Stay cool, stay frugal

By Mir
July 24, 2006

Kim asked a great question which I’m sure a lot of us have wondered about but been too lazy to really investigate. (That includes me. I made Otto look it up.)

Hi pretty girl,

In the midst of all of this insane heat, we have had quite a discussion here at the water cooler. Is it really cheaper to turn your A/C down during the day and have a programmable thermostat bring it back to temperature for your return? Please end this obnoxious debate. Thanks!

As someone who doesn’t have central AC, I guess I’d not thought about this too much. I have window units which I use as little as possible, and so if I’m not home, they’re off. But if you have central AC, what’s going to be the best bang for your buck?

After some searching, Otto found this great FAQ list about air conditioning. I recommend reading the whole thing just because it’s full of interesting and useful information about energy consumption and efficiency and all that good stuff.

I believe the information you need to end the water cooler debate, Kim, is in this bit:

Will I save energy by turning off my air conditioner when I leave home, or am I better off just letting it run?

If gone for four hours or more, more energy will be saved by turning off the air conditioner or turning up the thermostat.

During the day, keep windows shut and close curtains or blinds on any windows that will be exposed to sunlight.

The thermal mass of the house will probably keep the indoor temperature well below the outdoor temperature, and the house should cool quickly when the air conditioner is restarted. Use a programmable thermostat or timer to turn on the air conditioner 30 to 45 minutes before the expected arrival home. If the home is still warm upon arrival, turn on a fan to create air movement.

Moving air can make the air feel about four degrees cooler than it really is.

We also found that Mr. Electricity has discussed air conditioning as well (although, seriously, avert your eyes from his photo, because I was certain he was Weird Al Yankovic at first glance), adding this explanation:

It’s a myth that leaving the AC on while you’re away at work uses less energy than turning it on when you get home. Think about it: Heat constantly penetrates your home, and that’s what your AC removes. If you turn on the AC when you get home, then your AC has to remove the accumulated heat only once. If you leave it on during the day, your AC must repeatedly remove heat that enters your home.

You might think that there’s no difference, because your house should absorb the same amount of heat either way. Not true. With the AC off, at some point your house will be so hot that it can’t absorb any more heat. But with the AC on, your house will always be cool enough to absorb more heat — and you’ll be paying to remove that heat, over and over again.

Bottom line? Turn it off. Set it to cool the house back down before you get home, and enjoy the savings. Use your extra money to shop for deals here at Want Not! (Wow. Sorry about that. It must be a little too warm in here….)


  1. Wow — my service tech told me to leave it at a consistent temp all day. He said that it takes more energy to cool the house (remove all the heat) than it does to keep it cool. Of course, we keep it at a fairly high temp and only use it for hot spells.

  2. My husband is HVAC certified, and he’s always said the same thing Daisy’s tech said. He said the furniture absorbs the heat and that it takes more energy to cool not only the house but all the things in it than it does to just keep it relatively cool all day. My dad keeps his off all day long and only turns it on when he gets home. When I walk in his house after he’s been gone all day, I can hardly breathe! Of course, we live in the south, and we have to keep the a/c on at all times in the summer if we want the temp to stay below 100 degrees.

  3. I work at home, so the AC is on all day. My office is on the second floor, so when it’s cool downstairs, it’s a little too warm upstairs, and when the sun moves around to the front of the house and hits the blacktop of the road and the concrete of our driveway, all the curtains in the world won’t keep it cool in here if the AC is off. As a result, our electric bill has gone up a whopping $25 from last month, even with the recent much warmer temps. I think I can deal with that.

    I can hear the collective gasps right now, “She’s wasting money!” Don’t worry, I make up for it by crabbing at my husband to “just close the damn patio door! We’re not air-conditioning the neighborhood!”

  4. Dude. Mr. Electricity looks like Tiny Tim.

  5. My air conditioner repair guy also told me to leave it on all day. I have an older system that may not last the year (apparently). He said that by varying the temperature by anything more than four degrees I am making the system work too hard to cool and it is likely to keep it from lasting as long. Last summer I had several problems with the air not working. This year I have kept it at a consistent temperature, or gradually lowered it by two degrees at a time, and I haven’t had a problem.

  6. I have a programable thermostat and set it to 87 degrees during the day when nobody’s home and 76 degrees in the evening. My electric bills are definitely lower.

    I used to do energy audits for commercial businesses, and I’ve heard it both ways. I believe that newer units are most efficient running at their peak. When you look at a unit running all day (on and off, on and off) versus a unit that has to catch up in the 30 minutes before you get home, it’s clear to me that you use less electricity in the latter scenario. I think the happy medium is perhaps not letting your home get too hot by not setting your daytime temperature setback to more than 90 degrees.

  7. I can’t help but laugh at the people that state, “but my air conditioning guy said to leave it on.” The last place you want to go for information on efficiency for air conditioners is the man that is paid to repair it when it breaks down. (purely anecdotal of course).

    That being said, everybody’s situation is going to be different based on where they live, but I think ieatcrayonz (and the information Otto found) say it best. You don’t necessarily have to turn it OFF to save money, much like you heating bill in the winter, you can simply turn UP the temperature (if you have a programmable one – and really you should) during the day, it will prevent the house from getting “too hot” but still not draw as much electricity as it would if you kept it constant.

    And of course (for window models, particularly), making sure your air conditioner filter is clean goes a long way to cutting those energy bills down as well.

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