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I was recently recommended not to turn off the airconditioner (A/C) when leaving a room for two hours or less but instead to increase the temperature setting closer to the outside temperature. The explanation was that energy use of the A/C unit is very high when initially turning the unit on. However, I think when initially turning on the A/C, then of course the A/C has to work a lot harder to cool the room down. Therefore the explanation is not very satisfying.

  • It seems that if the A/C is kept running and the room is kept (even slightly) cooler for the two hours, then more heat will flow into the room in total.
  • If the total heat flow into the room is greater, then more heat has to be relocated in total if the A/C is kept running than if it is switched off for two hours.
  • If more heat has to be relocated if the A/C is kept running, then the only way that this costs less energy is that there is a difference in efficiency.

Thus my question could be rephrased at: is a heat pump less efficient in relocating heat when initially turned on?

In case it depends on the type of A/C: in this case it is a modern (2022) heat pump that cools only a single room.

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I was recently recommended not to turn off the airconditioner (A/C) when leaving a room for two hours or less but instead to increase the temperature setting closer to the outside temperature.

Entirely bogus recommendation for most units. The compressor is an on/off compressor on most units.

Where this could theoretically be true is if you have an inverter air conditioner that's sized such that it's working close to the full load. Then if you entirely turn it off, it may work harder when turning it back on, but if you increase the temperature setting closer to the outside temperature, it could theoretically run at a lower power level when cooling down the room, which theoretically could be more efficient. However, whether or not this happens depends on the exact details of the inverter AC. For example if outdoor temperature is 28C, and you cool your room to 22C, and set it to 25C when away, and the room is at this temperature, setting it back down to 22C might already switch to the highest power level. What may help is turning it gradually down: first from 25C to 24C, then to 23C, then to 22C. Also the energy efficiency savings from this are expected to be minimal, and if fact you would save the most money if you just turn the AC off, then when you arrive back, set it first to 27C, then to 26C, then to 25C, then to 24C, then to 23C and finally to 22C.

The explanation was that energy use of the A/C unit is very high when initially turning the unit on.

Indeed, it is very high.

In fact, so high that every user of inverter or a generator knows that you can't run an AC unit consuming 900 watts with a generator rated for 1000 watts.

However, this initial high energy consumption happens, maybe for few seconds. Not an issue when considering your electricity bill or environmental impact. Very big issue when considering fuse size, suitability of wiring, or suitability of a particular generator or inverter for running the AC unit.

An inverter AC unit will likely be different: they will likely have a soft-start feature that gradually increases the speed of the compressor from zero to the rated speed. But for units having a simple induction motor and no advanced drive electronics, this can't be done.

Also inverter AC units could be less efficient when working at full load and more efficient at partial load, but note most AC units are not inverter drive. Also note that when the AC unit is cooling down your house, not maintaining a set temperature, it's highly likely it anyway runs at the max power level, unless the temperature setpoint is only slightly below the current temperature.

It seems that if the A/C is kept running and the room is kept (even slightly) cooler for the two hours, then more heat will flow into the room in total.

This is true. More heat will flow into the room, and the AC unit will have to work harder to extract that heat. If you think about it, the most energy efficient temperature is the equilibrium temperature, because it has zero energy consumption.

If the total heat flow into the room is greater, then more heat has to be relocated in total if the A/C is kept running than if it is switched off for two hours.

Entirely true.

If more heat has to be relocated if the A/C is kept running, then the only way that this costs less energy is that there is a difference in efficiency.

There is a difference in efficiency: the cooler the indoor temperature and the hotter the outdoor temperature is, the lower the efficiency. You can't adjust the outdoor temperature but you definitely can adjust the indoor temperature. If you leave the room at a warmer temperature, the AC will have less trouble moving heat away from the room.

So the efficiency of AC unit is better when it's hot indoors. Not worse.

However, inverter AC units could be more efficient at part load, so definitely check whether you have an inverter AC unit. Also realizing the benefit when cooling down would require gradually decreasing the temperature setpoint one degree at a time.

Thus my question could be rephrased at: is a heat pump less efficient in relocating heat when initially turned on?

Yes, it is less efficient ... for maybe few seconds. This doesn't matter if you consider your environmental footprint. This doesn't matter if you consider your electricity bill. It definitely matters when considering wiring size, fuse size, and inverter or generator sizing.

Also inverter AC units may be less efficient at full load, but most AC units are not inverter units.

Besides, an inverter unit may have an "energy efficient cooldown" feature where it runs at the most efficient operating point despite the fact that the setpoint may be ten degrees below the current temperature. If you want to save money, then you should use that feature and turn off the AC entirely while away. Of course it's slower to cool down then.

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  • initial energy consumption is normally the first half cycle for a compressor in an air conditioner. Things that take a while to come up to speed like tablesaws spread that, but a compressor's first compression stroke is where the load is, and most stop at the start of that. They will usually draw whatever the circuit provides, which can be way over the nominal rating (and that's usually ok on grid-connected systems. But not generators or inverters)
    – Móż
    Aug 4, 2023 at 7:01
  • hard numbers for my "500W" aircon are a half cycle peak of about 24A on a 10A rated circuit. Which my chest freezer also managed, despite having a 90W continuous draw! I think 24A was the short circuit current...
    – Móż
    Aug 4, 2023 at 7:02

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