Is It Cheaper to Leave the AC On All Day or Turn It Off When You Leave?
You are leaving home for eight hours on a hot summer day.
Should you leave the air conditioner running at your normal temperature?
Turn the thermostat up?
Or shut the AC off completely?
There is a persistent idea that leaving the AC running all day can somehow be cheaper because the system will not have to “work as hard” when you come home.
For most homes, that is not a good reason to maintain the same cool indoor temperature while nobody is there.
ENERGY STAR recommends easing off heating and cooling when it is not needed, and smart thermostats are specifically designed to use energy-saving temperature settings when a home is unoccupied.
But that does not mean every home should simply shut the AC completely off for every absence.
The financially useful question is:
How much cooling can you avoid while you are away without creating comfort, humidity, health, pet, plant or building-protection problems?
My Take: Set Back the Thermostat Before You Leave the AC Running All Day
Short answer: For a typical empty home, I would not keep the same low cooling setpoint all day just to avoid a later cooling cycle. My default would be to raise the thermostat while away, then schedule a comfortable temperature before returning. Completely turning the system off is a separate decision that depends on humidity, pets, vulnerable occupants and building conditions.
My decision rule: I would compare the electricity avoided during the absence against any extra cooling after returning. I would use the calculator below as a scenario tool, not as proof that a particular home will save a fixed percentage. If humidity becomes excessive or indoor temperatures become unsafe, comfort and safety take priority over the estimated savings.
AC Away-Time Savings Calculator
Estimate how much an away-temperature strategy could save based on your AC's electrical demand and the cooling runtime you avoid.
The default inputs are illustrative scenarios, not U.S. household averages. Actual AC power and runtime depend on equipment size, efficiency, outdoor temperature, humidity, thermostat settings, insulation, solar gain and other home-specific conditions.
Short Answer: You Usually Don't Need to Cool an Empty House to the Same Temperature
If nobody is home, maintaining the exact same comfort temperature generally means your air conditioner is removing heat that you did not need removed for occupant comfort during those hours.
ENERGY STAR specifically recommends using thermostats that adjust to energy-saving temperatures when occupants are asleep or away.
That is the basic economic logic behind thermostat setback.
But there is an important distinction between:
- Keeping the AC at your normal occupied temperature all day
- Raising the thermostat while you are away
- Turning the cooling system completely off
The middle option is often the most useful place to start.
Why Does Raising the Thermostat Save Energy?
Heat naturally moves from warmer surroundings toward cooler spaces.
During summer, your house gains heat from outdoors, sunlight, air leakage, appliances and other sources.
The larger the temperature difference between the hot outdoors and the cooler indoors, the more heat the cooling system must ultimately remove.
If you allow the indoor temperature to rise while the house is empty, the temperature difference can become smaller.
That can reduce the cooling load during the away period.
This is why thermostat scheduling exists in the first place.
But Doesn't the AC Have to Work Harder When I Come Home?
It may need to run longer while bringing the house back to your preferred temperature.
That does not automatically erase the energy saved while you were away.
The useful comparison is not:
“Does the AC run for a long time when I get home?”
It is:
“How much total electricity did the AC use across the entire day under each strategy?”
A long recovery cycle can look expensive because the equipment is running continuously.
But the alternative may have been many shorter cycles throughout the entire period when nobody was home.
Compare total kWh, not how dramatic the recovery cycle looks.
Example: Leaving the AC at Your Normal Setting
Consider an illustrative central AC system averaging:
3 kW while operating.
Suppose you are away for eight hours and the system would otherwise accumulate four hours of compressor runtime during that period.
The cooling electricity used while you are gone would be approximately:
=
12 kWh
At 18.34 cents per kWh, that is about:
$2.20 for that day's away-period cooling.
The 3 kW and four-hour runtime assumptions are illustrative, not national averages.
Example: Raising the Temperature While You're Away
Now suppose the higher away setting reduces AC runtime during those eight hours from four hours to 1.5 hours.
When you return, suppose the system needs another half-hour of runtime beyond what it otherwise would have used while restoring your normal temperature.
The simplified total becomes:
=
6 kWh
Instead of 12 kWh in the simplified baseline scenario.
That would save about 6 kWh for that day under these assumptions.
At 18.34 cents per kWh:
6 kWh × $0.1834 = about $1.10.
If the same situation occurred on 20 workdays:
about $22 per month.
This is an illustration of the calculation method, not a promised savings figure.
Should You Turn the AC Completely Off?
Not necessarily.
Saving energy is only one part of the decision.
Indoor temperature and humidity can also matter.
In hot and humid climates, moisture management can become particularly important.
