How Smart Thermostat Precooling Saves Energy in a Heat Wave

Start with the rate plan, not with a favorite temperature. A smart thermostat can help save during a heat wave when it cools the house before the expensive or grid-stressed window, lets the indoor temperature rise during that window, then restores comfort after it ends. On a flat-rate plan, that same move is weaker: you may still improve comfort and avoid a brutal late-afternoon recovery, but you are not shifting runtime into cheaper hours.

ENERGY STAR’s 78°F belongs in this conversation as a cost-management starting point for cooling, not as a law for every person, room, medical condition, humidity level, or heat-wave afternoon [1][2]. Use it as the middle of the recipe, then adjust the schedule to your actual utility window.

The flat-rate recipe: keep the average temperature sane

If every kilowatt-hour costs the same, precooling is not a magic discount. Lowering the thermostat early can make sense when the house always falls behind at 4 p.m., but the savings case is narrower: you are trying to avoid emergency-style cooling, short cycling, and an uncomfortable hallway later in the day. You are not moving that cooling into a cheaper price block.

• Set the normal occupied cooling target around 78°F if that is safe and tolerable for your household [1][2].
• If the late afternoon routinely overheats the house, lower the setpoint modestly before the hottest indoor period, then come back to the normal occupied target instead of staying overcooled all evening.
• Do not copy a 4–9 p.m. time-of-use recipe just because it sounds efficient. On a flat-rate plan, aggressive pre-peak cooling can simply add runtime.
• If humidity is the real problem, do not let an Away or Eco setting keep the AC off so long that the house feels damp when you return.

For a broader flat-rate heat-wave routine—blinds, fans, runtime timing, and what to skip—use the fuller smart home cooling tips only after the thermostat schedule is no longer the bottleneck.

APS 4–7 p.m.: the short peak-window recipe

APS gives the most directly copyable pattern for a three-hour on-peak window: cool the house a few degrees lower before 4 p.m., raise the thermostat during the 4–7 p.m. on-peak period, and return to normal after 7 p.m. Its published precooling page includes a customer example that uses 72°F off-peak and 80°F on-peak [3].

The 72°F number is not an invitation to turn every house into a refrigerator. It is a sourced APS customer example inside an APS article, useful because it shows the size and direction of the move. If your house gets clammy, your upstairs bedrooms overshoot, or anyone in the home is heat-sensitive, narrow the swing before you chase a bill result.

SCE and PG&E-style 4–9 p.m.: the long coast

A 4–9 p.m. peak window asks more from the house. California ISO Flex Alert guidance uses the same basic shape—pre-cool to 72°F earlier in the day, then set AC to 78°F or higher from 4–9 p.m. if health permits [4]. PG&E time-of-use scheduling guidance described by Pearce HVAC uses the same pre-cool, coast, recover logic for a 4–9 p.m. weekday peak window and notes that off-peak electricity can be 10–30% cheaper [5]. For SCE households, SCE frames reduced peak-time usage as demand response, while an SCE-bill troubleshooting guide gives the practical 72–74°F before peak and 78–80°F+ during 4–9 p.m. thermostat range [6][7].

This is where many “set it to 78” answers become too thin. At 2:30 p.m., lowering the thermostat may be the right move for a 4–9 p.m. household because the expensive window has not started yet. The same tap may be pointless on a flat-rate plan, or too late on a house that needs several hours to shed heat.

Why precooling has better evidence than most heat-wave thermostat tips

The useful pattern shows up in more than one place: cool lower before the peak, coast higher during the peak, restore after. APS publishes it for a 4–7 p.m. on-peak window [3]. California ISO publishes the 72°F pre-cool and 78°F-or-higher 4–9 p.m. Flex Alert version, with the health-permitting caveat attached [4]. That does not prove every household saves money, but it is stronger than a comfort rule passed around without a rate window.

The harder measured evidence comes from demand-response work, not from a single household’s bill. Ecobee’s eco+ evaluation page describes a summer 2019 pilot with about 240,000 thermostats, using a Randomized Encouragement Design with climate-zone stratification. It reports average demand-response savings of 0.91 kW per opt-in thermostat in 2019 and 1.12 kW in 2020, with load reductions largest in the first event hour and increasing with outdoor temperature [8].

That is evidence of reduced load during events. It is not the same thing as a guaranteed percentage off your bill. A household bill depends on the rate plan, weather, insulation, equipment sizing, humidity control, occupancy, and how much comfort you give back after the event.

Ecobee’s more bill-shaped number is useful, but it needs its label. The company says eco+ Time of Use can save 7–26% on cooling costs, based on a third-party June–August 2020 U.S. and Canada study commissioned by ecobee [9]. That is not worthless; it is just not the same kind of proof as the measured demand-response load reductions above.

