How Much Do Smart Thermostat Settings Save in a Heat Wave?

Smart thermostat settings for a heat wave can cool a home and save energy, but the useful answer is not one magic number. The savings claims people hear in July and August are measuring different things: a degree-by-degree cooling estimate, an annual heating-and-cooling average, a manufacturer’s annual claim, or a one-event demand-response reduction. Those are not interchangeable.

The short version: a defensible expectation is much narrower than the marketing spread. ENERGY STAR’s smart thermostat figure is about 8% on annual heating and cooling bills, or about $50 per year, based on its certification approach and field-data requirements—not a heat-wave-only guarantee. Independent studies cited in consumer reporting tend to cluster around 10% to 15%. The often-repeated degree-by-degree rule is roughly up to 3% less cooling energy for each degree above 72°F, but even that should be treated as a dated, attributed estimate rather than a law of physics for every house. Demand-response programs are a separate category: they can reduce peak load by about 1 kW during an event, but that does not automatically translate into the same bill savings for every enrolled household.

First, audit the savings number

A thermostat claim is only useful after you know the denominator. Is it annual energy, summer cooling cost, compressor runtime, peak demand, or one utility event? The same thermostat can look impressive in one column and ordinary in another.

That table is the whole reason “set it to 78°F and save 10%” is too blunt. A 10% annual estimate may be reasonable in one context and misleading in another. A 3% per-degree estimate may help you compare 74°F with 76°F during an afternoon, but it does not say what your utility will bill after a week of humid nights, demand charges, or a late-evening price peak.

The old 75°F to 78°F range and a roughly 7°F setback belonged to dated federal guidance, and the relevant energy.gov page was reported removed in July 2026. That history explains why 78°F keeps appearing in advice columns, but it does not make 78°F the right comfort, safety, or savings number for every home during a heat wave [8]. Carnegie Mellon researchers writing in The Conversation have also cautioned that 78°F is not the right setting for everyone during extreme heat [9].

Where the savings physically come from

A smart thermostat saves cooling energy in a heat wave only when it reduces the work the air conditioner has to do, shifts that work into cheaper hours, or lets a utility briefly reduce load during a peak event. The app screen, occupancy sensor, and learning algorithm are just controls. The bill changes when compressor minutes, high-price kilowatt-hours, or peak-event demand change.

The cleanest mechanism is a higher cooling setpoint. If your thermostat maintains 72°F all afternoon while it is 98°F outside, the system fights a larger temperature difference than it would at 75°F or 76°F. The practical estimate often cited is about 3% cooling savings for each degree above 72°F, but the source trail matters: Consumer Reports and Trane discuss it as DOE-linked advice, while United Power attributes a similar claim to NREL [2][3][4]. That disagreement does not make the lever useless. It does mean the figure should stay in the “rough estimate” column.

Setback magnitude and duration matter more than the thermostat brand. A one-degree change for 20 minutes while someone opens the oven and the back door will not do much. A several-degree setback for a workday, a long errand, or an unused upstairs zone has a better chance of reducing compressor runtime. The thermostat’s job is to make that setback repeatable without forcing someone to remember it at the worst moment of the day.

This is where smart features can be useful without magic. Geofencing can avoid cooling an empty home to the same temperature used when people are cooking dinner. Adaptive recovery can start cooling early enough to avoid an uncomfortable return, but late enough to avoid running the compressor longer than necessary. Vacation and away modes prevent a forgotten 72°F setting from becoming a week-long cooling schedule. Those features save only if the household lets the setpoint float during periods when comfort and safety allow it.

A reasonable heat-wave schedule therefore starts with people, not percentages. Keep occupied hours at the warmest temperature that is safe and tolerable for the household. Use a warmer setting when the home is empty. Avoid deep setbacks if recovery would run the system continuously into an expensive peak period or leave vulnerable occupants overheated. If you are trying to compare models rather than settings, a separate smart thermostat savings comparison is the better place to sort device-level claims from heat-wave behavior.

The 78°F debate is less important than the shape of the day

The temperature printed in an advice article does not know your insulation, duct leakage, afternoon sun exposure, humidity, medical needs, or electric rate. During a heat wave, the shape of the cooling day often matters more than the single setpoint: when the compressor starts, how long it runs, whether it runs hardest during the utility’s expensive window, and whether the home ever gets too warm to recover safely.

For a flat-rate customer, a warmer setpoint mainly saves by cutting total cooling runtime. For a time-of-use customer, the same temperature choice may also save by avoiding expensive late-afternoon or early-evening kilowatt-hours. For a demand-response participant, the thermostat may add a temporary adjustment during a grid event. Those are three different paths to savings, and they should not be collapsed into one promised percentage.

A practical heat-wave setting plan looks something like this:

• Occupied hours: choose the warmest safe, comfortable cooling setpoint the household can actually live with.
• Unoccupied hours: raise the setpoint enough and long enough to reduce compressor runtime, while leaving time for recovery before people return.
• Sleep: use a separate setting if nighttime comfort or health needs differ from daytime comfort.
• Peak-price hours: if you have time-of-use rates, check whether the thermostat is running hardest during the expensive window.
• Extreme heat safety: override the savings schedule when indoor conditions become unsafe, especially for children, older adults, people with health risks, and pets.

The comfort clause is not decoration. Heat-wave advice that ignores indoor safety is bad energy advice. A setting that looks efficient in an app but leaves someone dizzy, unable to sleep, or unable to cool down is not a successful setting.

