Best smart thermostat: ENERGY STAR's own figure is 8% (about $50 a year), so the C-wire line on each maker's page decides more than the screen does

If your heater is a 120 V or 240 V baseboard, this is the wrong page: every thermostat below is a 24 V device, and the last section names the kind you need instead.

How was this shortlist built?

OwnerSpec compared manufacturer-published specifications and ENERGY STAR’s published list on 2026-10-03 and confirmed every linked listing in stock the same day. Nothing on this page was installed, run or measured.

The fields compared are the ones a maker or ENERGY STAR prints: the certified list row and its date, the C-wire requirement and the maker’s substitute, stages and heat pump support, remote sensor, display, voice assistant, Matter and warranty. “Certified” here means the model appears in ENERGY STAR’s own dataset, not that the maker says so.

Three candidates were dropped: Honeywell’s page marks the T9 kit “Discontinued and sold out,” the Google Nest Thermostat was not confirmed in stock on the day, and the Honeywell T5 was not buyable.

What does ENERGY STAR certified actually promise?

ENERGY STAR’s technical bulletin says certified smart thermostats “have been independently certified to deliver energy savings based on actual field data. In fact, the average home saves 8% on heating and cooling costs, or $50 per year.” That is the number to hold every maker claim against. The key product criteria page prints the floor a model has to clear:

The bulletin adds that “many do recommend a C wire for new installations,” and that “with variable-speed equipment, the thermostat recommended by the equipment manufacturer should be used.” The setback savings calculator turns the 8% into dollars at your own bill.

Which thermostats need a C wire, and what does each maker ship instead?

Honeywell Store’s guide defines the term: “A C-wire, also called a common wire, provides a continuous 24V power path from the HVAC system to the thermostat.” R supplies the 24 V, C completes the path. The C-wire guide walks through the wall plate.

ecobee’s Power Extender Kit page sets the kit’s limit: it “only works with 24 VAC systems. Millivolt, high-voltage systems, dual transformer and fan coil or communicating units are incompatible.”

How do the units compare on paper?

“Not printed” means the document read does not list it. “Not confirmed” means no maker document for that model was readable.

ecobee Smart Thermostat Premium: buy it or skip it?

Buy it if the wall has no C wire and you want a room sensor from day one. ecobee’s page prints 24 VAC through terminals C and Rc, the Power Extender Kit and a SmartSensor in the box, furnaces, air conditioners and heat pumps up to 2-stage, a 4 in 540 x 540 touchscreen, Alexa built in, Siri through an Apple HomePod, dual-band Wi-Fi and a 3-year limited warranty. ENERGY STAR’s list carries the EB-STATE6 row from 2023-04-25.

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The flaw is the stage count: two stages of heat is the ceiling ecobee prints, so a 3-stage system is the Nest’s territory. Matter is not on the page read.

Skip it if the furnace is variable-speed: ENERGY STAR’s bulletin defers to the equipment maker.

Google Nest Learning Thermostat (4th gen): buy it or skip it?

Buy it if the system has more stages than a two-stage thermostat can drive, or for Matter. Google’s technical specifications page prints 20-30 VAC input, W1-W3 heating and Y1-Y3 cooling terminals, a 2.68 in 600 x 600 display, Wi-Fi on 2.4 and 5 GHz, Google Home and Matter, the Nest Temperature Sensor (2nd gen) and a rechargeable lithium battery. Google’s compatibility page adds up to 2 cooling and 3 heating stages, and up to 4 heating stages on a heat pump including 2 heat pump stages; the linked bundle includes one sensor.

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The flaw is the C-wire answer, which is “it depends.” Google’s help page says only that the Google Home app “will often tell you during setup if you need to connect a C wire,” with the Nest Power Connector as the fallback. No warranty figure appears on the spec page read.

Skip it if you want Alexa or Siri inside the thermostat: Google’s page lists Google Home and Matter, nothing else.

ecobee Enhanced and Essential: buy them or skip them?

Buy the Enhanced for the Premium’s wiring answer without the sensor in the box. ecobee prints the Power Extender Kit included, 2H/2C conventional systems and heat pumps with 1 stage of AUX heat, SmartSensor compatibility, Apple HomeKit, Alexa and SmartThings, and a 3-year warranty that becomes 5 with professional installation. Its ENERGY STAR row, EB-STATE6L, is dated 2025-12-16.

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Buy the Essential only if a C wire is already there, or you will add the kit. ecobee’s page says it “requires a common wire,” sells the Power Extender Kit separately, and prints 2 cooling plus 1 heating stage or 2 heating plus 1 cooling, dual-fuel heat pumps, radiant heat and a 3-year warranty.

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The flaw with the Essential is a paperwork gap: ecobee says ENERGY STAR certified, yet the ENERGY STAR dataset queried on 2026-10-03 returned no Essential row among its 120 rows. Until it does, this page credits the claim to ecobee, not to ENERGY STAR.

