If it’s cooler outside than in, then in general opening the windows and having the AC on will cool things off faster than just one or the other.

Caveats might include the AC output being near a window and it being cooler inside near the window, or humidity. But absent that, sure, both will cool it down faster.

Once the inside temperature drops below outside, though, probably want to close the windows or turn off the AC.

I’d also add that if your AC has a ventilation-only mode that can pull outside air in — a window unit or ducted unit probably does, split mini won’t, portable may not — that’ll help cool the inside and be more energy-efficient than running the compressor, if you don’t care about getting inside colder than ambient temperatures. Might also consider putting a box fan in a window or two, which could also drastically increase air turnover rate.

I live in a pretty comfortable climate and normally always have an open window and a small fan near the ceiling blowing air out the window, to have more airflow than would normally be the case with purely-passive ventilation.

If you’re in an environment that doesn’t get much humidity — the Southwest in the US is a good example — you might also consider an evaporative cooler, which won’t give you the potential very cold temperatures of an air conditioner (given enough power), but will cool things below ambient temperatures without needing much power.

You provided an image of a mini split. They are so efficient, if it is colder outside than in it is pretty negligible that it will be any less efficient. Unless the window is near the outside unit and the heat just comes back in. But you need to take into account the humidity, if it’s much more humid outside you definitely should keep the windows shut.

As long as having the windows open and the AC running has the same effect, it makes sense to do both. As soon as the AC has to work against the air coming in from outside, it probably makes sense to either close the windows or turn off the AC.

Even then it might make sense to keep doing both though. For example, if the AC only changes air temperature and moisture, but does not reduce volatile organic compounds, and it’s hot and humid outside. You keep the window open to keep VOC low, while keeping the AC on to keep temperature and moisture pleasant.

Thanks for this question. As the weather cools, I’ve started considering the same thing. Leaving the windows open overnight never seems to cool the house down enough on its own. I’ve been considering using the A/C to help, that way I don’t have to run it as much during the day. Glad I’m not the only one thinking this way.

I’m wondering if this will harm the operation of the air conditioner in any way, given that I have a whole-home HVAC system, and they’re designed to work with the windows closed.