When you’re home, 78°F is the best AC setting, the DOE comfort-and-efficiency anchor. But “best” changes with what you’re doing. There’s a different right answer for sleeping, for an empty house, and for a heat wave. Here’s the whole day.
Best AC temperature by scenario
| Scenario | Setpoint | Notes |
|---|---|---|
| Home, active | 78°F | DOE recommendation — the anchor for all other numbers |
| Home, sleeping | 65–68°F | Sleep research: core body temp drops 1–2°F during sleep onset; a cooler room supports that |
| Away (work/errands) | 85–88°F | The biggest savings opportunity — don’t cool an empty house |
| Vacation (<1 week) | 82–85°F | Don’t turn it off entirely; humidity spikes fast |
| Vacation (>1 week, humid climate) | 80°F max | Mold risk rises above 60% RH; keep it running low |
| Baby/infant in home | 68–72°F | AAP-recommended comfort range for infants |
| Elderly/heat-sensitive | 75–78°F | Lower threshold reduces heat stress risk |
What each degree costs you
Every degree you raise the setpoint saves roughly 3% on cooling costs. That’s the commonly cited figure from ENERGY STAR and DOE guidance, and it’s directionally right even if your exact number varies by climate, insulation, and system efficiency.
Run the math on a real gap:
- 72°F vs. 78°F = 6 degrees = roughly 18% lower cooling cost
- 72°F vs. 85°F (away setpoint) = 13 degrees = roughly 39% reduction while you’re out
If your cooling bill runs $200/month in summer, moving from 72°F to 78°F while home saves about $36/month. Moving to 85°F for the 9 hours you’re at work saves another $20–25 on top. Over a three-month cooling season, those two changes together are $150–180.
The DOE’s better-anchored figure: a 7–10°F setback for 8 hours a day saves about 10% per year on total HVAC cost. Not just cooling — the full year. The per-degree math gets you there: a 7°F difference held for a third of the day compounds over months.
Treat 1–3% per degree as directional. The direction is always the same: every degree lower costs you money, and every degree higher saves it.
Night temperature: the sleep exception
This is the one place where “colder is genuinely better” has solid science behind it. Your core body temperature drops 1–2°F during sleep onset. A bedroom too warm interferes with that drop and delays sleep.
Most people sleep best with bedroom air around 65–68°F. That’s cooler than the 78°F daytime anchor, and it’s colder than most people set their thermostat at night.
The cheaper way to hit it: hold 75–78°F at the thermostat and run a ceiling or box fan moving air directly over the bed. The airflow effect lets you stay comfortable at a higher (cheaper) setpoint. If you sleep hot even with a fan, drop the setpoint — but spend on airflow first.
Heat pump note on overnight setbacks: If you have a heat pump, check your aux heat lockout settings before setting a large overnight setback. In shoulder-season months (May, September), a heat pump in Auto mode may read a big temperature drop as a heating call and fire resistance strips. If you see “AUX” on the display in the morning, that’s what happened. Either stay in COOL mode overnight or set the setback modestly (2–3°F rather than 10°F).
Why not just set it to 72°F and forget it?
Lower setpoints make the system run longer against a hotter outdoor temperature. The unit doesn’t cool faster at 72°F — it just runs more. And on hot days, it may never fully recover if the outdoor temp climbs high enough.
The DOE’s 10%/year savings figure for a 7–10°F setback is real money over a cooling season, and you probably won’t notice the difference between 72°F and 76°F after about two weeks of adjustment.
How to set the temperature on your thermostat
Nest (Learning Thermostat / Thermostat E / Thermostat 2020)
- Wall unit: Turn the outer ring. The current setpoint displays in the center.
- App: Open the Nest app, tap the thermostat, tap the temperature bubble, then drag up or down.
- Schedule: On the wall unit, go to Settings > Schedule. In the app, tap Schedule under your thermostat.
ecobee (SmartThermostat Premium / SmartThermostat Enhanced)
- Wall unit: Tap the temperature displayed on screen. Use the + and – buttons to adjust.
- App: Tap your thermostat card, then tap the temperature to adjust.
- Schedule: Main Menu > Schedule. You can set different setpoints for each time block.
Honeywell T6 Pro
- Wall unit: Use the Up/Down arrow buttons on the face of the unit. Hold for faster changes.
- Schedule: Press the Mode button, then follow the on-screen prompts to set time blocks and temperatures for each.
Honeywell T9 / T10 Pro
- Wall unit: Tap the temperature display, use arrows to adjust.
- App: Open the Resideo app, tap your thermostat card, set the temperature directly. For scheduling, tap the Schedule tab and add or edit time blocks.
Heat pump in summer: set it to COOL, not Auto
A heat pump in cooling mode works the same as central AC for setpoint purposes — set it to what you’d set a conventional system. But in spring and fall, a thermostat in Auto mode may flip between heating and cooling based on the setpoint, which can cause unexpected heating calls on a warm afternoon.
During summer months, put the system in COOL (not Auto). This prevents the heat pump from switching to heating when a cloud passes or the evening cools down. If you leave it in Auto and come home to a warm house, check whether the system was heating rather than cooling — it happens more than you’d expect during shoulder season.
If it won’t reach the setpoint
On days above 95–100°F outdoor temperature, a correctly sized system may only be able to maintain about 20°F below the outdoor temp. If it’s 100°F outside and the house sits at 80°F, that’s physics, not a malfunction.
But if the system held 74°F last summer and can’t this summer at the same setpoint, that’s a different problem. Refrigerant loss, a dirty coil, or a blocked filter will all reduce capacity over time. That’s not a settings issue — see Why Your AC Isn’t Cooling.
If it can’t get below a specific number (say, it stops at 74°F no matter what you set), see AC won’t go below a certain temp.
Best temperature for AC in summer: FAQ
What temperature should I set my AC in summer to save money?
78°F when you’re home, 85–88°F when you’re away. Those two numbers together get you close to the DOE’s 10%/year savings benchmark. Add an overnight setback to 75–78°F if you’re comfortable, and you’ve covered the main levers.
Is 72°F too cold for AC in summer?
Not dangerous, but expensive. Running at 72°F instead of 78°F costs roughly 18% more on your cooling bill. Most people adapt to 76–78°F within a week or two and stop noticing the difference.
What temperature should AC be set at night in summer?
65–68°F in the bedroom is the sleep science target. If whole-home setback to 65°F is too expensive, run a fan in the bedroom and hold the thermostat at 74–76°F. The airflow does most of the work.
Does turning AC up when away actually save money?
Yes, and it’s the single highest-impact change most people can make. Every hour the house sits at 85°F instead of 78°F is an hour the compressor isn’t running. Over a workday, that’s 8–9 hours of reduced runtime. A programmable or smart thermostat does this automatically.
What’s the highest temperature you can set AC and still be comfortable?
For most adults at rest, 80°F with good airflow is manageable. Above 82°F it gets uncomfortable for most people. Above 85°F for extended periods is where heat stress risk rises, particularly for elderly adults or anyone with cardiovascular conditions. If you’re trying to hold 88°F with people home, add fans and keep an eye on how people feel.
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Related: Best Thermostat Settings for Summer · Fan On or Auto · Vacation Settings
