Best smart thermostat

How to choose

Check that your system can run one

Turn the system off at its switch or breaker, then take the cover off your current thermostat.³ The Nest, ecobee and X2S need a standard 24-volt system.^{2 3 4} Amazon's help page states no voltage, so run its compatibility checker.²⁷ Thick wires joined with wire nuts, or a thermostat labeled 120 or 240 volts, mean a high-voltage system, usually electric baseboard heat. Google says only a professional should install or remove one of those thermostats, and you need a line-voltage model, which this guide doesn't cover.³

Google also rules out millivolt wall heaters and proprietary systems, whose wires have odd labels such as 1, 2, 3 or A, B, C.³ If you have a variable-capacity heat pump or air conditioner, ask your installer first, because ENERGY STAR says these generally perform best with the thermostat their maker recommends.²⁸ Ductless mini-splits are outside this guide.

Look for a C wire

Look for a wire on the C terminal, or a spare wire tucked behind the plate.³ If you find one, all four picks are open to you. If you don't, your choices are the Nest, or a kit, adapter or new wire for the others.

If you have a heat pump, start with the backup heat

HVAC School says many heat pump problems trace to backup heat that a thermostat can't stage or that was set up wrongly.²⁹ Energy Vanguard warns that deep setbacks, where you let the house cool a long way, can trigger electric strip heat during recovery and raise the bill.³⁰

Expect a small saving

Energy Trust of Oregon compared 924 gas-heated homes with similar homes and found a saving of 3 to 4 percent of gas.⁶ A Northwest study of 678 homes found 5 percent, but electricity use in those gas furnace homes rose 2.4 percent, possibly from furnace fans or cooling.⁷

The strongest study found no saving. In two randomized field experiments with nearly 1,400 volunteer households in Northern and Central California, reported in a working paper, electricity fell 0.09 percent and gas rose 1.70 percent over 18 months, and neither change was statistically significant. People overrode their settings nearly 1.7 times a day on average, usually toward using more energy.⁵

Advertised numbers use a smaller base than your bill. ecobee's up to 23 percent comes from its own 2021 analysis of select customers, compared with holding 72°F.² ENERGY STAR says about 8 percent of heating and cooling bills.²⁸ A national lab notes that a 10 percent cooling saving may equal only 2 percent of a home's electricity.³¹

If your electricity price changes through the day, automation can help. In another randomized study, a feature that responds to time-of-use prices cut afternoon air conditioning.³²

Don't pay more to save more

No independent study shows that a pricier model saves more. At ENERGY STAR's average saving, the Nest Learning takes about five to six years to pay back at list price, the ecobee about three, and the X2S or Amazon under two.^{1 2 24 26 28} If your saving is closer to the billing studies, it will take longer.

Buy a certified model, but don't read certification as a ranking. A Northwest study tried to predict billing savings from ENERGY STAR's runtime measures and found very weak or no correlation.⁷ Certification can make a thermostat eligible for rebates. Federal rebates are largely gone, but some utilities still pay them, so ask yours before you buy.⁹

Read the terms, because some programs enroll you in remote adjustments on peak days. In Texas in 2021, some owners who had opted in were surprised when settings rose by up to 4 degrees in a heat wave.³³ If you enroll, learn how to override an event.

Buy sensors for comfort, not savings

A 2026 study of 20 homes found an average spread of 4°F within a house, and a wall thermostat reads one spot.³⁴ Room sensors shift comfort around the house, and nobody has shown that they cut bills.

How many you need

You need one thermostat for each zone, meaning each part of the house heated and cooled separately. For a hot or cold room inside a zone, add a room sensor rather than a second thermostat.

Don't buy an old Nest

Google ended app support for the 1st and 2nd gen Nest Learning on October 25, 2025, so avoid them used.³⁵ They still work as thermostats on the wall.³⁵ ENERGY STAR requires certified models to work as a basic thermostat without a connection, so the risk with any of these is losing the app, not your heat.³⁶

After you install it

Set a schedule, turn on home and away detection, and go easy on manual changes. In those California experiments, homes that used schedules the way engineers assume saved as much as 8 to 19 percent of gas, though those homes weren't randomly assigned.⁵

If your old thermostat contains mercury, keep it out of the trash. Take it to a household hazardous waste site.³⁷