Answer Man

June 17, 2014

Answer Man: Does turning off AC during the day save money?

Answer Man

Got questions? You've come to the right place

Q. My roommate and I have had an ongoing disagreement, so we've finally agreed to abide by your answer since you seem to be very smart and research your answers well. Here it is: We set our air conditioner at 74 degrees when we're home. We both work and are gone from the house from roughly 7:15 a.m. to 5:15 p.m. on weekdays. I say it's cheaper to leave the thermostat alone rather than making the air conditioner cool down a hot house in the evening when we get back. She insists on jacking it up to 78 during the day. Which practice will save us more money?

-- R.B., of Belleville

A. I hope this doesn't make you lose your cool, but your roommate has a better grasp of the cold, hard facts when it comes to minimizing your summer power bills. In fact, Michael Bluejay -- who calls himself Mr. Electricity -- bluntly says your roomie is not going far enough.

"There are lots of crazy myths about energy use," he writes on "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 percent (more) energy than turning it on after work and cooling it down to the same temperature."

You have to understand one simple scientific principle to realize why: Heat goes to where it's not. That's why an ice cube lying on a table melts. It's basically absorbing heat and the resulting liquid eventually will wind up at room temperature.

So think of your house as a big ice cube. After the air conditioner kicks off at 74, heat from the outside is going to warm your house, so the air conditioner constantly has to turn on and off all day to keep it at your favorite temperature that nobody's home to enjoy. Ameren will love you but it doesn't do you any good.

If you turn it up to 78, you're going to save some money because the AC won't turn on until the house heats up to 78. Also, as I learned when I answered a similar question about turning down furnaces in winter, there's a smaller difference between 78 and the outside temperature than 74. Studies have shown the smaller the differential, the less work your unit will do.

But experts seem to agree that unless you're going to fry your cats, you should try turning the air conditioner completely off when you leave for an extended period -- and Bluejay has the numbers to back him up.

In his experiment, he picked two days when the outside conditions were similar. On Sept. 18, 2012, he set his window air conditioner to 78 degrees when the outdoor reading hit 86 during the day. In just under 11 hours, he had used 920 watts of energy.

On the following morning, he turned his air conditioner off when the inside temperature was just under 78. After the outdoor temperature reached 87, he came home at 5:30 and switched on the AC again. By 6:30 p.m., he said, it had cooled his apartment back to 78 -- but used only 290 watts, a nearly 70 percent savings over the previous day.

"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," he admits. "But the point is, there's definitely a penalty for running the AC all day."

Don't think Bluejay is some wild-eyed tree-hugger, either. Jennifer Thorne Amann, with the American Council for an Energy-Efficient Economy, concurs with his finding.

"Turn it off when you leave," she flatly told Rodale News. "It may seem like a waste of energy to turn your air conditioner on and off, but doing so actually saves you a fair amount of money."

Air conditioners are more efficient when they run at full tilt than they are when they keep cycling on and off for short periods at a time, she says. They also dehumidify better that way. Ameren Illinois estimates you will use 3 percent to 5 percent more energy for every degree below 78. They recommend setting the thermostat 5 degrees higher when away for an extended period; otherwise, keep the temperature constant.

Other suggestions: Buy a programmable thermostat, add ceiling fans if you don't have them, close the blinds and drapes, make sure your filters are clean, have your units serviced regularly and invest in a modern high-efficiency unit if you don't have one. I've noticed a whale of a difference in my bills the past four years.

So chill out -- but not during the day while you're gone.

Today's trivia

He's 10th on the list of most career strikeouts -- but he admits he failed to fan Tony Gwynn in more than 100 tries. Who is he?

Answer to Tuesday's trivia: Seems hard to believe now, but Michael Jordan was passed over twice in the 1984 National Basketball Association draft. Enjoying first dibs, the Houston Rockets went local by drafting University of Houston junior Hakeem "The Dream" Olajuwon, who did eventually lead the Rockets to back-to-back crowns in 1994 and 1995. But because they had drafted Clyde Drexler the year before, the Portland Trail Blazers opted for 7-foot-1 Sam Bowie, who played 10 seasons, but often was slowed by foot and leg injuries. As a result, his Airness was still available when the Chicago Bulls picked third.

Send your questions to Roger Schlueter, Belleville News-Democrat, 120 S. Illinois St., P.O. Box 427, Belleville, IL 62222-0427 or or call 618-239-2465.

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