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Here’s the short version: Google’s Nest Protect and First Alert’s Onelink Safe & Sound were both excellent smart smoke detectors, but both have since been discontinued by their makers. Google now directs shoppers to a First Alert model instead. X-Sense’s SC07-WX is the only one of these three you can still order new today. At around $33, it detects the same carbon monoxide and sends the same phone alerts for a fraction of the price, though its own product listing doesn’t display the UL certification the two bigger names carry. That gap, more than any single spec, is what separates a $35 alarm from a $100-plus one.
I’ve been through enough storm-season power outages to know battery backup matters more than any app feature. A smoke and CO alarm that keeps working when the grid doesn’t beats one with a speaker built into the ceiling. That’s the lens I bring to comparing these three below.
Which Smart Smoke Detector Is Best?
After enough storm-season power outages, I’d point a budget-conscious friend to the X-Sense SC07-WX, because it’s the only one of the three you can still buy new and it still catches both smoke and carbon monoxide.
- Best overall (if you can still find one): Google Nest Protect (2nd Gen). The most polished self-test and app experience of the three, but Google has stopped selling it new.
- Best budget pick you can actually buy today: X-Sense SC07-WX — roughly $33, still in active production, with app alerts and CO detection included.
- Best for a built-in speaker and voice assistant: First Alert Onelink Safe & Sound — Alexa and music streaming baked into the alarm, while remaining stock lasts.
Smart Smoke Detector Feature and Certification Matrix
| Feature | Google Nest Protect (2nd Gen) | First Alert Onelink Safe & Sound | X-Sense SC07-WX |
|---|---|---|---|
| Power source | Battery (6× AA lithium) or 120V wired with battery backup | Hardwired (AC required) with 10-year sealed lithium backup | Battery only — replaceable 3V CR123A lithium |
| CO detection | Yes — 10-year electrochemical CO sensor | Yes — electrochemical CO sensor | Yes — electrochemical CO sensor |
| Self-test | Manual test button; automatic self-check per Google’s spec sheet | Test/silence button plus app-initiated remote test | Manual test/mute button; remote one-tap test via app |
| Interconnect | Wireless, up to a full-home mesh over its own 2.4GHz radio | Wireless, up to 18 compatible detectors (12 smoke max) | Wi-Fi-based, up to 49 devices per household via the app |
| Hub/app required for phone alerts | Google Home app + Wi-Fi (no separate hub) | Onelink Home app + Wi-Fi (no separate hub) | X-Sense Home Security app + 2.4GHz Wi-Fi (no separate hub) |
| Subscription | None | None | None for core alerts; optional $4.16/mo Protect+ Elite fire-dispatch add-on |
| UL listing cited | UL 217 (8th ed.) & UL 2034 (4th ed.), per Google’s spec sheet | UL 217 & UL 2034, per First Alert’s product page | Not shown on the SC07-WX product listing itself |
| Current availability | Discontinued by Google; sold only while retailer stock lasts | Discontinued by First Alert; sold only while retailer stock lasts | Currently in active production and sale |
What “Smart” Buys You Over a Basic Smoke Detector
A basic $35 smoke alarm from any hardware store does one job: it sounds a local horn when it senses smoke. It doesn’t know whether you’re home, and it can’t tell you it’s dying until it starts chirping at 3 a.m. Unless it’s a dual-sensor model, there’s also a good chance it doesn’t check for carbon monoxide at all. Every alarm in this comparison adds three things on top of that baseline: a phone notification when it trips, a way to test or silence it remotely, and a built-in CO sensor rather than a separate unit you’d have to buy and mount yourself.
None of the three requires a separate smart-home hub. Each one just needs a 2.4GHz Wi-Fi connection and its maker’s free app. None of them charges a subscription for that baseline functionality, either. Where they diverge is in build quality, interconnect range, extra hardware (a speaker, a display, a voice assistant), and — as you’ll see below — whether the maker is even still selling the thing. If you want a broader look at how “smart” pricing plays out elsewhere in the house, our water leak sensor comparison runs a similar hub-versus-no-hub breakdown for flood detection.
