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Efficiency of Fluorescent Lights Revisited
Name: Suart
Status: Student
Grade: 9-12
Location: MA
Country: United States
Date: July 2006
Question:
Is it true that it is better to
leave a fluorescent light on rather than turning it
on or off over a short period of time? If this is
true, does the rule hold true for all kinds of
fluorescent lights? What is the threshold point in
terms of time for leaving fluorescent lights on or
turning them off? Thanks for answering these
questions.
Replies:
Hi Stuart,
The practical answer is, no, it makes no sense to leave a
fluorescent light (of ANY type) on instead of turning it off.
This is an story that seems to have started over 50 years ago
when someone noticed that the starting current for the first
second or less, of a fluorescent lamp, was higher than its
current draw after that time.
Starting current of a typical fluorescent lamp is in the
neighborhood of 50% more than its normal operating current.
But as I stated above, this only lasts for around a second or
so. Even much less with many types of fluorescent lamps.
Let us take a standard 40 Watt fluorescent tube as an example.
When you turn it on, it draws perhaps 60 Watts for the first
second, then 40 Watts from then on. So, for the first second,
it is using 20 Watts more than normal.
Now, let us say you are walking out of the room for just 10
minutes, and you decide to leave the lamp on, because someone
claimed it would use less energy than if you turned it off,
then on again 10 minutes later. That lamp will be wasting 40
Watts of electrical energy for 10 minutes, before you return
to the room.
But if you switched it off, it would consume no power at all
for that 10 minutes, then when switched back on, it would
consume an extra 20 Watts or so for a mere second or less. So
in this example, leaving it on would use "waste" 40 Watts for
10 minutes. Switching it off would "waste" 20 Watts for a
second or less. Clearly, that old story about leaving
fluorescent lamps running to save power is utter nonsense!
Regards,
Bob Wilson
Stuart-
I really do not know; it depends on details.
I bet different lights differ in that regard.
I suspect the 8-foot-long instant-on fluorescents having only one
pin at each end
are better for multiple restarts,
than the usual type with two pins on each end.
But I have not really investigated whether they, too have some little
curly-cue pigtail of filament in each end, which might erode.
The filaments enable starting and make the voltage drop from
metal-to-gas smaller even after warm-up,
and they are the things which age worst usually.
If you investigate those innards on the Internet and by breaking old bulbs
(careful of broken glass, wear gloves and dust mask, wash hands,
phosphors can be toxic and there is a bit of mercury too.)
then you will actually be learning something and teaching others.
My idea of a fluorescent tube would have no filaments,
instead have sturdy metal hollow-cathode cups on each end with
magnets to enhance them,
and a little radioactive gas inside or a little Tesla coil on the
outside to help it start.
I bet this bulb would not wear out until metal atoms sputtered off the cathode
finally buried all the mercury vapor the tube came charged with.
Hard to stop that process. Maybe the "metal cup" would only be
outside the glass,
and the AC voltage would be 100KHz to go through the glass by capacitance.
Sputtering happens very gradually during hours of use.
So this kind of tube, if you could find and afford it,
would definitely be worth turning off as often as possible,
both to save power and extend life.
But like I said, they do not make it yet.
I lean towards just turning the thing off, if the switch is convenient.
The power savings you know exists, the replacement penalty you do not.
The power penalty of start-up is almost non-existent, only
equivalent to 1-10 seconds of on-time.
and often the light's intensity and color are not quite up to par
for the first minute or so of warm-up,
but usually that does not stop you from using the room.
Jim Swenson
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Update: June 2012
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