12 min read
I’ve installed seven smart thermostats across three houses and my parents’ place over the past six years. I’ve used every major brand — Nest, Ecobee, Honeywell, and the Amazon Smart Thermostat. And I’ve tracked the energy bills before and after each installation because I wanted to know, with actual numbers, whether these things deliver on their savings promises.
The short answer: yes, smart thermostats save money. But the amount varies wildly depending on which thermostat you buy, how your HVAC system is set up, and — more than anything — how you were managing your temperature before. Someone upgrading from a basic manual thermostat with no schedule will see dramatically bigger savings than someone replacing a decent programmable unit they were already using well.
This guide breaks down the four major smart thermostats on the market, compares their real-world energy savings, walks through installation (including the infamous C-wire problem), and tells you exactly which one pays for itself fastest based on your situation.
The Contenders: Four Smart Thermostats Head to Head
Google Nest Learning Thermostat (4th Gen) — $280
The Nest kicked off the smart thermostat revolution back in 2011, and the fourth generation (released in 2024) is the most refined version yet. The hardware is gorgeous — a polished metal face with a large, borderless LCD display that shows temperature, weather, and energy usage at a glance. It looks like a piece of jewelry on the wall.
Key features: Automatic schedule learning (it watches when you adjust the temperature and builds a schedule around your habits), occupancy sensing via built-in Sensa radar, geofencing through the Google Home app, energy dashboard with monthly reports, and Matter support for cross-ecosystem compatibility.
Compatibility: Works with most 24V HVAC systems including forced air, heat pumps (with auxiliary heat), radiant, and dual-fuel systems. Requires a C-wire or can use the included power connector adapter for most systems. Tight integration with Google Home ecosystem; limited Apple HomeKit support through Matter.
What I liked: The learning algorithm is genuinely impressive. After about two weeks, it had nailed my schedule better than I would have programmed manually. The energy reports are detailed and actually useful — they break down runtime by heating and cooling and show you exactly when the system ran. The Sensa radar for room occupancy is a meaningful upgrade over the older model’s basic motion detection.
What I didn’t like: The Google Home app is required and it’s become bloated with smart home features that slow down simple temperature adjustments. No remote room sensors available (a significant gap compared to Ecobee). And at $280, it’s the priciest option on this list.
Ecobee Smart Thermostat Premium — $250
Ecobee has been the smart thermostat I recommend most often, and the Premium model is why. It’s the most feature-complete thermostat available — built-in Alexa and Siri, air quality monitoring, and a room sensor system that actually solves the “one thermostat for a whole house” problem.
Key features: Built-in Alexa speaker and Siri support, one SmartSensor included (additional two-pack $80), air quality and humidity monitoring, occupancy detection in every sensor, Smart Home/Away detection, bi-directional HVAC monitoring for system health, and comprehensive energy reports.
Compatibility: Works with virtually every 24V HVAC system. Includes a Power Extender Kit (PEK) in the box for systems without a C-wire — this is a huge deal and I’ll explain why in the installation section. Works with Apple HomeKit, Google Home, Amazon Alexa, Samsung SmartThings, and supports Matter.
What I liked: The SmartSensor system is the standout feature. Place sensors in the rooms you use most, and the thermostat averages their readings (or prioritizes occupied rooms) to decide when to run the system. In my two-story house, the upstairs bedrooms were always 4-5 degrees warmer than the main floor where the thermostat sat. Adding two sensors upstairs meant the system actually kept those rooms comfortable instead of overcooling the main floor to compensate. The air quality monitor is a nice bonus — it flagged high VOC levels when I was painting, and the humidity readings help me manage the whole-house humidifier.
What I didn’t like: The built-in Alexa speaker sounds terrible — tinny and quiet. It’s fine for voice commands but useless as a music speaker. The touchscreen interface, while functional, isn’t as intuitive as Nest’s rotary dial. And the unit is physically larger than competitors, which matters if your wall space is tight.
Honeywell Home T9 — $170
Honeywell has been making thermostats since 1906. The T9 is their answer to Nest and Ecobee — a smart thermostat with room sensors at a lower price point. It doesn’t have the polish or learning features of the top two, but it gets the core job done reliably.
