What this calculator does
Smart-thermostat marketing loves a round number — “save up to 23%!” — but the honest answer is that your savings depend on two things you control: how much you spend on heating and cooling, and how aggressively you let the temperature drift when no one needs it. This calculator takes both, applies the federal setback rule, and returns an annual dollar estimate plus the payback period on the hardware.
It’s deliberately conservative. Rather than quoting a best-case number, it models the one behavior that reliably saves energy — setback — and caps the result at a realistic ceiling so you don’t walk away expecting savings that only show up in a lab.
How to use it
- Average monthly energy bill. Use your all-in number. If you heat with gas and cool with electricity, add both bills together and divide by 12.
- Share from heating & cooling. HVAC is the biggest slice of most home energy use — roughly 45–55%. Nudge this up if you run a furnace and AC hard, down for a mild climate.
- Setback amount and hours. How many degrees you let the house drift, and for how many hours a day. Eight degrees for twelve hours (overnight plus an empty workday) is a realistic default.
- Thermostat cost. Used only for the payback estimate — subtract any utility rebate you qualify for.
The formula, in plain English
The Department of Energy’s guidance is that you can save “as much as 10% a year on heating and cooling by simply turning your thermostat back 7°–10°F for 8 hours a day.” That works out to a clean rule of thumb:
We multiply that percentage by the part of your bill that actually goes to heating and cooling. The ENERGY STAR benchmark shown beside the result — a flat 8% — is what the average certified thermostat delivers across thousands of real homes, so you can see how your schedule compares to the typical install.
How accurate is it?
It’s an estimate, and it’s honest about that. The 1%-per-degree relationship is a heuristic the DOE itself publishes; real homes vary with insulation, how long the building holds heat, and how often you override the schedule. Two guardrails keep the number grounded:
- Savings are capped at 25%. Setback alone rarely beats that, even with an aggressive schedule.
- Plug in 8°F for 8 hours and you’ll land exactly on the DOE’s published 8% — a quick sanity check that the math matches the source.
How to actually hit these numbers
A smart thermostat doesn’t save anything on its own; the schedule does. To get the figure this tool shows:
- Turn on geofencing so the system sets back the moment the last phone leaves, then recovers before you’re home.
- Let the overnight setback run — most people lose savings by overriding it the first cold night. Add blankets, not degrees.
- In summer, do the reverse: let the house drift warmer when empty rather than holding one temperature around the clock.
- Check for a utility rebate before you buy — many US utilities knock $50–$100 off a qualifying smart thermostat, which can halve the payback period.
Frequently asked
How much does a smart thermostat really save?
ENERGY STAR puts the average certified smart thermostat at about 8% off heating and cooling costs. With a deliberate setback schedule — turning the temperature back 7–10°F while you sleep and when nobody is home — the Department of Energy estimates up to 10% or more. For most US households that lands somewhere between $80 and $200 a year.
What setback schedule saves the most without being uncomfortable?
A common winter pattern is 68°F while you are awake and home, dropping to about 60°F overnight and while you are out — roughly an 8°F setback for 12–16 hours a day. In summer you do the reverse: let the house drift warmer when empty. Geofencing and learning schedules automate this so comfort returns before you do.
Does a smart thermostat save money in every climate?
Savings scale with how much you spend heating and cooling. Homes in hot or cold climates with big seasonal swings save the most; mild coastal climates save less because there is simply less HVAC spend to trim. Adjust the "share of bill from heating & cooling" slider to match your home.
How long until a smart thermostat pays for itself?
Divide the installed cost by the annual savings. A $130 thermostat saving $115 a year pays back in just over a year; add a utility rebate (many offer $50–$100) and payback can drop under a year.