Is it more efficient to turn my AC unit off periodically vs. keeping it on only while in the room?

Question

I have an AC unit in my bedroom + workspace. I use its "auto" fan speed and "energy saver" mode (rather than "cool" or "fan only"). I'm wondering, which strategy is more energy efficient and by roughly how much?

Strategy 1: When the room temperature exceeds 73 F / 22.8 C, turn it on and set it to 70 F / 21.1 C. When the room reaches that temperature, turn the unit off and wait until the temps rise to 73 F again and repeat.

Strategy 2: Same conditions, except keep the unit running as long as I am in the room.

It takes about 20-30 minutes on the average day for temps to rise from 70 F (set) to 73 F (trigger).

Is there a general best practice for this or is it dependent on the unit?

Answer 1

With strategy 1, you're basically emulating what your AC's thermostat in energy saver mode would do when set to a temperature of about 72 degrees -- it will cool to a bit beyond the set point, then switch off for a while, and let the temperature rise a bit above the set point, and then switch on again. Most ACs have a range of about 2-3 degrees around the set point they try to stay within.

Whether you manually or automatically switch the AC on and off as the temperature fluctuates shouldn't matter all that much. However, since with strategy 2, you have specified a lower set point of 70 degrees, if you're in the room for long enough, that strategy will use a bit more energy (and provide more cooling). If you adjust your set point up to 71 or 72 degrees, it will operate almost identically to strategy 1.

One note: ACs don't like to be cycled on and off rapidly, so in some cases the electronics will add an extra delay before cycling on, beyond what the temperature set point calls for. Usually this is only about 5 minutes though, so since you say it takes about 20-30 minutes for the temperature to rise from 70 to 73, this won't apply to you.