Key features: Room sensor support (one included, extras about $40 each), geofencing, flexible scheduling, and integration with Alexa, Google Home, and SmartThings. Multi-zone system support for homes with zone dampers. The Resideo app provides basic energy reports.
Compatibility: Broad HVAC compatibility including conventional, heat pump, and dual-fuel systems. Requires a C-wire — no adapter or power extender included, which is a drawback for older homes. No Apple HomeKit support and no Matter support as of 2025.
What I liked: The room sensors work well and the multi-zone support is unique at this price point. If your house has a zoned HVAC system with dampers, the T9 can optimize each zone based on sensor readings. The Honeywell brand also carries weight with HVAC technicians — they’re familiar with the wiring and won’t give you grief during service calls.
What I didn’t like: The app is functional but dated compared to Nest and Ecobee. No learning capability — you program the schedule manually. The display is basic. And the lack of a C-wire solution in the box is frustrating, especially at $170 where Ecobee includes one at $250. If you don’t have a C-wire, factor in $20-$50 for an adapter or $100+ for an HVAC tech to run one.
Amazon Smart Thermostat — $80
At $80, the Amazon Smart Thermostat is less than a third the price of the Nest and Ecobee. It’s a rebranded Honeywell product with a clean design and tight Alexa integration. The question isn’t whether it’s good — it’s whether it’s good enough.
Key features: Alexa voice control (requires an Echo device — no built-in speaker), basic scheduling, geofencing through the Alexa app, Hunches feature that uses Alexa’s occupancy detection from Echo devices to adjust temperature, and Energy Star certification.
Compatibility: Most 24V systems. Requires a C-wire — includes a C-wire adapter in the box (good). Works with Alexa only — no Google Home, no HomeKit, no Matter. This is the biggest limitation.
What I liked: The price-to-functionality ratio is unbeatable. For $80, you get a programmable smart thermostat with geofencing and Alexa integration. The Hunches feature — where Alexa detects that everyone has left the house and turns down the temperature — actually works surprisingly well if you have Echo devices in a few rooms. I installed this at my parents’ house and their heating bill dropped 18% the first winter. At $80, it paid for itself in under three months.
What I didn’t like: No room sensors. No learning algorithm. The display is basic and the interface is minimal — you’ll do most control through the Alexa app, which isn’t great for quick adjustments. And the Alexa-only ecosystem lock-in is a real limitation. If you ever switch to Google Home or Apple HomeKit, this thermostat becomes a basic programmable unit without smart features.
Real Energy Savings: What the Data Actually Shows
Manufacturer claims are marketing. Here’s what I’ve measured and what independent studies have found.
Google claims the Nest saves “an average of 10-12% on heating and 15% on cooling.” Ecobee claims “up to 26% savings.” Both cite internal studies. Those numbers aren’t wrong, but they’re based on comparisons against homes with no programmable thermostat and no temperature management at all — basically, the best-case scenario.
A more realistic picture comes from independent research. A DOE study found that properly used programmable thermostats (smart or not) save about 10% on heating and cooling annually by reducing temperature 7-10 degrees for 8 hours per day. The “smart” part adds savings through occupancy detection, geofencing, and optimization algorithms — typically an additional 3-8% beyond basic scheduling.
Here’s what I measured across my installations:
Nest (replacing a manual thermostat, 2,100 sq ft house, gas furnace + central AC): 16% reduction in gas usage over the first heating season, 12% reduction in cooling electricity the following summer. Annual savings: approximately $320 on a $2,400 HVAC spend. Payback period: about 10 months.
Ecobee with 3 sensors (replacing a basic programmable, 2,400 sq ft two-story, heat pump): 11% reduction in total HVAC energy over a full year. The sensors were the difference — the system stopped overrunning because it could finally read the actual conditions in occupied rooms instead of just the hallway where the thermostat was mounted. Annual savings: approximately $210 on a $1,900 HVAC spend. Payback period for the thermostat plus two extra sensors ($410 total): about 23 months.