DOE research on hot-humid homes notes that controlling indoor humidity can be challenging even in efficient homes because moisture loads do not necessarily fall in proportion to sensible cooling loads.
So the financially cheapest theoretical strategy is not automatically the best operating strategy for every home.
A moderate away temperature can reduce cooling demand while still allowing the system to manage indoor conditions.
When Turning the AC Completely Off May Not Be a Good Idea
Be more cautious if the home contains:
- Pets
- People remaining in the home
- Temperature-sensitive medications or materials
- Humidity-sensitive belongings
- Conditions where excessive indoor heat could create a safety concern
Local climate and the building itself also matter.
CostCandid is comparing energy cost strategies, not prescribing a universal indoor temperature.
What About Humidity?
This is one reason “just shut it off” is too simplistic.
An air conditioner does two things during normal cooling operation:
- Reduces indoor temperature
- Removes some moisture from indoor air
In a hot, humid climate, completely eliminating cooling for a long period may allow indoor humidity to increase.
The appropriate strategy therefore depends partly on climate, building enclosure, ventilation and HVAC design.
If humidity control is a known problem in your home, monitor indoor relative humidity rather than making the decision solely from electricity cost.
How Long Will You Be Gone?
The length of the absence matters.
Leaving for 30 Minutes
The potential savings from changing the thermostat may be small.
Leaving for Several Hours
A scheduled higher cooling setpoint can become more meaningful because the system has more time to avoid maintaining the occupied temperature.
Leaving for a Full Workday
This is exactly the type of recurring absence that programmable and smart thermostats are designed to manage.
Leaving for Several Days
Vacation settings can reduce unnecessary HVAC operation while still maintaining whatever temperature and humidity protection your home requires.
ENERGY STAR specifically identifies away and vacation control as useful smart-thermostat functions.
Is a Smart Thermostat Worth It for This?
A smart thermostat is not required to change your temperature when you leave.
You can do it manually.
The advantage is automation.
ENERGY STAR certified smart thermostats can provide features such as:
- Scheduling
- Remote temperature control
- Occupancy-based adjustments
- Geofencing
- Equipment-use information
- Automatic away settings
ENERGY STAR certified models must demonstrate energy savings using real-world field data.
That does not mean every household will save the same percentage or dollar amount.
The result depends on your existing thermostat habits, climate, HVAC equipment and schedule.
What If You Have a Variable-Speed AC?
This is where simple internet rules become less useful.
Modern variable-speed and inverter-driven systems can operate at different output levels instead of simply switching between full power and off.
That means rated maximum electrical demand multiplied by clock hours can substantially misrepresent actual electricity consumption.
The same caution applies to heat pumps and mini-splits.
Use actual utility data, equipment monitoring or measured consumption when available.
The calculator above is deliberately based on estimated average electrical demand and equivalent runtime rather than claiming every AC consumes the same number of watts.
What If Your Utility Uses Time-of-Use Rates?
The cheapest strategy can become more complicated when electricity prices change by time of day.
For example, your highest electricity price might occur in the late afternoon when you are returning home.
In that situation, the question is no longer only:
How many kWh did I save?
You also need to ask:
When were those kWh consumed?
A smart thermostat or precooling strategy may sometimes shift cooling to a different period, depending on the home, rate structure and utility program.
Check your actual utility tariff before assuming every kWh has the same price.
Don't Confuse Fans With Air Conditioning
Ceiling fans and air conditioners work differently.
DOE notes that fans primarily improve occupant comfort rather than cooling an empty room.
So if nobody is in the room, leaving a ceiling fan running usually provides no comfort benefit to anyone.
This logic should not simply be transferred to air conditioning, because AC also controls indoor temperature and contributes to moisture removal.
What Should You Actually Do Before Leaving Home?
A practical approach is:
- Do not maintain an unnecessarily low occupied cooling temperature in an empty home.
- Use an away temperature or schedule appropriate for your climate and household.
- Consider humidity and home-protection needs before shutting cooling completely off.
- Restore your preferred temperature before or when you return.
- Compare actual daily or monthly kWh before and after the change.
Your utility data is more useful than arguing about universal thermostat rules.
Run Your Own 2-Week Test
If you want to know what works in your house, test it.
For one week, use your normal cooling strategy.
Record:
- Daily kWh
- Outdoor weather
- Hours away
- Thermostat settings
For another comparable week, use an away setback.
Then compare electricity consumption on days with similar weather and occupancy.
That will not create a laboratory-quality experiment, but it can provide much more household-specific information than a generic claim that one thermostat setting always saves a certain percentage.
