Should You Turn the AC Off While Away?

In bed one night, I asked Google Gemini whether I should turn the AC off while away or leave it on. It told me leaving it on is more efficient. It didn't ask how long I'd be gone, what the temperature was outside, or what I'd set the AC to while away. It struck me as a wild oversimplification.

I lay there arguing with it. How could me turning it off by hand magically waste more energy than the unit doing the same thing itself?

The AC already turns itself on and off

An air conditioner does not make cold. It moves heat. The compressor squeezes refrigerant so the system can dump indoor heat outdoors, and that pressure work uses a lot of electricity.

What turns the compressor on and off? The setpoint. Say the room is at 74°F and you ask for 68°F. The compressor runs until the room reaches 68°F. In practice it runs a bit past, so it can rest while the room drifts back up instead of rapid-cycling. Engineers call that gap hysteresis. Then the room warms a degree or two and it runs again. A conventional single-stage compressor has no other move: it is either on at full capacity or off. (ENERGY STAR)

So the AC is already turning itself on and off all day. Flipping it off by hand just makes one off stretch longer. What changes is how warm the room gets in the meantime.

The startup surge

I had heard the same advice before, backed by a reason. Turning the AC off and on draws more power than leaving it at a warmer setting, because the compressor pulls a rush of current when it first starts. The rush is real. Startup current can run several times the running current, and it lasts a fraction of a second. (Copeland)

But your bill is energy: power over time. A huge spike for a fraction of a second is nothing next to a compressor running for hours. And the unit already pays that spike every time the thermostat calls for cooling.

So the AC does not remember that it was off and punish you later. What decides the bill is how much heat leaked into the room while you were gone, and what it cost to move that heat back outside.

Why off usually wins

Heat leaks into a cool home faster when the temperature difference is larger. The smaller the gap between indoors and outdoors, the lower the cooling bill. That is why the standard advice says keep the house warmer than normal while you are away. (DOE)

If it is 95°F outside, a 75°F room fights a 20°F difference all day. Once it has drifted up to 85°F, the difference is 10°F, so heat leaks in at roughly half the rate.

This difference is the delta.

Delta is why you wait a few minutes before adding milk if you want your coffee to cool fast. When the coffee is hottest, the delta is highest, so it cools the fastest! Once you add milk, the delta drops, so it cools more slowly.

Yes, the AC runs hard when you come home. That is the part people feel. But the recovery run removes a smaller pile of heat, the one that trickled in while the room was warm. Holding the room cool means fighting the big delta all day long. Modeling across several test cases found the same shape: setbacks used less energy than holding the home cool, with savings that shifted with insulation, equipment, climate, and setback length. (PBS)

What changes with an inverter AC

Inverter and variable-speed systems can slow the compressor down instead of cycling between full power and off. Running at part load can be more efficient, and it helps comfort and humidity control.

This is where I traded more blows with Gemini. Every answer waved COP at me as if the number settled the question. COP is heat moved divided by electrical input. (DOE) A COP of 3 means the unit moves three units of heat for every one unit of electricity it eats. An inverter cruising at low speed can post a high COP, sure. But a COP doesn't matter if your room isn't getting colder.

Low speed is a maintenance gear. It can hold a room that is already cool, skimming off the heat as it leaks in. Ask it to rescue a hot room and it may never get there, because heat can enter faster than a slow compressor removes it. So the unit shifts up toward full capacity, and you're back to full blast.

For an inverter, then, the question turns on time. For a short absence the house barely warms, so turning it off dodges little heat. The efficient hum can win. Across a full workday, holding the room cool means fighting the full delta for hours. The extra heat can outgrow the efficiency edge. Somewhere between the errand and the workday the answer flips, and where it flips depends on your house.

So the honest answer is not “always off” or “always leave it on.” It is a small model:

heat entering = leakiness × delta (outdoor temp - indoor temp)
electricity  = heat removed ÷ efficiency at that load

Then add the human parts: humidity, pets, sleep, noise, time-of-use rates, and whether you need the room comfortable the second you walk in.

The answer I would use

For a normal workday and pure kWh, turning the AC off or setting it much warmer usually beats holding the room cool. That matches the Department of Energy's guidance and the basic heat-transfer story.

For a short errand, a humid home, pets, a hard comfort deadline, or a good inverter system running near its efficient part-load range, a warmer setback can be the better practical choice.

For your own numbers, go back up to the calculator. Change the hours away, the outdoor temp, and the away setpoint. Watch whether the energy you skip while gone beats the recovery run when you get home.