Amazon Smart Thermostat (replacing a manual thermostat, 1,400 sq ft ranch, gas furnace + window AC): 18% reduction in gas usage. The Hunches feature was especially effective here — my parents frequently forgot to turn down the heat when leaving the house, so the automated detection created big savings. Annual savings: approximately $280. Payback period at $80: under 4 months.
Honeywell T9 (replacing a 10-year-old programmable, 1,800 sq ft, zoned forced air): 8% reduction in total HVAC energy. The zone optimization helped, but the savings over an already-programmed thermostat were modest. Annual savings: approximately $150 on a $1,850 HVAC spend. Payback period: about 14 months.
The C-Wire Problem: What It Is and How to Fix It
The C-wire (common wire) is the single biggest installation headache with smart thermostats, and if your house was built before 2000, there’s a decent chance you don’t have one.
Here’s what’s going on: traditional thermostats run on the 24V power from your HVAC system’s transformer, using a few thin wires — R (power), W (heat), Y (cooling), G (fan), and sometimes others. They draw so little power that the signaling wires provide enough juice to operate. Smart thermostats have WiFi radios, displays, processors, and sensors that need continuous power. The C-wire provides a return path to the transformer that allows the thermostat to draw constant power without interrupting HVAC signals.
Without a C-wire, smart thermostats try to “steal” power from the signaling wires. This can work, but it can also cause problems: the furnace or AC clicking on and off erratically, the thermostat losing WiFi or rebooting, or in some cases, the thermostat slowly draining the transformer and shortening its lifespan. I’ve dealt with all three of these issues.
How to Check If You Have a C-Wire
Pull your current thermostat off the wall (most pop off the base plate). Look at the wires connected to the terminals. If there’s a wire on the “C” terminal, you’re good. If you see an unused wire tucked into the wall (often blue), check the other end at your furnace or air handler — it might be connected to the C terminal there but never hooked up at the thermostat. If so, just connect it at both ends and you’re done.
If you only have four wires (R, W, Y, G) and no spare, you have three options:
Option 1: Use an add-a-wire adapter ($20-$40). Products like the Venstar Add-A-Wire or the Fast-Stat Common Maker multiplex signals over the existing wires to free up one for use as a C-wire. They’re installed at the furnace end and work with most systems. I’ve used the Venstar ACC0410 twice and it worked perfectly both times. Takes about 20 minutes to install if you’re comfortable working at the furnace control board.
Option 2: Use the Ecobee Power Extender Kit. Included free with every Ecobee thermostat, the PEK installs at your furnace and uses smart relay technology to provide C-wire functionality over existing wiring. It’s purpose-built for Ecobee and well-documented. This alone is a reason to choose Ecobee if you don’t have a C-wire.
Option 3: Run a new thermostat wire. The permanent solution. 18/5 thermostat wire (five conductors) costs about $0.30 per foot. If the run from your thermostat to the furnace is accessible through a basement or attic, an electrician or HVAC tech can pull new wire in under an hour ($100-$200). If it’s through finished walls, it gets more expensive and invasive.
Installation Guide: What to Expect
I’ve installed all four thermostats covered here, and all of them are DIY-friendly if you have a C-wire (or an adapter solution). The process takes 20-45 minutes depending on the model and your comfort level with wiring.
Before you start: Turn off power to your HVAC system at the breaker. Take a photo of the existing thermostat wiring — wire colors and terminal labels — before disconnecting anything. This photo is your lifeline if something goes wrong.
Step 1: Remove the old thermostat from its base plate. Label each wire with the terminal letter it was connected to (most kits include labels). Disconnect the wires.
Step 2: Mount the new thermostat base plate. Most include a built-in level and wall anchors. If the new plate doesn’t cover the old mounting holes, you’ll need to patch and paint or use a wall plate (included with Ecobee, available as an accessory for others).
Step 3: Connect wires to the matching terminals on the new base plate. R to R, W to W, Y to Y, G to G, C to C. The colors of the wires don’t matter — only the terminal labels from the old thermostat matter. A red wire connected to W on the old unit connects to W on the new one, regardless of color.