If you want the same Cal ISO 72°F/78°F logic in a heat-dome format, compare the related heat-dome thermostat schedule and the per-brand California heat-wave settings after you have confirmed your actual rate window.

How to run the recipe on ecobee, Nest, and Home Assistant

Ecobee: use eco+ Time of Use if your utility and plan match

Ecobee is the cleanest advanced path when eco+ Time of Use supports your utility and rate plan. The feature is designed to shift cooling away from expensive periods, and ecobee’s own savings claim is specifically tied to Time of Use cooling-cost reductions [9]. Set your comfort bounds first, then let the feature do the price-window work. If you override it every afternoon because the peak setpoint is too high, lower the ambition rather than pretending the automation failed.

Nest: schedule the window, then leave Airwave alone

Nest can run the same recipe with a normal schedule: a pre-cool period before peak, a higher setpoint during peak, and recovery after the window. Keep the fan on Auto unless you have a separate reason to circulate air; running the fan continuously can work against dehumidification in some homes.

Airwave is worth leaving enabled, but it is not a precooling engine by itself. Google says Airwave turns the compressor off slightly before the target temperature, then runs the fan on residual coil cooling for about 5–10 minutes, and it only works when indoor humidity is low enough [10]. On sticky days, Airwave may disable itself. That is a humidity protection behavior, not a sign that the thermostat forgot the schedule.

For Nest-specific feature behavior—Airwave, Eco Temperatures, schedule holds, and Smart Recovery conflicts—use the deeper copyable thermostat heat-wave settings guide.

Home Assistant: automate the window, not the weather drama

A Home Assistant setup should make the rate window explicit. Keep the logic boring: one helper for the peak window, one pre-cool action, one peak setpoint, and one recovery action. Do not bury the 4–9 p.m. rule inside a tangle of outdoor-temperature conditions until the plain version works.

alias: TOU heat-wave precool example
mode: single
trigger:
  - platform: time
    at: "14:00:00"
  - platform: time
    at: "16:00:00"
  - platform: time
    at: "21:00:00"
action:
  - choose:
      - conditions: "{{ now().hour == 14 }}"
        sequence:
          - service: climate.set_temperature
            target:
              entity_id: climate.downstairs
            data:
              temperature: 73
      - conditions: "{{ now().hour == 16 }}"
        sequence:
          - service: climate.set_temperature
            target:
              entity_id: climate.downstairs
            data:
              temperature: 79
      - conditions: "{{ now().hour == 21 }}"
        sequence:
          - service: climate.set_temperature
            target:
              entity_id: climate.downstairs
            data:
              temperature: 76

Treat that as an example shell for a 4–9 p.m. plan, not as a universal set of temperatures. Replace the entity, times, and setpoints with the schedule that matches your rate plan and household limits. If you are still choosing hardware for this kind of setup, check smart thermostat and cooling-device compatibility before building automations around features the device does not expose.

Mistakes that erase the savings

• Overcooling below the sourced recipe range. The schedules here bottom out around 72°F because that is where APS and Cal ISO examples land, not because colder is always better [3][4].
• Letting a flat-rate house copy a 4–9 p.m. bill-shifting schedule. If there is no cheaper off-peak block, the pre-cool period has to justify itself through comfort or reduced runtime—not through time-of-use arbitrage.
• Letting the house climb so high that humidity and recovery take over. A high peak setpoint that looks elegant in the app can feel miserable if the AC stays off long enough for indoor moisture to build.
• Assuming Nest Airwave is broken during muggy weather. Airwave requires low indoor humidity and may stop using residual coil cooling when humidity is too high [10].
• Counting vendor savings ranges as guaranteed bill cuts. Ecobee’s 7–26% cooling-cost number is a vendor-commissioned third-party study; the demand-response pilot numbers measure load reduction during events [8][9].

Precooling is the best-supported heat-wave thermostat tactic because it gives the thermostat a job it can actually do: move some cooling earlier, coast through a defined peak, and recover after. The copyable part is the pattern. The part you must not copy blindly is the window.

References

1. Smart Thermostats — ENERGY STAR
2. Why 78 degrees isn’t the right thermostat setting for everyone during a heat wave — The Conversation
3. Precooling — APS
4. Flex Alerts can play an important role during extreme conditions — California ISO
5. PG&E Time-of-Use HVAC Scheduling Guide — Pearce HVAC
6. What is Demand Response — Southern California Edison
7. Why Nest & Ecobee Smart Thermostat Increased SCE Bill: Los Angeles & Ventura County Fixes — Affordable Air LA
8. eco+ EM&V — ecobee
9. eco+ — ecobee
10. Learn about Nest thermostat Airwave — Google Nest Help