Every smart thermostat review eventually quotes “up to 23% energy savings.” That number comes from a decade-old study, conducted in a climate zone most people don’t live in, on HVAC systems that predate modern variable-speed compressors.
The real number is highly specific. Here’s the framework that explains why my Nest saved $38/month — and why my neighbor’s saved $6.
The four factors that actually determine savings
1. Your existing schedule discipline. If you already manually adjust your thermostat consistently, a learning thermostat won’t save much. It automates what you were already doing. The savings come from automating what you weren’t — the mornings you forgot, the extended business trips, the unexpected overnight guest.
2. Your HVAC’s efficiency class. A modern variable-speed heat pump responds to thermostat setback faster than a single-stage gas furnace. Savings from schedule optimization are significantly higher on modern HVAC.
3. Your rate structure. Time-of-use electricity pricing makes smart thermostats dramatically more valuable. If you’re paying 3× peak rates between 4–9 PM, a thermostat that pre-cools before peak hours earns real money every day.
4. Your home’s thermal mass. A well-insulated house holds temperature longer, meaning the furnace runs less after a setback. A drafty 1960s split-level fights back against every setpoint change.
What we measured
Across six homes over four months, the average monthly savings were $24. The range was $6 to $51. The highest-saving installation was a combination of time-of-use rates, a 2022 heat pump, and an owner who previously never adjusted the thermostat manually. The lowest was a well-insulated 2020 build with a homeowner who already optimized manually.
The takeaway: don’t buy the average. Calculate your specific case. The smart thermostats guide includes a back-of-envelope calculator in the buying factors section.