Three Situations That Change What I Would Do
1. A quick 30-minute errand
I would not expect a dramatic bill difference from a very short absence. I might leave a reasonable cooling setting in place rather than aggressively change the thermostat twice. The exact result depends on the home's heat gain and system controls.
2. An eight-hour workday in a reasonably dry home
I would test a warmer away setpoint and restore comfort shortly before arriving home. For an illustrative calculation only, suppose the AC averages 2 kW while running, the away strategy avoids 1.5 hours of runtime, and recovery adds 0.5 hours later. Net avoided runtime is 1 hour, or 2 kWh/day. At an assumed marginal rate of $0.20/kWh, that is $0.40/day or $12 over 30 identical days. Real outcomes vary, and this is not a measured savings claim.
3. A humid climate, pets at home, or moisture-sensitive rooms
I would use a moderate setpoint rather than switch cooling off completely if indoor humidity or temperature could become problematic. A thermostat setback that causes condensation, mold risk, or unsafe temperatures is not a worthwhile financial trade.
What Would Make Me Change My Recommendation?
I would be less aggressive with setbacks if measured indoor humidity rises too high, the home contains people or animals needing a controlled environment, or the equipment has operating constraints. I would also consider time-of-use electricity prices and whether a variable-speed system's controls respond differently. I would not assume that running the AC at a fixed setting all day is cheaper merely because recovery cooling feels intense.
My Final Decision
Short trip: small adjustments may have little impact. Several hours away: I would usually use a warmer setback and test the bill and indoor conditions. Extended absence: I would protect humidity, plumbing, pets and building materials before maximizing savings. If the calculator shows a large saving, I would validate it with actual meter or thermostat runtime data rather than treat it as guaranteed.
Frequently Asked Questions
Is it cheaper to leave the AC on all day?
Maintaining the same occupied cooling temperature while nobody is home generally creates unnecessary cooling demand. ENERGY STAR recommends adjusting heating and cooling to energy-saving settings when a home is unoccupied.
Should I turn my AC off when I leave for work?
You do not necessarily need to shut it completely off. Raising the thermostat while you are away can reduce cooling demand while still allowing temperature and humidity control. The appropriate setting depends on climate, household needs and the home.
Does turning the AC off make it work harder later?
The system may run longer when you return, but a longer recovery cycle does not automatically use more electricity than maintaining the lower temperature all day. Compare total kWh across the full period.
Does leaving the AC on save money because it runs less later?
Not by itself. Avoiding a later recovery cycle requires the AC to maintain the cooler indoor temperature while you are away. The meaningful comparison is total electricity consumption under both strategies.
How much should I raise my thermostat when I leave?
There is no single setting that is financially optimal for every home. Climate, humidity, equipment, pets, occupancy and comfort requirements differ. ENERGY STAR supports using energy-saving away settings rather than maintaining the same occupied setting unnecessarily.
Can turning the AC off cause high humidity?
It can contribute to reduced moisture removal because the cooling system is not operating. This can matter more in hot-humid climates and homes with existing moisture-control challenges.
Does a smart thermostat actually save electricity?
ENERGY STAR certified smart thermostats must demonstrate energy savings based on field data. Actual household savings vary depending on climate, HVAC system, previous thermostat behavior and occupancy patterns.
Methodology
This article evaluates AC away strategies by comparing estimated cooling electricity consumption rather than assuming a universal percentage savings.
The calculator uses:
Average AC Electrical Demand (kW) × Equivalent Runtime (hours)
Electricity Cost
Electricity Use (kWh) × Electricity Rate
The away strategy includes both estimated AC runtime while the household is away and any additional recovery runtime after occupants return.
This is a simplified decision-support model, not a building-energy simulation.
Actual cooling consumption depends on equipment capacity, efficiency, part-load operation, outdoor temperature, humidity, solar gain, insulation, air leakage, thermostat settings and building thermal mass.
The calculator defaults of 3 kW, four hours of baseline runtime, 1.5 hours of away-strategy runtime and 0.5 hours of additional recovery runtime are illustrative assumptions only. They are not national averages.
The default electricity rate of 18.34 cents per kWh uses the June 2026 U.S. residential average from the U.S. Energy Information Administration. Your actual utility rate may be substantially different.
Sources
- ENERGY STAR — Smart Thermostats
- ENERGY STAR — Smart Thermostat Energy-Saving Tips
- U.S. Department of Energy — HVAC Programmable Thermostats
- U.S. Department of Energy — Advanced HVAC Humidity Control for Hot-Humid Climates
Last reviewed: September 2026. Cooling costs vary by climate, equipment, electricity rate, home characteristics and thermostat strategy. Calculator inputs are illustrative unless replaced with your household-specific values.
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