Google Nest Protect (2nd Gen): The One Google Stopped Selling
The Nest Protect helped define what a “smart” smoke detector even meant when it launched. The 2nd-generation model is still the alarm a lot of people picture when they hear the phrase. It’s available in battery (six AA lithium cells) or wired versions, both with a 10-year electrochemical CO sensor. It interconnects with other Nest Protect units over its own wireless mesh, rather than relying on your home Wi-Fi for unit-to-unit communication — only the app features need Wi-Fi. Per Google’s own technical specifications, it’s rated to UL 217 (8th edition) and UL 2034 (4th edition), and it satisfies the National Fire Alarm and Signaling Code, NFPA 72.
The catch, confirmed on Google’s own support and store pages, is that sales of Nest Protect have been discontinued. The product page for the 2nd-gen unit now states it’s “no longer available” and points shoppers toward a First Alert model instead, a shift also reported by How-To Geek. Existing units keep working and keep getting security updates. But you’re now buying leftover retailer stock rather than a current product, and pricing on what’s left is inconsistent. We saw a battery unit listed at Best Buy for $149.99, but marked as no longer purchasable in new condition.
What it gets right
- Wireless interconnect that doesn’t depend on your home Wi-Fi to alert other units in the house
- A 10-year CO sensor lifespan backed by Google’s published spec sheet
- UL 217 and UL 2034 compliance, plus NFPA 72 conformance, documented directly by the manufacturer
- No subscription required for any core function
Where it falls short
- Discontinued — you can’t order a new one directly from Google anymore
- Remaining retail stock is inconsistent in price and availability
- Requires migrating from the old Nest app to Google Home, which some existing owners have found a rough transition
Worth it if…
You already own Nest Protect units and want to add one more to a room. Or you find unopened retailer stock at a fair price, and you’re already in the Google Home ecosystem.
Skip it if…
You want a product you can reliably reorder in a year if a unit fails. Or you’re starting from scratch, and would rather buy something still in active production.
First Alert Onelink Safe & Sound: The One With a Speaker Built In
Onelink Safe & Sound was First Alert’s answer to the smart-speaker era: a hardwired smoke-and-CO alarm with a real speaker built in and Amazon Alexa Voice Service embedded. The same puck on your ceiling could stream music or answer questions, in addition to sounding an alarm. It requires AC power (it’s not a battery-only unit), with a 10-year sealed lithium backup battery for outages. It uses an electrochemical CO sensor, and can wirelessly interconnect with up to 18 compatible detectors — up to 12 of them smoke alarms — over Wi-Fi and Bluetooth. First Alert’s own product documentation lists UL 217 and UL 2034 conformance.
Like Nest Protect, this one has also been discontinued. First Alert’s parent company lists the SKU as discontinued in its own support database. The product page we checked showed it at $261.95, with no current stock guarantee. If you want First Alert’s currently-produced smart smoke-and-CO alarm, the company’s SC5 line has effectively taken its place. It’s sold in both battery and hardwired versions, lists at $129.95, and is explicitly built to interconnect with any Nest Protect units a household already owns — presumably why Google recommends it.
What the Onelink Safe & Sound Gets Right
- A genuinely useful built-in speaker and Alexa integration, not a gimmick bolt-on
- A 10-year sealed battery backup means no user battery swaps for the life of the alarm
- Broad interconnect ceiling — up to 18 devices on one wireless network
- UL 217 and UL 2034 conformance documented by the manufacturer
The case against it
- Discontinued, with pricing on remaining stock running well above $200
- Requires hardwiring — not a drop-in retrofit for every ceiling box
- You’re paying for speaker hardware you may not want just to get the smoke/CO sensor
Buy it if…
You specifically want the Alexa-speaker combo and can find remaining stock at a reasonable price. And your home already has AC wiring at the alarm locations.
Pass if…
You just want a smoke-and-CO alarm without paying for a speaker. Or you’d rather buy First Alert’s currently-produced SC5 line, instead of hunting for discontinued stock.
