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Does turning on a light use more power than running a light?

J

James

Jan 1, 1970
0
Does turning on a light use more power than running the light?
 
D

Don Klipstein

Jan 1, 1970
0

Although true, I feel the urge to expand on this because of a bit of
truth to the contrary that has been expanded into myths/legends. I hope
I don't cause confusion more than anything by doing this!
In case I need to say a "bottom line" that skips the details, it is
always true that you use more energy keeping an incandescent lamp on than
you use giving the lamp a break or any number of breaks, no matter how
short or how long or anywhere in between.
(There is a special and more significant "submyth" for fluorescent
lamps, and the truth there is that a start causes as much wear as a few to
maybe several minutes of steady operation. However, with most 4-foot
lamps in most areas of the USA electricity cost is so much higher than
cost of replacing the lamps that it can pay to turn them off for only a
minute rather than to keep them on. That is why I have seen occupancy
sensors in some fluorescent-lit restrooms in a few office buildings! The
"break even time" is longer for lower wattages.)

When an incandescent lamp starts up, it draws more power while the
filament is cold than while the filament is hot. It can draw 10-15 times
as much power when cold as when hot, but only for a fraction of a second.
The energy consumed while the filament is warming up is less than that
consumed in 1 second of steady operation.
This surge in power consumption by a cold filament has received enough
notice, sometimes with embellishment in distribution of such notice, that
I feel a need to explain that it both exists and always fails to cause a
lightbulb being turned off and back on to consume more energy than it
would if it was left on.

Even with these "surges", the lamp only consumes as much energy as
it gives off in the form of light and heat. The filament's temperature
(averaged over the length of one in any condition, and throughout a
filament in good condition) does not "overshoot" during a start. So an
interruption in operation can only cause heat output to drop. Light
output certainly drops. Infrared output certainly drops. I have yet to
see on-off operation of an incandescent produce more ultraviolet than
steady operation (normally drops a little more than visible output does
in on-off operation), and ultraviolet is a very small slice of the "pie
chart" of the output of an incandescent lamp no matter what. Output of
X-rays, radio waves, etc. is close enough to zero under all conditions,
including the most abnormally aged lamps, to give zero serious
consideration. And if you turn on and off an incandescent lamp in
whatever way results in the most sound, that's still a fraction of a
milliwatt (1 milliwatt of sound in audible frequencies, which is a wide
roughly 10-octave range, radiated isotropically, is about 82 dB at 1
meter!).
Bottom line: Less energy is coming out of an incandescent
lamp that is getting interruptions of operation than is coming out of
one that is operating continuously.

If anyone wants to do an experiment, then get a stopwatch, a lamp, and
maybe an extension cord and a digital camera. Experimentation is to be
done where you can see (and photograph) the visibly turning dial of your
electric meter. See how much the visibly turning dial turns in a specific
time period (10 seconds or whatever), or determine the amount of time it
takes for the dial to make exactly one full rotation. Do this with the
experimental lamp on, with that lamp off, and while turning the lamp
on/off in any way you suspect could increase your electricity consumption.
Just watch for your refrigerator or any climate control equipment or
anything else turning on or turning off and contaminating your readings.

- Don Klipstein ([email protected], http://www.misty.com/~don/index.html)
 
V

Victor Roberts

Jan 1, 1970
0
Although true, I feel the urge to expand on this because of a bit of
truth to the contrary that has been expanded into myths/legends.

[Excellent discussion snipped]

My simple answer assumed that the time frame of the question was more
than 1 second :)
 
D

Don Klipstein

Jan 1, 1970
0
I (Don Klipstein) wrote said:
(1 milliwatt of sound in audible frequencies, which is a wide
roughly 10-octave range, radiated isotropically, is about 82 dB at 1
meter!).

Yes, I sometimes screw up a bit in ways like this late at night when I
have gone a couple years without doing any work in acoustics. A milliwatt
of sound radiated isotropically in air at "standard sea level conditions",
assuming no echoes adding to the SPL, to the nearest dB, results in 79 dB
rather than 82 dB SPL 1 meter from the sound source. 82 dB at 1 meter
being the figure for 1 mW of sound is the figure for a "half space
environment" which is a common theoretical ideal for loudspeakers.

Oops! :)

- Don Klipstein ([email protected])
 
D

David J. Mann, LC

Jan 1, 1970
0
The inrush to an incandescent lamp will, for a virtually instantaneous
period of time, thus the need for fast-make contacts in switches
intended for switching tungsten loads. Beyond that virtual instant, the
answer to your question is "no".
 
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