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Is it better to run the AC all day or turn it off when going
There are lots of crazy myths about energy use, and one of
them is that somehow it takes less energy to run the air
conditioner all day while you're at work, than it does to have it off all day and then turn it on when you get home.
The theory is that the AC has to work "harder" when the house
So let me give it to you straight: Turning the AC off
when you leave definitely takes less energy.
Period. In my own test, having the AC run all day used
317% as much energy as turning it on after work and cooling it
down to the same temperature. My test was a bit crude
and I won't be surprised if the penalty for having your AC run
all day is actually a bit less, but the point is, there's
definitely a penalty for running the AC all day. The
reason is simple: If an AC is constantly cooling your
home, the cooler house is a heat magnet, which invites
more heat into the house, which the AC then has to remove,
over and over.
To see why this is so we need only remember something from
high school physics: heat goes to where it's
not. If you bake a potato and then set it on the table,
what happens? Eventually it cools down to room
temperature. That's because the room was cooler than the
potato, and heat goes to where it's not, so the heat from the
potato transfers to the room. Once the potato gets down
to room temperature, the heat transfer stops.
Next example: Put an ice cube on the table.
What happens? It melts, of course. But why?
It melts because heat goes to where it's not. The room
is warmer than the ice cube, so the heat from the room goes
into the ice cube, which absorbs the heat. That's what
melts it. The ice cube is basically a heat magnet.
Okay, so now think about your house. You leave
for work, and you shut the AC off. It's warmer outside
than inside, so heat enters your home. You come home,
turn on the AC, and the AC removes all the extra heat,
once. Now let's try it the other way: say when you
leave for work you leave the AC on. The AC constantly
removes heat from your home, making it cooler, so you've just
made your house a heat magnet, just like the ice
cube. The cooler it is in the house, the more heat wants
to enter the house. By constantly cooling the house,
you're inviting more heat into the house, which the air
conditioner has to remove, over and over.
[an error occurred while processing this directive] Now,
you might wonder, "So why does my house sometimes get warmer
than it is outside?" One reason is that the energy
from the sun is hitting the house, especially the
roof, which makes the attic super-hot, and that heat migrates
into the house. If heat goes to where it's not, then why
doesn't the heat from the house go outside where it's
cooler? The answer is that it does, but you
don't get heat equilibrium instantaneously. When you put
the ice cube on the table, it doesn't instantly turn to water,
and when you take the potato out of the oven, you have to wait
a bit before it cools to room temperature.
But that's neither here nor there about whether to turn the
AC off when you leave home. It definitely saves
energy to turn the AC off while you're gone.
Here's the test I ran: I picked two days in which
the outside temperature was projected to be similar. On
9/18/12 with an actual high temperature of 86°F, at 7:45am
with a starting inside temp of 77.7°F I set a window-unit AC
to cool to 78°F and put it on the Energy Saver mode, which
means that the fan turns off when it's not cooling. By
6:30pm it had used 0.92 kWh of electricity. The next
day, with a starting inside temp of 77.5°F a high of 87°F, I
turned on the AC at 5:30pm, simulating turning it on when
getting home from work. By 6:30pm it had cooled the room
down to 78°F, and used only 0.29kWh to do so.
You can do the same test yourself. Here's how:
Pick two days with similar projected temperatures and
On Day A, turn your AC off when you go to work.
When you get home, write down the number on your
Turn on the AC. Don't turn any other big energy
hog on, like laundry machines, electric oven, etc.
When it gets down to the desired temperature and shuts
off, go check the electric meter again and see how much
energy it used.
Also record the time that the AC shut off (e.g.,
On Day B, set your AC to cool all day when you
leave. Make sure everything else in the house is
Write down the numbers on your electric meter, then go
When you come home, don't turn on any big energy hogs.
At the same time as Day A, check the electric meter to
see how much energy you used.