Precooling can help, but only under the right rate and building conditions

Precooling is the heat-wave tactic most likely to be oversold. The idea is simple: cool the house earlier, before the expensive or hottest hours, then let the temperature drift upward while the air conditioner runs less during the peak. On a time-of-use plan, that can lower the bill even if total cooling energy does not fall much. On a flat rate, the same tactic may simply move or increase energy use.

The evidence is conditional. NPR’s 2025 reporting on NREL work described roughly 32% cooling-cost savings from precooling in high-performance building envelopes, while also noting mixed or no savings elsewhere and the importance of time-of-use rates [6]. That is a narrower and more useful claim than “precool your house to save money.” A tight, well-insulated home can store some of that earlier cooling. A leaky home with strong solar gain may give it back quickly, leaving the system to run both early and late.

The test is not whether the house feels wonderfully cold at 1 p.m. The test is whether the thermostat avoids enough expensive or peak-hour runtime later to justify the earlier cooling. If your utility charges the same price all day, the bar is higher: precooling must reduce total compressor work, not merely shift it. If your rate has a sharp afternoon peak, a controlled morning or early-afternoon precool may be worth testing.

A cautious precooling experiment should be modest. Drop the setpoint by a few degrees before the peak window, then let it rise during the peak. Do not turn the home into a refrigerator in the morning and call the later discomfort “savings.” If the thermostat app shows longer total runtime and the bill does not benefit from off-peak pricing, the tactic is probably solving the wrong problem.

Demand response is a grid tool first

Utility demand-response programs deserve a separate bucket. In these programs, the thermostat can be adjusted during a grid event, usually with advance notice and an override option. The household may receive a rebate, bill credit, or program incentive. The grid gets a coordinated reduction during the hour when everyone’s air conditioner wants to run.

The event numbers are meaningful, but they come from operator-reported case studies. Google reported that Consumers Energy saw about 0.9 kW of load reduction per thermostat event in 2020, and that five utilities averaged about 1.06 kW per thermostat event from 2020 to 2022. The same Google Sustainability story said Nest demand-response programs involved more than 110 utilities and more than 1 million enrolled thermostats, and described a 75 MW peak reduction during a 2022 California heat wave [7].

That is useful grid evidence. It is not the same as an independent finding that your summer bill will fall by a specific percentage. For a household, the trade is more concrete: you allow limited thermostat adjustments during stressed hours in exchange for the program’s incentive and possible bill effects. Before enrolling, read the event rules, override policy, incentive structure, and whether the program can change your setpoint during the exact hours your home is hardest to keep comfortable.

How to verify whether your own heat-wave settings saved money

The only savings number that can settle the household question is your own usage under comparable conditions. One bill will rarely prove perfect causality, because weather, occupancy, humidity, utility rates, and billing-cycle length all move at once. But a disciplined comparison is still much better than borrowing a percentage from a manufacturer page or a utility press release.

Start by saving four things in the same place: the thermostat schedule or runtime screenshots, the utility bill, the rate plan, and notes about the heat wave. If your utility offers daily or hourly usage data, use that instead of waiting for the monthly bill. If your thermostat app reports cooling runtime, keep screenshots before changing the schedule and after the heat wave passes.

Then compare like with like. A useful comparison might be this heat-wave week versus a similar hot week before the setting change, or this billing cycle versus last year’s similar period if occupancy and equipment have not changed much. If you changed the thermostat from 72°F to 75°F during occupied afternoons, look for two signals: lower cooling runtime and lower kWh during the relevant hours. If you enabled precooling, check whether peak-hour kWh fell enough to matter under your rate plan. If you joined demand response, separate the incentive or credit from the energy-use change.

A simple worksheet can be enough:

1. Write down the old schedule and the new schedule.
2. Mark the heat-wave dates and any days when the house was unusually occupied or empty.
3. Record whether the rate is flat, time-of-use, or tied to a demand-response program.
4. Compare cooling runtime before and after the change, if the thermostat provides it.
5. Compare kWh for similar days or hours, not just the final dollar total.
6. Subtract or separately note rebates, demand-response credits, and rate-plan effects.
7. Decide whether the comfort trade was acceptable enough to repeat.

For households building automations around this, the same logging habit matters. Heat-wave automations can close shades, adjust setpoints, or prepare for an extreme heat warning, but the thermostat savings still need to be checked against runtime and bills. If you want to turn the measurement into repeatable rules, companion guides on smart-home automations that save money and extreme heat safety automations are better next steps than chasing another universal setpoint.

The trustworthy answer is conditional, but it is not empty. Higher setpoints can reduce compressor runtime. ENERGY STAR-certified smart thermostats have a defensible annual savings basis. Independent savings findings are more modest than the largest manufacturer claims. Precooling belongs mainly to time-of-use rates and tighter homes. Demand response can cut about a kilowatt during events, while proving something different from household bill savings. The percentage worth keeping is the one you can tie back to your own schedule, rate, weather, runtime, and bill.

References

1. Smart Thermostats FAQs for EEPS, ENERGY STAR
2. Best Setting for Your Central Air Conditioning, Consumer Reports
3. Should You Really Set Your Thermostat to 78 in the Summer?, Trane
4. Save Energy During Summer Heat Waves, United Power
5. How much money can a smart thermostat realistically save during a heat wave?, CBS News
6. How effective is precooling your home during off-peak hours? It depends, NPR
7. One click away: your home's thermostat can save energy and the grid, Google Sustainability
8. DOE Deletes Webpage Instructing People To Lower Thermostat To 78, Newsweek
9. Why 78 degrees isn’t the right thermostat setting for everyone during a heat wave, The Conversation