If you grew up in the 1980s, 1970s, or earlier, when air conditioning was common but not as efficient as it is now, you likely remember being told to shut the air conditioner off whenever you left home and no one would be in the building. It made sense, right, because if no one was there for eight, nine hours, or more, there was no need to run the air conditioner.

Then a few years ago, a new piece of advice proliferated across the internet: leave the air conditioner on at a few degrees higher than normal even when you're not home so that you don't shock the system by turning it on and forcing it to suddenly cool a house that could be in the 90s or low 100s indoors. The idea was that, by leaving the system on — and these systems were light-years more efficient than older systems were — you'd see less wear on the system as a whole.

But leaving the system on means using energy and spending more money. So, for those who want to be both energy-efficient and cost-efficient, what's the best thing to do?

It Doesn't Mean Leave It Constantly Running

Leaving your air conditioner on all the time doesn't mean having it run all the time. As the advice said, turn that thermostat up. If no one's home, you don't need your home to be 73 or even 74 indoors. But putting the thermostat at, say, 78 or 80 means that on really hot days, the house isn't going to get so unbearably hot that the air conditioner struggles to cool it once you get home. If you live in a region where temperatures can be in the upper 90s regularly, a home that stays around the upper 70s is easier to cool down to the lower 70s when you finally go inside.

So the point is to adjust the thermostat so that the system isn't running as much, but leaving the system as a whole on so that, if the interior temp does get too high, the system will turn on and cool things back down. You'll need to set the thermostat daily if your area is having a heatwave to ensure the best combination of cooling and not using energy unnecessarily.

Do Take Other Steps to Cool Your Home, Too

With the air conditioning system on all the time — not running, but on — you will see a slight increase in utility bills. You can mitigate this to an extent by ensuring that your home itself is energy-efficient. Fill gaps, install ceiling fans, get dual-pane windows if you can, and so on. By keeping your home cooler overall through these steps (often referred to as being part of "passive solar"), you won't need to use your air conditioner as often even if you're home. That saves energy.

Above all, make sure your air conditioner is functioning as well as it can. Have older systems replaced with new ones that conform to the latest efficiency standards, and have newer systems inspected to ensure they don't have any hidden problems.

For more tips, contact a local AC service.