Smart Thermostat Savings Calculator

ENERGY STAR says certified smart thermostats save more than 8% of heating and cooling energy, about $50 a year for the average household. DOE says a 7 to 10 F setback for 8 hours a day saves up to 10% a year. On a $1,000 heating and cooling bill that is $80 to $100.

Price divided by yearly savings. At $72 a year, a $100 thermostat pays back in about 17 months and a $250 one in about 3.5 years. Bigger bills pay back faster.

Many need one. The ecobee Enhanced and Premium include a Power Extender Kit for homes without one. The Nest Learning Thermostat (4th gen) runs without a C-wire in most homes. The Amazon Smart Thermostat needs a C-wire or its C-Wire Power Adapter, also sold as a bundle. The Google Nest Thermostat runs without one in most homes, but heat pump, heating-only, cooling-only and zoned systems need a C-wire or the Nest Power Connector. The tool asks about your wiring and adds the part when you need it.

Be careful. DOE says a setback on a heat pump in heating mode can trigger inefficient operation and cancel the savings. Keep setbacks small or use a thermostat with heat pump logic.

They are best cases under the maker's conditions. We use ENERGY STAR's independent average (more than 8%) and DOE's setback figure (up to 10%) instead.

About $6 a month for the ENERGY STAR average home: $900 a year of heating and cooling x 8% = $72 a year. On a $180 monthly energy bill with a nightly 7 to 10 F setback, the tool's math gives about $112 a year, a little over $9 a month. The savings land mostly in the months the heat or AC runs hardest, not evenly.

DOE suggests 68 F in winter while you are awake and lower while you sleep or are away, and 78 F in summer when you are home and need cooling. A 7 to 10 F setback for 8 hours a day saves up to 10% a year on heating and cooling, per DOE. Heat pump homes should keep winter setbacks small.

No. DOE explains that a house loses heat more slowly when it is cooler inside, so the energy saved during the setback is more than the energy needed to warm back up. The same logic applies to raising the setpoint in summer. The exception is a heat pump in heating mode, where a big recovery can bring on electric backup heat.

In practice, the smart one. ENERGY STAR ended its programmable thermostat label in 2009 because many people never set the schedules, and it now certifies smart thermostats on field data showing more than 8% average heating and cooling savings. A programmable thermostat that you actually program can still earn DOE's setback savings of up to 10%.

Usually, if your system uses a low-voltage (24 V) thermostat, which most furnaces, central ACs and heat pumps do. Turn off the HVAC power at the breaker, photograph and label the wires, and check for a C-wire before you buy. Line-voltage systems like electric baseboard heaters need a different kind of thermostat, and the Amazon Smart Thermostat, for example, is not for baseboard heat.