Sensi Touch 2: buy it or skip it?

Buy it if the wall has a C wire and the system is a dual-fuel heat pump. Copeland’s spec sheet R-5530 prints 20 to 30 VAC, 1.5 A per terminal and 2.5 A total, single and multi-stage gas, oil, electric and heat pump systems, “automatic dual fuel control without an outdoor sensor,” a 45 to 99 F setpoint range, the RS01-SG room sensor, Wi-Fi on 2.4 or 5 GHz, Alexa, Google Assistant and SmartThings, and ENERGY STAR CERTIFIED. The list row is ST76, dated 2022-06-28.

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The flaw is the first line on the sheet: “All systems require a common wire (c-wire).” The sheet names a common wire kit, SA11, as an accessory, not as box contents. The sheet prints a 3-year limited warranty; Matter is not printed.

Skip it if you have four wires and no C: the ecobee kits above answer that wall.

Amazon Smart Thermostat and Honeywell Home RTH9600WF: buy them or skip them?

Buy the Amazon Smart Thermostat if a C wire is present and the ENERGY STAR row is all you need. The certified list carries model S6ED3R from 2023-06-21, and the listing title prints “C-wire required” and “Works with Alexa and Ring.”

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The flaw is that this page can say nothing else about it: Amazon’s compatibility help page returned an error every time, so heat pump, dual-fuel and adapter details are unconfirmed.

The Honeywell Home RTH9600WF sits in the same position: an ENERGY STAR row (RTH9600WF2022, 2019-08-01) and a listing title printing “C-Wire Required” and “Alexa Ready,” with no Resideo spec page opened this session.

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Skip both if there is no C wire at the wall; neither listing names an adapter in the box.

Who should skip every thermostat here?

Electric baseboard, wall or ceiling heat on 120 V or 240 V. Google’s compatibility page says “high voltage systems, such as electric baseboard heaters, aren’t compatible with Nest thermostats.” ecobee’s support page says a high-voltage thermostat is labelled 120 V or 240 V, or has L1/L2 labels and thick wire nuts, and is not compatible unless a professional fits a step-down transformer. Mysa’s line-voltage baseboard thermostat is rated for 120-240 VAC at a 16 A maximum load, 3,800 W at 240 V and 1,900 W at 120 V; Mysa says regular low-voltage smart thermostats are not compatible.

Millivolt heaters. Google and ecobee both list millivolt wall and floor heaters as incompatible.

Proprietary, communicating and variable-speed systems. ecobee lists non-standard wire labels (1, 2, 3, 4 or A, B, C, D) and communicating units as incompatible; ENERGY STAR’s bulletin covers variable-speed equipment, above.

What goes wrong?

The thermostat reboots or the battery drains. Google’s C-wire page says a Nest Thermostat E or a 3rd-gen Learning Thermostat with power issues or a draining battery needs a C wire.

Power was never switched off. ecobee’s support page says to cut power at the HVAC switch or the breaker first, then confirm by changing the setpoint at least five degrees.

The filter is the real problem. A furnace that cannot hold temperature is not a thermostat fault; the furnace filter interval is the first and cheaper check.

The savings never show up. ENERGY STAR’s bulletin says owners “will need to connect their smart thermostats to WiFi to realize the full benefits,” and 8% is an average, not a promise.

Frequently asked questions

Sources

1. ENERGY STAR, Technical Bulletin: ENERGY STAR Certified Smart Thermostats (PDF) (government or regulatory source)
2. ENERGY STAR, Smart Thermostats Key Product Criteria (government or regulatory source)
3. ENERGY STAR, Certified Smart Thermostats dataset (data.energystar.gov) (government or regulatory source)
4. ENERGY STAR, Smart Thermostats product page (government or regulatory source)
5. ecobee, Power Extender Kit (manufacturer or vendor document)
6. ecobee, Smart Thermostat Premium product page (manufacturer or vendor document)
7. ecobee, Smart Thermostat Enhanced product page (manufacturer or vendor document)
8. ecobee, Smart Thermostat Essential product page (manufacturer or vendor document)
9. ecobee support, What types of HVAC systems are ecobee thermostats compatible with? (manufacturer or vendor document)
10. Google Nest Help, Nest thermostat compatibility (manufacturer or vendor document)
11. Google Nest Help, Learn about the common or C wire (manufacturer or vendor document)
12. Google Nest Help, Nest thermostat technical specifications (manufacturer or vendor document)
13. Copeland, Sensi Touch 2 spec sheet R-5530 (PDF) (manufacturer or vendor document)
14. Honeywell Home, T9 Smart Thermostat with sensor and C-wire adapter (manufacturer or vendor document)
15. Honeywell Store, Thermostat C-Wire and Adapter Guide (manufacturer or vendor document)
16. Mysa, Smart Thermostat for Electric Baseboard Heaters (manufacturer or vendor document)