Is it Better to Leave the Lights On, or Turn Them Off, When You Leave a Room?

Is it better to turn a light off every time youworks out to far less energy than the usually
leave a room, or leave it on if you'll be comingquoted five or fifteen minutes of leaving the light
back to the room shortly?on.
If you're into energy conservation, or trying toAll right, you say, but won't the light burn out if I
cut your home energy bills, you have probablykeep flicking it on and off?
asked yourself this question. And chances are youYou just have to watch that toddler in action for
have accepted the conventional wisdom, that it isa while to know the answer: I've seen kids wreck
better to leave the light on for short periods, thana light bulb in a matter of minutes with the on-off
turn it off, then on again.trick. The more times you turn a bulb on or off,
In this case, the conventional wisdom is deadthe sooner it burns out.
wrong.But even if each time you turn a light on you
Here is how the argument goes: When you firstshorten its life by an hour - and the figure is
power a light on, it will use as much as five (orprobably far lower than that - you will still save
fifteen) minutes of the regular consumption of theenergy and money if you turn off lights
bulb, within the first second. So if a three-year-oldwhenever you leave a room.
flicks the switch continuously for a minute, on orAgain, consider the lowly incandescent. You can
off every second, they are actually burning 5buy a cheap 100 watt bulb for around 25 cents
minutes worth of electricity every other secondand it lasts about 1,000 hours. They burn 0.1
(30 times in one minute). That works out to 30 xkilowatt hours each hour they are on. If we
5 minutes, or 150 minutes, worth of electricity inassume we burn a bulb out in 1,000 on-off cycles,
that one minute.and electricity costs us 10 cents a kilowatt hour,
It's not hard to demonstrate that this is nonsense.that means it costs us 1 cent to run the bulb for
Suppose the kid is toggling a 100 watt light. Overone hour (100 watts = 0.1 kilowatt, X 10 cents =
the course of sixty seconds, if we accept that1 cent).
switching on the light on uses the equivalent ofSo, each time the light gets switched off (which
what the light normally uses in five minutes, weentails switching it back on later) you are spending
have used 100 watts times 150 minutes.a thousandth of the 25 cents you spent on the
Now, 150 minutes worth of electricity at 100bulb, or one twentieth of a cent (a mere $0.0005!)
watts is the same amount of power as 1 minuteAnd every time you turn a bulb off for five
of electricity at 15,000 watts. And since the lightminutes you are saving 5/60 of the $0.01 it costs
was turned on and off over the course of oneto run the bulb for an hour, or 0.08 of a cent.
minute, it means that if our assumption about theSo switching the light off for five minutes cuts
size of the initial power surge is correct, duringyour electricity costs by more than three times
that one minute the light bulb behaved as if itthe extra you'll be spending on shortened bulb life.
were burning 15,000 watts continuously.And remember, we assumed that each flick of
Remember your high school physics class, wherethe switch uses an hour of the bulb's life, but it's
you learned the rule: Watts = Amps X Volts? Inprobably far less than that - we just chose an
this case, we know both the Volts and the Wattshour to prove the point.
so we can fill in:There is one other flaw with the leave-the-light-on
15,000 watts = Amps X 110 voltsconventional wisdom: it fails to take into account
(Let's suppose the mischievous kid lives in Canada,what happens when we get distracted.
where power in homes is normally 110v). To solveYou leave the room for a few minutes to put
for Amps, we divide both sides by 110v, whichsomething away, but you leave the light on as
yields:you plan to return shortly. But a neighbor at the
15,000 watts / 110 volts = Ampsdoor, a friend on the phone, or some other
Which means that the light was drawing 136distraction, keeps you away from the room
amps of power.where you left the light on - and half an hour or
Now I don't know about your house, but mine ismore, you remember that light left on. Even
certainly not going to be able to handle a 136 ampworse, if the light was in a room you don't visit
current on one light for a whole minute, since theoften - the basement work room or that empty
whole house has a power supply of just 100third bedroom, you might not discover the light
amps. And my circuit breakers are all 15 or 30has been left on until several days later. Forgetting
amp breakers - which means they trip off whento turn a light off in one case like that can eat up
the power surges to much more than their ratedway more money and energy than shortening the
amperage of 15 or 30 amps. So that toddlerbulb's life by an hour.
turning the light on every other second for aSo make it your philosophy to turn off lights. Not
minute, yielding a 136 amp draw, would blow theonly will you save electricity when you turn off
circuit breaker for the circuit the light is on, andlights, and save money overall, but it will remind
possibly blow the main circuit breaker for theyou to be an energy saver in other ways. And
house.you will be setting a visible example to others,
So what's the scoop? Yes, there is a powerwho will become more conservation conscious as
surge when a light is turned on. But that surgewell.
lasts only a tiny fraction of a second, and it