X-Sense SC07-WX: The Budget Pick You Can Still Buy New
X-Sense’s XS01-WX — the model this comparison originally set out to check — turns out to be smoke-only with no CO sensor, according to its own product page. So it doesn’t actually belong in a smoke-and-CO comparison. The closer match is the SC07-WX, X-Sense’s current Wi-Fi-connected combination smoke and CO alarm. It runs on a single replaceable CR123A lithium battery, rated for roughly a year per charge and about 10 years of total device life. It also uses an electrochemical CO sensor, and connects to the free X-Sense Home Security app over 2.4GHz Wi-Fi, with support for linking up to 49 devices across a household through the app rather than a dedicated radio mesh.
Pricing starts at $32.99 for a single unit, which is remarkably close to what a basic non-smart alarm costs. That’s the headline of this whole comparison.
Does the X-Sense SC07-WX Have UL Certification?
The SC07-WX doesn’t display a manufacturer UL certification on this specific SKU’s product listing. Sibling X-Sense models in the same SC07 family (the non-Wi-Fi SC07 and SC07-W) are marketed as UL 217 and UL 2034 certified on their retail listings. An independent reviewer at Reviewed specifically flagged a related app-connected X-Sense variant for lacking UL certification, recommending the certified SC07-W or a First Alert alarm instead. We didn’t find UL 217/2034 language on the SC07-WX’s own product page during our research, which is worth confirming for yourself before buying.
Reviewers who’ve used the SC07-WX day to day — including a hands-on test at The Gadgeteer — found the smoke detection itself responsive (candle smoke triggered an alert in about 20 seconds). It also flagged sluggish account setup and app login issues, plus a replaceable CR123A battery that some U.S. states now restrict for legal compliance in favor of sealed 10-year batteries.
What you gain at this price
- An actively sold, currently in-production smart smoke-and-CO alarm you can reorder
- App alerts and CO detection for close to basic-alarm money
- An optional professional fire-dispatch subscription (Protect+ Elite) if you want it, for $4.16 a month billed annually
What you give up
- No UL certification shown on this specific product listing
- A replaceable battery you’ll swap roughly once a year, versus a sealed 10-year battery on the other two
- Reported app account-setup friction from independent reviewers who used the device day to day
- No speaker, voice assistant, or radio-mesh interconnect independent of Wi-Fi
A good fit for…
Budget-conscious buyers who want phone alerts and CO detection without paying $100-plus. And buyers who are comfortable confirming certification details themselves before purchase.
Not a fit for…
Anyone who wants a manufacturer-documented UL listing on the exact unit they’re buying. Or anyone who lives somewhere that restricts replaceable-battery smoke alarms.
The Real 5-Year Cost of Owning a Smart Smoke and CO Detector
Sticker price alone hides the real cost gap between these three. We normalized each one to a 5-year window per single unit: unit price, plus any battery replacements the manufacturer’s own specs imply, plus subscription cost if you opt in. That gives a cost-per-protected-year figure. This is our own calculation, not a number published by any manufacturer.
| Detector | Unit price | Battery cost over 5 years | Subscription over 5 years | Total 5-year cost | Cost per protected year |
|---|---|---|---|---|---|
| Nest Protect 2nd Gen (battery) | $149.99 (last listed price; discontinued, stock-dependent) | Not published by Google for this model — treated as $0 for this calculation | $0 (no subscription) | $149.99 | ≈$30.00 |
| First Alert Onelink Safe & Sound | $261.95 (last listed price; discontinued, stock-dependent) | $0 (10-year sealed battery outlasts the 5-year window) | $0 (no subscription) | $261.95 | ≈$52.39 |
| X-Sense SC07-WX | $32.99 (current, 1-pack) | ≈$32 (about four CR123A replacements at typical retail pricing, per X-Sense’s ~1-year battery-life spec) | $0 baseline / ≈$249.60 with optional Protect+ Elite | ≈$64.99 (no add-on) / ≈$314.59 (with dispatch add-on) | ≈$13.00 (no add-on) / ≈$62.92 (with dispatch add-on) |
Even with battery swaps factored in, X-Sense’s SC07-WX comes out cheapest per protected year. Add the optional dispatch subscription, though, and it lands close to Nest Protect’s territory. First Alert’s Onelink Safe & Sound is the most expensive per year by a wide margin. That tracks with it being the only one of the three built around a full speaker and voice assistant, rather than just a sensor.
