Best Summer Thermostat Setting: 78°F Explained
Written and checked with AI using the references below. This is not human expert review.
Quick answer
The U.S. Department of Energy recommends 78°F when you are home in summer, and raising the thermostat when you leave — every degree set below 78°F can increase your cooling bill by up to 8%, so two degrees lower could add up to 16% (illustrative: 2 × 8%) to your monthly cooling cost.
The 78°F Recommendation — and the 8% Cost Penalty
The U.S. Department of Energy recommends setting your thermostat to 78°F or higher when you are home during summer. That figure balances indoor comfort against what your air conditioner spends to hold a steady temperature on a hot day.
The cost of going lower is steep. A University of Florida Extension document citing the U.S. Department of Energy states that every degree you lower the thermostat can increase your cooling bill by up to 8%.
Illustrative example: If your summer cooling bill is $150 a month at 78°F, dropping two degrees to 76°F could add up to 16% — roughly $24 extra per month, or about $72 over a three-month cooling season (calculation: 2 degrees × up to 8% per degree = up to 16%; $150 × 0.16 = $24). Your actual result depends on your climate, home size, and system efficiency.
One common belief is that setting the thermostat lower is always better for comfort. The evidence contradicts this: lower costs more per degree and may not fix clamminess if humidity is the real problem — more on that below.
Raise It When You Leave: The Away-Setback Strategy
Holding 78°F all day while the house is empty wastes energy. The smaller the gap between indoor and outdoor temperatures, the less work your AC does. Raising the thermostat when you leave directly shrinks that gap.
A programmable thermostat makes this automatic. You set time blocks — a higher temperature during work hours, then a return to 78°F before you get home. You can store different schedules for workdays and weekends. Look for a model with a manual override so you can adjust on the fly without erasing your whole schedule. Make sure the thermostat is designed to work with your specific system before you buy.
ENERGY STAR reports that homes with high heating and cooling bills that are empty for much of the day can save roughly $100 a year with an ENERGY STAR certified smart thermostat. That figure applies only to homes that fit that specific profile — savings vary by household. ENERGY STAR and equipment sellers may have a financial interest in promoting certified products, so treat that figure as a general benchmark, not a guarantee.
Evidence gap: The available sources support raising the thermostat when you are away or asleep but do not name a specific numerical setpoint for those periods. Check your thermostat manufacturer’s guide or the heating and cooling topic hub for model-specific scheduling guidance.
Humidity: Why 78°F Feels Different in Every Home
Temperature alone does not determine comfort. High indoor humidity makes 78°F feel much warmer and stickier than it should. This is where many homeowners make a costly mistake.
When a home feels clammy, the instinct is to lower the thermostat. But that costs up to 8% more per degree and leaves the root problem unsolved. Some high-efficiency systems still remove moisture poorly — they cool the air fast and shut off early. The house hits 78°F on the dial but still feels uncomfortable, and the bill runs higher than expected.
Air conditioners remove humidity as a side effect of cooling, not as a direct goal. An oversized system cools too fast, shuts off early, and leaves excess moisture behind. If your home feels clammy at 78°F with windows closed, the fix is a system evaluation — not a lower setpoint. A licensed HVAC contractor can check whether your system is sized and performing correctly. This is not a DIY check.
How to Make 78°F Feel More Comfortable
Several low-cost steps reduce the heat your AC has to fight, so 78°F stays livable without touching the thermostat dial.
- Shade east and west windows. Morning and afternoon sun through unshaded glass adds significant heat. Blinds, curtains, or exterior shading cut that load directly.
- Delay heat-generating tasks. Running the dishwasher or oven during the hottest part of the day forces your AC to work harder. Shift those tasks to evening when outdoor temperatures drop.
- Keep the house closed during the day. Opening windows lets in hot, humid outdoor air. Keep the house closed tight during most of the cooling season to avoid letting in unwanted heat and humidity.
- Use bath and kitchen fans sparingly. These fans pull conditioned air out and draw in unconditioned air. Run them only as long as needed when the AC is on.
- Add ceiling fans. A ceiling fan does not lower the air temperature — it moves air across your skin, which makes you feel cooler. That effect can help you tolerate 78°F more easily. Turn fans off when you leave the room; they cool people, not spaces.
Common Beliefs vs. What the Evidence Shows
| Common belief | What the evidence shows |
|---|---|
| Lower is always better for comfort | Lower costs up to 8% more per degree and may not fix clamminess if humidity is the real problem |
| Leaving the AC at one setting all day is fine | Raising the temperature when you leave saves energy; a programmable thermostat automates this |
| A high-SEER system automatically handles humidity | SEER measures cooling efficiency, not moisture removal; a separate metric (SHR) governs dehumidification |
| Bigger AC units cool and dehumidify better | Oversized units cool too fast, shut off early, and leave excess humidity behind |
When to Stop and Call a Professional
Some comfort problems cannot be solved by adjusting the thermostat. Call a licensed HVAC contractor if:
- Your home feels clammy at 78°F even with windows closed and fans running — this points to a dehumidification or sizing issue.
- The system runs constantly but never reaches the set temperature on a normal summer day.
- You notice ice forming on the indoor unit or refrigerant lines. Do not try to diagnose or fix this yourself. Refrigerant work must be done by a certified technician — handling refrigerants without EPA Section 608 certification is illegal.
- You smell burning or hear unusual sounds from the air handler or outdoor unit.
Do not bypass safety switches or open electrical compartments inside your HVAC equipment. Those areas carry dangerous voltages. For broader home efficiency questions, the Home & Household section covers related topics.
Frequently asked questions
Should I turn the AC off completely when I leave the house?
Turning it off entirely can let indoor temperatures and humidity climb so high that the system works harder to recover when you return. Raising the thermostat to a higher setpoint while away — rather than switching it off — is generally more efficient. The available sources support raising the temperature when you leave but do not specify an exact off-versus-setback threshold; your system's manual is the best guide.
Does running ceiling fans let me set the thermostat higher?
Yes, in practice. Ceiling fans create a wind-chill effect that makes you feel cooler without actually lowering air temperature. That perceived cooling can make a 78°F setting feel more comfortable. Turn fans off when you leave the room — they cool people, not spaces, so running them in empty rooms wastes electricity.
Why does my house feel humid even though the thermostat reads 78°F?
An oversized or poorly matched AC system cools the air quickly and shuts off before removing enough moisture. High SEER ratings measure cooling efficiency, not humidity removal — that is governed by the Sensible Heat Ratio (SHR). If clamminess persists with windows closed, a licensed HVAC contractor should evaluate whether your system is correctly sized and performing to its SHR rating.
Is a programmable thermostat worth it if I have an irregular schedule?
A programmable thermostat works best when your schedule is consistent enough to set reliable time blocks. If your hours vary widely, a smart thermostat with remote app control may be more practical — you can adjust the setpoint from your phone before heading home. ENERGY STAR notes that homes unoccupied for much of the day can save roughly $100 a year, though savings vary by household.
Does using a dehumidifier alongside the AC help at 78°F?
Running a standalone dehumidifier while the AC is operating actually increases the cooling load, forcing the AC to work harder. The evidence recommends against using both at the same time. If humidity is the core problem, the better fix is having a contractor verify that your AC system's latent (moisture) load capacity matches your home's actual humidity load.
Sources
- Heat & Cool Efficiently · ENERGY STAR (energystar.gov)
- FCS3262/FY1026: Energy Efficient Homes: Air Conditioning · Ask IFAS - Powered by EDIS (edis.ifas.ufl.edu)
Written by AI; checked through automated evidence and editorial review. References retrieved October 11, 2026.
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