What should I set my thermostat to in summer to save money?

There are two versions of this answer: the poster on the wall, and the one that works in a town that hits 107.

The reference numbers: 78 when you are home is the setpoint the efficiency programs are built around, and the Department of Energy’s long-standing math says setting the temperature back 7 to 10 degrees for 8 hours a day, the workday, saves up to 10% a year on heating and cooling. Every degree you hold below 78 on a hot afternoon costs real compressor runtime, because the cost of cooling scales with the gap between inside and outside.

The Bakersfield version: do not fight the afternoon, outsmart it. Run the system harder in the morning while the outdoor air is merciful and cooling is cheap and easy, get the house and everything in it genuinely cold, then let the setpoint drift up through the brutal hours. A pre-cooled house coasts. Thick walls, furniture, and floors hold cold the way they hold heat, and the system that would have run flat out from 3 to 8 pm instead cycles gently. Ceiling fans buy you another 2 to 3 degrees of comfort in occupied rooms because moving air feels cooler on skin. Fans cool people, not rooms, so turn them off when you leave.

What not to do: big dramatic swings in the wrong direction. Setting 68 on a 107 degree day does not cool faster, it just guarantees continuous runtime chasing a number the system cannot reach. And shutting the AC off entirely for the workday in July usually backfires here, because the recovery from a 95 degree house through the hottest hours costs more than holding a moderate setback would have.

If you are on a time-of-use electric plan, and most PG&E households now are by default, the pre-cooling strategy gets a second payday, because it shifts your heavy usage into the cheap hours. That one gets its own answer.