy savings. Any extra energy used during 2 seconds of turn-on would be compensated by less than 1 minute of being turned off.

The reason is simply that tube life _may_ be shortened if they are turned on very often, something like ten times a day or more. It is a trade-off between electric bills on one hand, and on the other hand the price of new tubes plus the maintenance attention needed to change them. In my opinion, it is a little difficult for most small institutions to actually know their best position in that trade-off. You never know how much real reduction of tube-life is occurring, and the on/off patterns are often unstable, and changing one tube at a time defies scheduling and cost-accounting. Extreme micro-record-keeping might help, but I think it needs to be extraordinarily convenient to use and clearly distinguished from micro-management, which can be obstructive and demoralizing.

Paraphrasing, Nobody really knows the "best" answer yet. It probably varies. If you are willing to ignore maintenance costs and manufacturing energy consumption, yes, your lowest electric bill is definitely to turn the lights off promptly. Perhaps it is a good habit to teach.

It can be valid experiment to try the two strategies you mentioned, if you can track the results of each. Suppose your class got a ladder and changed your own bulbs for a year. And got an "on-time meter" for the lights. Then you could assign a cost to your time changing bulbs and a cost to the presumed wattage the lamps draw. I suppose that is probably an experiment for a higher grade than yours, if only because of the math involved. In about 8th grade, perhaps simply recording on paper each bulb that is seen to fail, and later be replaced, would be enough. Kids that age could probably estimate with you the hours of on-time in your frequent-turn-off practice. I am not sure what subject to put this experiment under. Maybe economics.

Jim Swenson


I had heard a limit less than your 30 minute criteria, but I am no expert so I did a search and got this answer. The answer sounds reasonable, and fits a theory of mine that the focus of most questions is too limited. Here, the answer expands to the question of saving money. The repair of the light. This aims at my pet criteria, time should be our ultimate criteria. What tactics save us the most time.

The energy portion of your question is urban myth. The energy needed to start a fluorescent lamp is not significantly higher than the energy needed to operate it. Even if we assume that a fluorescent lamp uses twice the normal amount of power during the starting phase, this phase lasts for only about 1 or 2 seconds. So, the crossover time to save energy would be less than 2 seconds.

But the situation regarding the true cost of operation, including not only energy costs but also the cost of replacement lamps and the labor to change them is more complex because frequent starting can reduce the life of fluorescent lamps. The crossover time for lowest total cost depends upon your labor costs, what types of fluorescent lamps you are using and, most importantly, what types of ballasts they are operating on. Fluorescent lamps operated on instant start ballasts will have the most significant decrease in lamp life if started frequently - defined as more than one start per three hours of operation. For T8 lamps operating on electronic instant start ballasts the crossover point for turning off lamps to achieve lowest total cost of lighting is about 10 to 20 minutes - though opinions on this issue may differ. If you are using electronic programmed rapid start ballasts, which are far more rare than instant start ballasts, there is virtually no starting damage, so lamps should be shut off if the space is going to be unoccupied for even less than 10 minutes.

James Przewoznik


Turning light bulbs on and off uses essentially no extra electricity. The only disadvantage is that the bulb might wear out faster with many on/off cycles.

Nowadays, light bulbs are inexpensive and reliable, and it is thought best to turn them off when leaving the room.

Bob Erck



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