# Turning Off Lights

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• #249533

> …thoughts on turning lights off and on…you save
>by turning them off when you are out of a room….
>(possibly) not a real saving, as it (might) cost more
>to turn them back on. So, not sure of what is the
>truth…

The TRUTH, as in so many cases, is: It sort of
depends.

Most people in the U.S. pay in the vicinity of \$0.07
per kilowatt hour (that is, using a 1,000 watts of
electricity for one full hour costs a dime or less –
as does using 100 watts for 10 hours or 10 watts for
100 hours).

So, if you leave a 100 watt bulb burning all night (or
day) you’ll find an extra dime on your bill at the end
of the month.
If you turn it off for, say, 15 minutes while you
aren’t in a particular room for a little while, you’ll
save in the order of a quarter of a penny (given that
it takes a dime to run the 100 watt bulb for 10 hours
which is a penny for 1 hour and
15 minutes is one-quarter of an hour).

And you’ll notice that more and more it is becoming
common to find that the lights are off in restrooms
when you first go in,
copy machines are “sleeping” until you wake them up,
and there
are timers or sensors on lights in public places. It

But life is never quite so simple. The typical light
bulb has a low
resistance when it is “cold” (turned off and not
giving off light)
and a higher resistance when it is “hot” (turned on
and illuminating
the room). When you first turn on a filament-style
electric light
there is a surge of current larger than normal through
the cold
bulb until it warms up. So a person would be correct
in saying
that for some short period of time it takes a little
more energy
(which is turning your meter) to warm up the bulb than
it would
to just have left the bulb on in first place.

Opinions vary as to where the break-even point is
between just
leaving the bulb burning versus turning it off and
then back on
but it’s most likely a time measured in seconds…not
minutes.

But, there is another “gotcha” in all this. Turning a
light off, letting
it cool down and then turning it back on again
subjects the bulb, as
we said before, to a momentary surge of current
through the filament
which stresses the bulb both electrically and
earlier failure (I’m sure that you have noted that
many times a light
bulb will fail with a blinding flash of intense
blue-white light just when
you flip the switch on…the cold surge at work and
the filament will
be broken – you can sometimes hear it rattling around
inside – and there
may be dark smudges visible even through the frosting
that indicates
that something violent happened inside the bulb).

So,what does a new bulb cost and how much did we
shorten the
life of the bulb by turning it off and on to save a
little electricty and
will we have to replace the light switch sooner than
we would have
if we weren’t always flipping it off and on…off and
on…off and on…
until it too fails mechanically.

From what I’ve seen a lot of really sharp folks have
put a pencil to the
numbers for one reason or the other and come up with
some
interesting thoughts but it kind of gets down to where
the
assumptions are driving the results.

I really struggle to conserve electricity (and gas and
water and…)
by putting in the compact flourescents and turning off
lights in rooms
that nobody is currently occupying and hardly have a
monthly electric bill
over \$15.00 for about 1200 square feet (2 BR, 2 Bath
apartment) but
I’m not sure that everyone would actually want to live
like that.

For me, the lights go off as I leave a room unless I’m
absolutly sure
that I will be back in the room within a couple of
minutes (not
5 or 15 but 1 or 2). Could I prove that it’s EXACTLY
the cheapest
way to do things beyond a shadow of a doubt? Not very
likely
(and convenience and reduced family stress is worth
something…
just not very much…).

Jay

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