How We Compared These Smart Smoke Detectors
This comparison is built from each manufacturer’s published technical specifications and product listings, cross-checked against independent, non-commercial and editorial sources where available. Those sources include UL’s own standards documentation, NFPA’s home fire safety guidance, and hands-on reviews from independent outlets, which we’ve linked in-line above. Where a spec, price, or certification claim couldn’t be verified against a manufacturer or independent source, we said so explicitly rather than guessing.
For general background on how smoke alarms should be installed, tested, and maintained, the National Fire Protection Association’s guidance is a useful independent baseline. Its carbon monoxide safety page covers CO-specific placement advice. For what UL 217 and UL 2034 certification actually test for, see UL Standards & Engagement’s UL 217 listing. Every alarm here should still be installed, tested, and replaced according to its manufacturer’s instructions and your local fire code. Certification and specs tell you what a device is designed to do, not a guarantee of how it performs in your specific home.
Published August 13, 2026. Updated September 13, 2026.
Prices are approximate, current as of publication, and change frequently — confirm the latest price with the retailer before buying. Read more about how we research and vet products on our editorial standards page. For a broader look at how we approach smart-home comparisons, browse our Smart Home category.
Frequently Asked Questions About Smart Smoke Detectors
Do Smart Smoke Detectors Test for Carbon Monoxide?
Yes. All three use an electrochemical carbon monoxide sensor alongside their smoke sensor, according to each maker’s own spec sheets. Google’s Nest Protect, First Alert’s Onelink Safe & Sound, and X-Sense’s SC07-WX are all built as combination smoke-and-CO alarms, not smoke-only units.
Do smart smoke and CO detectors require a monthly subscription?
Not for the core features. Phone alerts, remote self-testing, and app-based hush are included free with all three brands’ apps. The only recurring cost here is X-Sense’s optional Protect+ Elite plan. It adds professional fire-dispatch monitoring for $4.16 a month billed annually, per X-Sense’s own pricing page.
Is the Google Nest Protect still available to buy new?
No. Google has discontinued the Nest Protect and stopped selling it directly. Retailers are working through remaining stock, and several listings show it as sold out. Google now recommends First Alert’s SC5 Smart Smoke & Carbon Monoxide Alarm as the successor. It can interconnect with any Nest Protect units you already own.
Do you need a separate smart-home hub for phone alerts?
No. Each of these alarms only needs a 2.4GHz Wi-Fi connection and its maker’s free app: Google Home, Onelink Home, or X-Sense Home Security. That’s enough to send push notifications to your phone. None of them require a separate hub device to get remote alerts.
Is a cheaper smart smoke detector as safe as a pricier one?
We can’t say one brand detects fire or CO more reliably than another without independent lab testing of each unit. What we can confirm from each maker’s documentation is UL listing status. Nest Protect and Onelink Safe & Sound both cite UL 217 and UL 2034 compliance, while X-Sense’s SC07-WX product listing doesn’t display that certification. Check the specific certification of whatever you actually buy before deciding on price alone.
Which Smart Smoke Detector Should You Buy?
The jump from a $35 basic alarm to a $100-plus smart one buys you phone alerts, remote self-testing, no separate CO detector to install, and — in First Alert’s case — a speaker and voice assistant thrown in. It doesn’t automatically buy you better fire or CO detection. And two of the three best-known names in this category, Nest Protect and Onelink Safe & Sound, aren’t something you can walk into a store and reorder anymore. If you want the smart features without hunting down discontinued stock, X-Sense’s SC07-WX or First Alert’s current SC5 line are the practical starting points. Just confirm UL certification on the exact model and pack you’re buying before you check out.
If smoke and CO coverage is one piece of a bigger smart-safety setup, our indoor security camera comparison and smart lock comparison cover two of the other pieces most people add next.
