Answer first, context after
Is it cheaper to leave the AC on all day than to cool the house back down?
No. The myth survives because the catch-up blast feels expensive, but heat leaks into a house faster the bigger the indoor-outdoor gap, so a home held at 72 all day absorbs more heat than one allowed to drift warm. The Department of Energy puts setback savings at up to 10 percent a year. In Bakersfield, just time the recovery around PG&E peak hours.
The logic sounds airtight: the AC “works so hard” cooling the house back down that you might as well have left it running. It is one of the most durable myths in home cooling, and the physics disagrees with it politely but completely.
Why the myth is backwards: heat flows into your house at a rate set by the temperature difference between inside and out. A house held at 72 against a 105 afternoon is absorbing heat at full speed all day, and your AC removes every unit of it. A house allowed to drift up to 85 while you are at work absorbs heat more and more slowly as the gap narrows. When you cool it back down, you remove only the heat that actually got in, which is less than the held-at-72 house took on. The catch-up run is long and feels dramatic, but it is doing less total work. The Department of Energy’s figure: a 7 to 10 degree setback for 8 hours a day saves up to 10 percent a year.
The Bakersfield asterisk that actually matters: PG&E’s default time-of-use plan charges peak rates from 4 to 9 p.m. every day, which is exactly when you would come home and slam the thermostat down. Doing the whole recovery at peak prices gives back some of the setback savings. The play is pre-cooling: have the system start recovery in the early afternoon at off-peak rates so the house is already comfortable when peak pricing begins, then coast.
The two-line version of the strategy: let the house drift warm while nobody is home, and never do the catch-up between 4 and 9. A smart thermostat runs this schedule without you thinking about it, $240 to $420 installed.
When holding steady is actually right: on the handful of days so hot the system barely keeps up, skip the setback, because the recovery may not finish before evening. Same if the house holds pets, plants, or anyone sensitive to heat. The myth is not that steady cooling is ever sensible, it is that it saves money. It costs money and occasionally buys something worth paying for.
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