The basic idea is simple: cool the house before the hottest part of the day arrives, so the AC runs mostly during cheap, off-peak electricity hours rather than during expensive peak ones. Banks of cool air in the walls and furniture buy you time.
The schedule that works
This is a two-part setup in your smart thermostat:
Off-peak window (typically midnight to 3 PM, but check your utility): Pull the setpoint down to 74-76°F for a few hours before the heat peaks. The exact window depends on when your utility charges peak rates. Most TOU plans price peak between 4-9 PM. Starting pre-cool by noon gives two to three hours of building up thermal “credit.”
Peak window (typically 4-9 PM): Let it float up to 78-80°F. The AC may still run, but much less than if you hadn’t pre-cooled. You’re drawing down the cool-air reserve rather than fighting outdoor temps with expensive electricity.
After peak (9 PM onward): Return to 78°F or wherever you’re comfortable. Off-peak rates are back; running the system at 78°F costs less per hour.
A Nest, ecobee, or Honeywell T6 Pro can hold this schedule automatically. Look for the “Time-of-Use” or peak savings mode, or just build it as a manual weekday schedule: 74°F at 11 AM, 78°F at 4 PM, 78°F overnight.
Heat wave mode: different problem, same strategy
A heat wave (95°F+ days) adds a wrinkle: the outdoor temperature may be high enough that even a correctly sized system can only maintain about 20-25°F below outdoor temperature. At 100°F outside, holding 75°F indoors is the system working at full capacity. Holding 72°F is asking it to do more than it can.
During a heat wave:
- Pre-cool to 74°F by 10-11 AM, before outdoor temp climbs past 85°F. The system can actually hit that number in the morning. By 3 PM it may not be able to.
- Accept 78-80°F at peak without fighting it. Blocking every window with blinds, limiting appliance use (ovens, dishwashers), and running fans to even temperatures helps more than cramming the setpoint down.
- Don’t turn the system off. Off gives the heat a head start it’s very hard to recover from once the thermometer reads 100°F.
If your system absolutely cannot hold 74°F by 11 AM on a mild day, that’s a separate problem. See Why Your AC Isn’t Cooling.
Does your current system hold up?
Pre-cooling only works if the equipment can hit the early-morning setpoint. If your AC short cycles, ices up, or can’t pull below 76°F even on a 75°F morning, pre-cooling reveals a system problem more than solves it.
Spring is the right time to evaluate this, before you’re trying to manage a heat wave with a marginal system. If you’re thinking about replacing aging central air before next summer, the SEER rating matters more than most people realize for a pre-cooling strategy — a higher-SEER system moves more heat per dollar of electricity, which shows up directly in off-peak pre-cooling cost. Get multiple itemized quotes (equipment efficiency tier, electrical scope, and warranty depth typically explain more of the spread than contractor markup) and use BidSmart to compare them. Getting those bids in spring gives you time to understand the tradeoffs before the heat forces the decision.
TOU on a heat pump vs central AC
If you already have a heat pump, pre-cooling is even more effective because:
- The system can also handle dehumidification overnight at low loads, arriving at peak hours with humidity already managed
- Heat pump efficiency (COP) is highest at lower outdoor temperatures, so early-morning cooling costs less per BTU
- Variable-capacity heat pumps modulate output and can run extended low-power cycles during off-peak windows efficiently
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Related: Best Thermostat Settings for Summer · Best Temperature for AC in Summer · AC Short Cycling
Related reading
Pre-cooling works best alongside a correctly-set away/vacation schedule the rest of the week; see thermostat settings for summer vacation for the setpoint range that balances savings against humidity risk when you’re not actively pre-cooling for a peak-rate window.
