AC thermostat settings: the numbers that actually save money
Contents (7)
No single household number is argued about more than the summer setpoint. One camp runs 68 °F and complains about the bill; the other holds 80 °F and complains about the heat; a third insists that leaving the thermostat alone all day is cheaper than turning it up. Only one of those positions is settled by physics, and it is not the third one.
The reference point published by ENERGY STAR for the cooling season is 78 °F while you are home and awake, with the temperature allowed to drift upward when the house is empty or everyone is asleep. That figure is not a comfort verdict handed down from Washington — it is the anchor of a schedule, and the schedule is where the savings live.
The reference schedule
| Period | Recommended setting | What it does |
|---|---|---|
| Awake and home | 78 °F | The comfort anchor — everything else is expressed relative to it |
| Away (work, school, errands) | +7 °F or more (≈85 °F) | The single largest saving, because the house is empty for the longest block |
| Evening, home | 78 °F | Recovery back to the anchor before occupancy |
| Asleep | +4 °F or more (≈82 °F) | Smaller offset: bodies tolerate less drift overnight than an empty house does |
The asymmetry matters. The away setback is aggressive because nobody is made uncomfortable; the sleep setback is modest because sleep quality degrades above roughly 80 °F, and a saving that costs you a night is not a saving. If 78 °F is not liveable in your house, do not abandon the schedule — move the whole thing down a degree or two and keep the offsets. The offsets, not the anchor, do the work.
What one degree is worth
The rule of thumb circulated by utilities is that each degree of setpoint costs or saves in the region of 3 % of cooling energy over a season. It moves with climate: in a mild coastal summer the machine barely runs, so a degree is nearly free; in a Phoenix or Houston August a degree is worth noticeably more. What makes the rule directionally right everywhere is the mechanism — heat crosses your walls at a rate proportional to the indoor-outdoor gap, so raising the indoor number shrinks the load you pay to remove.
Work your own figure rather than trusting a percentage: multiply your monthly cooling kilowatt-hours by your rate, and compare a month at 74 °F with a month at 78 °F on similar weather. The method for isolating the cooling share of a bill is in our running cost guide, and how many hours your climate demands in the first place is visible in the cooling degree days on every state page.
Four myths worth retiring
« Holding a constant temperature is cheaper than setting back »
False for conventional AC, and it is the most persistent of the four. A house loses heat gain proportionally to the indoor-outdoor gap, so a warmer house all afternoon leaks in less heat all afternoon. The energy spent re-cooling in the evening is less than the energy saved by not fighting the peak. The myth survives because the evening recovery is visible while the six quiet hours before it are not.
« Setting it to 65 cools the house faster »
False. A single-stage compressor has one output: full. Dialing 65 °F rather than 76 °F does not increase capacity, it only guarantees the system overshoots your actual target, runs longer than needed, and leaves the room cold and clammy. Variable-speed equipment does modulate, but it modulates toward the setpoint you asked for — the fastest route to 76 °F is still to ask for 76 °F.
« Leave the fan on AUTO or ON, it makes no difference »
It makes a real one, and mostly in the wrong direction. With the fan set to ON it runs continuously, including after the compressor stops — which blows air across a wet evaporator coil and re-evaporates the moisture the system just removed, sending humidity back into the house. AUTO is the default answer in humid climates. Continuous circulation has legitimate uses — filtration, evening out a two-storey house — but belongs to a duty-cycling circulate mode, not plain ON.
« Closing vents in unused rooms saves money »
Almost never. Closing supply registers raises static pressure across a system that was balanced for all of them open, which reduces total airflow, pushes more air out of every duct leak upstream, and in the extreme drops the evaporator temperature toward freezing. Closing a couple of registers in a large house is harmless; closing a third of them is a slow way to buy a repair.
Humidity moves the comfortable number
Setpoint is only half of comfort. At 50 % relative humidity, 78 °F reads as pleasant to most people; at 65 %, the same 78 °F reads as sticky and the instinct is to drop the thermostat two degrees — which costs energy to fix a moisture problem with temperature. The cheaper lever is dehumidification: a right-sized system running longer, gentler cycles removes far more water than an oversized one that satisfies the thermostat in eight minutes and shuts off. This is the practical case for two-stage and variable-speed equipment in the Southeast, and the practical case against oversizing generally — the threshold arithmetic sits in our BTU sizing guide.
Heat pumps and smart thermostats: two caveats
The setback logic above is written for cooling, where it holds without exception. In heating season a heat pump deserves gentler treatment: a deep overnight setback can trigger electric resistance backup during recovery, and resistance heat costs three to four times what the heat pump costs per unit delivered. Most modern thermostats detect this and ramp recovery gradually — if yours does not, keep winter setbacks to a few degrees. Cooling season has no equivalent trap.
Smart thermostats earn their keep less through clever algorithms than through occupancy detection and geofencing: they apply the away setback on the days your schedule breaks, which is precisely when a programmable model is silently cooling an empty house. Many utilities discount them, and demand-response programs pay for the right to nudge your setpoint a couple of degrees on peak afternoons — worth checking against the incentive landscape in our credits and rebates guide. What no thermostat can do is fix a system that is not delivering; if the house cannot reach its setpoint at all, the problem is upstream and belongs in the not-cooling walkthrough.
When to stop optimizing
Setpoint discipline is a cost strategy, not a health strategy. During an extreme heat event the indoor temperature stops being a budget line: heat illness risk rises steeply for older adults, infants, people on certain medications and anyone without acclimatization, and the correct move is to cool the house properly and let the bill be what it is. Comfort thresholds are personal too — 78 °F in a top-floor room with an unshaded west window is not the same 78 °F as in an interior one.
Frequently asked questions
Is 72 °F unreasonable?
It is not unreasonable, it is simply more expensive — roughly 15–20 % more cooling energy than 78 °F on the 3 %-per-degree rule of thumb, more in a long-season climate. If 72 °F is what your household needs, keep it and take the savings from the away and sleep offsets instead, which cost you nothing in comfort.
Should I turn the AC completely off when I leave for the day?
For a workday, no — set back rather than off. A house allowed to climb to 90 °F soaks heat into furniture, drywall and floors, and recovering that thermal mass takes long enough that the humidity comes back with it. For a week-long absence, off (or a high holding setpoint around 85 °F to protect against extreme humidity) is fine.
Does a ceiling fan let me raise the setpoint?
Yes, and it is the best value on this page. Moving air over skin raises the temperature that feels comfortable by around 4 °F, and a ceiling fan draws a small fraction of what a compressor draws. The condition: fans cool people, not rooms, so turn them off when you leave — a fan running in an empty room is pure added load.
Why does my house feel cold and damp at 74 °F?
That combination usually means an oversized system: it satisfies the thermostat quickly, shuts down before it has run long enough to condense much water, and leaves you cool but humid. Lowering the setpoint further makes it worse. The fixes are longer run times — via staged equipment, corrected airflow, or a dedicated dehumidifier — not a colder number.
Do these numbers apply outside the US?
The physics does; the reference point differs. European guidance tends to be framed in Celsius around 25–26 °C with a « no more than about 7 °C below outdoor » rule of thumb, which we cover in the European setpoint guide.