Step 4: Attach the thermostat to the base plate, turn on the breaker, and follow the on-screen setup. Every smart thermostat guides you through WiFi connection, HVAC system configuration, and schedule setup during initial startup.
Common gotcha: Heat pump systems use the O/B wire to control the reversing valve. If you have a heat pump, make sure you configure the thermostat for heat pump operation and connect the O/B wire to the correct terminal. Getting this wrong means the system heats when it should cool and vice versa. The setup wizard on all four thermostats asks about this — pay attention to that step.
Check the ASHRAE guidelines for thermostat placement if you’re moving the location — it should be on an interior wall, about 5 feet high, away from windows, direct sunlight, drafts, doorways, and supply vents. Bad placement gives the thermostat false readings and undermines all the smart features.
Which Thermostat Pays for Itself Fastest?
Based on my measurements and typical energy costs, here’s the payback ranking:
1. Amazon Smart Thermostat ($80) — 3 to 6 months. If you’re coming from a manual thermostat, have Echo devices already, and are comfortable in the Alexa ecosystem, nothing beats this on pure ROI. The savings percentage is comparable to more expensive units for the simplest use case: automated scheduling and away detection.
2. Google Nest Learning Thermostat ($280) — 8 to 14 months. The learning algorithm and polished geofencing deliver strong savings with zero ongoing effort from you. The higher price extends the payback, but the hands-off optimization means savings persist even if you stop thinking about it. You can plug your own energy costs into an ROI calculator to estimate the exact payback period for your household. Best for single-story homes where one temperature reading is representative.
3. Honeywell T9 ($170) — 12 to 18 months. Middle of the pack on price, middle on features, middle on payback. The sensor system helps, but without learning features, you’re doing more manual optimization. Best for zoned HVAC systems where its multi-zone support is a unique advantage.
4. Ecobee Premium ($250 + $80 for extra sensors) — 18 to 26 months. Longest payback because the full system (thermostat plus sensors) costs the most. But in multi-story homes with uneven temperatures, the sensor-based optimization delivers savings you can’t get from other thermostats. The payback is longer, but the total lifetime savings are often the highest because it solves problems other thermostats can’t address.
My Recommendation Based on Your Situation
If budget is the priority and you use Alexa: Amazon Smart Thermostat. At $80, the risk is essentially zero. Even modest savings pay it off within months. I bought this for my parents and would do it again without hesitation.
If you want set-it-and-forget-it: Google Nest Learning Thermostat. The learning algorithm genuinely works. After two weeks of normal use, it handles scheduling better than most people would manually. The 4th generation hardware is excellent, and Matter support means you’re not fully locked into Google’s ecosystem.
If you have a multi-story home with hot and cold spots: Ecobee Premium with extra sensors. No other thermostat solves the uneven temperature problem as well. The sensors are the key differentiator, and for larger or multi-level homes, they make a measurable difference in both comfort and efficiency.
If you have a zoned HVAC system: Honeywell T9. Its zone support with room sensors is unique in this price range. It’s not as flashy as Nest or Ecobee, but it talks to zoned systems in a way the others don’t.
For even greater energy savings, pair your smart thermostat with proper home insulation — a well-insulated house lets the thermostat run your HVAC system less often, compounding the savings from both upgrades.
The Bottom Line on Smart Thermostat Savings
Every smart thermostat on this list will save you money compared to a manual or basic programmable thermostat. The differences between them are real but secondary to the fundamental shift of having an automated system that adjusts temperature based on occupancy, schedule, and weather data.
The single most cost-effective move is replacing a manual thermostat with any smart unit — even the $80 Amazon option. If you’re already running a programmable thermostat effectively, the incremental gains from going smart are smaller (3-8%) and the payback period stretches accordingly.
Pick the thermostat that matches your ecosystem, your home layout, and your budget. Install it on a weekend afternoon — it’s one of those satisfying home projects, like fixing a clogged drain, where a small time investment delivers an immediate payoff. Then check your energy bill in 90 days — the numbers will speak for themselves.
