Last reviewed 31 October 2026
Carbon monoxide does its worst work during power outages.
When the grid goes down, the generator comes out. So do the space heaters, the grill that never belongs near a door, and the car warming up in the attached garage. The CDC estimates unintentional CO poisoning sends more than 100,000 people to emergency rooms in the U.S. every year and kills more than 400 — and a big share of those cases cluster around storms and outages, when fuel-burning backup equipment is running and homes are sealed up tight.
Now think about what a plug-in CO alarm does when the power cuts. If it has no battery backup, it does nothing. It becomes a plastic box on the wall at the exact moment your home's CO risk is at its highest.
The backup is the whole point
A plug-in alarm is only as good as what happens when the outlet dies. Listed plug-in alarms solve this in one of two ways:
- A sealed lithium battery built into the unit, sized to back the alarm up for years of outages and to last the alarm's full service life (Kidde's 10-year models work this way).
- Replaceable backup batteries — commonly two AAs — that carry the alarm through an outage (First Alert's plug-in models work this way). These only protect you if the batteries are actually in and fresh.
Either design is fine. What is not fine is a plug-in unit with no backup at all, or a backup claim you have never verified. Which brings us to the only test that matters.
The 10-second test
Unplug your CO alarm.
- Screen still lit, unit still alive? Your backup works. Plug it back in.
- Screen dark, unit dead? You do not have outage protection, whatever the box said. That alarm is fair-weather only.
Do this for every alarm in the house, and repeat it whenever you do your monthly test-button check. While you're standing there, look for the manufacture or replace-by date printed on the unit. CO sensors wear out — most alarms are rated for 5 to 10 years — and an expired sensor can read “safe” while the air isn't. If there's no date or it's past, replace the alarm.
Look for the listing
A primary CO alarm should be UL 2034 listed (or listed to the equivalent standard by another recognized lab). That listing is what makes it a life-safety alarm: defined alarm thresholds, defined end-of-life warnings, tested battery backup. Placement follows the standard rule — outside every sleeping area and on every level of the home — and check your local building code, because many jurisdictions require alarms in homes with fuel-burning appliances, fireplaces, attached garages, or generators.
What about plug-in “monitors” like the one we bought?
Full disclosure, because we learned this the hard way: we bought four plug-in monitors from a company called Primo Goods (the model is the Sentivo). The appeal was real — a color screen showing a live CO number starting at 0 PPM, plus natural gas, temperature, and humidity readings in one little plug-in unit.
Then we read the fine print. In its own launch materials, the maker states the device is not UL 2034 listed and describes it as a supplemental monitor to be used alongside a listed alarm — not in place of one. Worse, the company's own pages disagreed with each other about the feature that sold us: the sales page claims a built-in rechargeable battery backup, while the company's help center says the unit runs on wall power only. That contradiction is exactly why the 10-second test above exists.
None of this makes supplemental monitors useless. Watching a CO number creep upward in real time — long before a listed alarm's threshold — is genuinely valuable information, and so is a gas-leak sensor in a kitchen or laundry room. But treat a monitor as a second layer, an early-warning display sitting next to the alarm that is actually certified to wake you up. Never let it be the only layer. If we'd stocked our whole house on monitors alone, we'd have had screens everywhere and certified protection nowhere.
The buying checklist
When you replace or add CO alarms, check five things:
- UL 2034 listed (or equivalent) — non-negotiable for a primary alarm.
- Battery backup, specified — sealed lithium or replaceable cells, and a stated backup behavior. Vague claims deserve the unplug test.
- A digital display with peak-level memory — so you can see if CO spiked while you were out.
- An end-of-life warning — the alarm should tell you when its sensor is done.
- A date you'll actually respect — write the replace-by year on the unit in marker the day you install it.
Then run the numbers for your household the way you would for water or food: one alarm outside the bedrooms, one per level, backups verified before storm season, not during it.
The bottom line
Your CO alarm's job peaks when the power is out. A plug-in alarm without a working backup is protection that resigns at the worst possible moment. Test yours today — it takes ten seconds — and make sure the alarm standing guard over your family is a listed one, with a battery you've personally watched keep it alive.
Sources: CDC carbon monoxide poisoning guidance and statistics (cdc.gov/carbon-monoxide); UL 2034 standard for single- and multiple-station carbon monoxide alarms; manufacturer specifications, Kidde and First Alert plug-in CO alarms; Primo Goods product and help-center pages and May 2026 launch release (primo-goods.com), checked October 31, 2026.