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Is it better to run the AC all day or turn it off when going to work?

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 is hotter.

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 86F, at 7:45am with a starting inside temp of 77.7F I set a window-unit AC to cool to 78F 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 77.5F a high of 87F, 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 78F, and used only 0.29kWh to do so.

You can do the same test yourself.  Here's how:
  1. Pick two days with similar projected temperatures and cloud cover.
  2. On Day A, turn your AC off when you go to work.
  3. When you get home, write down the number on your electric meter.
  4. Turn on the AC.  Don't turn any other big energy hog on, like laundry machines, electric oven, etc.
  5. When it gets down to the desired temperature and shuts off, go check the electric meter again and see how much energy it used.
  6. Also record the time that the AC shut off (e.g., 6:30pm).
  7. On Day B, set your AC to cool all day when you leave.  Make sure everything else in the house is turned off.
  8. Write down the numbers on your electric meter, then go to work.
  9. When you come home, don't turn on any big energy hogs.
  10. At the same time as Day A, check the electric meter to see how much energy you used.
If you run such a test yourself, please let me know the results.

Last update:  July 2013

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All advice is given in good faith. We're not responsible for any errors or omissions. Electricity can kill you; if you're not competent to work on your electrical wiring then hire a professional to do it.
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