The power goes out and the house goes quiet — no fridge hum, no furnace fan, no charger lights. On the counter, one small screen is still lit. Whether that screen is *supposed* to be lit is a question most people cannot answer, because the answer depends entirely on how the detector gets its power, and that is written in small print on the back of a unit installed years ago.
This guide answers it directly, then covers the part that matters more: an outage is not a neutral event for carbon monoxide. It changes how you heat, how you cook, and how your house ventilates — usually all at once. That combination is exactly when a working detector earns its place.
The Short Answer
It depends on the power source:
| Detector type | Runs on mains | Runs without mains | Notes |
|---|---|---|---|
| Battery-only | No | Yes | Independent of the grid, but battery condition is entirely on you |
| Plug-in with battery backup | Yes | Yes, for a limited window | Backup runtime is finite and stated per model |
| Plug-in, no backup | Yes | No | Goes dark with the outage |
| Hardwired with battery backup | Yes | Yes, for a limited window | Backup batteries are the failure point most often missed |
| Hardwired, no backup | Yes | No | Common in older installations |
Three words do the work here: backup battery. A plug-in detector is not automatically a detector that works in an outage, and a hardwired detector with a dead backup cell is not one either. The unit has to be both powered *and* backed up.
If you are shopping and this is a deciding factor, the plug-in versus battery comparison lays out the trade-off in full — including the case where battery-only is genuinely the better answer.
Why an Outage Raises CO Risk
An outage does not create carbon monoxide. It changes behaviour, and behaviour creates it. Four things typically happen in the first few hours:
Portable generators come out. A generator run too close to the house — on a porch, in a garage with the door cracked, under an open window — puts exhaust where it does not belong. This is the single largest source of CO incidents associated with outages, and it is a placement problem, not an equipment problem. Our generator safety guide covers distances and the specific configurations that cause trouble.
The fireplace or wood stove becomes the heat source. A chimney that was fine in normal operation can behave differently when the house is closed tight and the pressure balance changes. Dampers left partly closed, a cold flue, or a fire built in a fireplace that has not been swept all change the picture. More detail in fireplaces and CO.
Portable space heaters get pressed into service. Electric ones are not a CO source. Fuel-burning ones are, and a kerosene or propane heater in a closed room is a real source. See space heater risk for the distinction.
Mechanical ventilation stops. Any fan that was moving air out of the house stops moving it. A house that was diluting a small, steady source now accumulates it. This is why CO levels can rise during an outage from equipment that was never actually malfunctioning.
That last point is the reason a *display* matters more than usual. An alarm tells you a threshold was crossed. A readout tells you something is trending upward before it gets there — useful when you are deciding whether to crack a window or leave.
What Battery Backup Actually Covers
Backup is not unlimited runtime, and the number is smaller than most people assume.
A plug-in or hardwired detector with a rechargeable backup cell typically keeps the sensing and alarm function alive for a period measured in hours, not days. As a concrete reference point, the VASAIKO HD16 carries a 1000 mAh internal cell rated at roughly eight hours of standby operation — enough for a storm night, not enough for a three-day restoration.
Two things are worth understanding about that number:
It covers monitoring, not comfort features. While on backup, the unit keeps sensing and will alarm. Screen backlight behaviour, brightness, and refresh behaviour may change to conserve the cell. The sensing function is the one that matters.
It is a bridge, not a plan. Backup exists to cover the gap between losing mains power and either getting it back or making a different arrangement. If an outage is going to run long, the sensible move is a battery-only unit as a second layer rather than expecting backup to stretch.
Runtime also degrades. A rechargeable cell that has cycled through several hundred outages and several years of trickle charging does not deliver its original capacity. If your unit has been in service a while, treat the published runtime as an upper bound and check the cell as part of your replacement and end-of-life planning.
If backup runtime is the specification you are buying on, read it the way you would read a fuel tank: what matters is hours of sensing, not the capacity figure on the box. The HD16 specification page states standby behaviour explicitly rather than leaving it implied.
Reading the Display When the Power Is Out
If your detector shows a live ppm figure, an outage is one of the few times that number is genuinely actionable rather than decorative.
A few practical notes:
Expect a re-read, not an instant number. Detectors need a short stabilisation period after power-up — on the order of sixty seconds for electrochemical CO sensors. If a unit was dark and comes back on backup, the first minute is not a measurement.
Watch for a trend, not a single reading. One reading of 12 ppm means very little. 12, then 19, then 31 over twenty minutes with a generator running outside means something. The thresholds and what they generally correspond to are laid out in the safe ppm levels chart.
The display is illustrative unless you are looking at the real thing. Any figure shown in marketing imagery or product photography is a representation, not a measurement. Judge the unit by whether it shows *a* number reliably, not by what number appears in a photo.
If you want the full argument for why a readout changes how you respond, digital display units covers it — including the case where an alarm-only unit is fine and a display would not change your behaviour.
How to Tell Whether Your Detector Still Has Backup Power
Most people find out during an outage, which is the worst possible time. Three checks you can do while the power is on:
Press the test button — and understand what it proves. The test button exercises the alarm circuit and, on most units, the sensor. It tells you the unit is alive. It does not always tell you the *backup cell* is good, because a plugged-in unit may be running on mains while the cell sits dead.
Unplug it for thirty seconds. This is the honest test for a plug-in unit. Pull it from the wall with the power on and see whether it stays lit and responsive. If it goes dark immediately, it has no usable backup — or none at all. Do this once a season, not once a decade.
Listen for the low-battery chirp — and do not confuse it with an alarm. A periodic single chirp is typically a battery or end-of-life signal; four rapid chirps followed by silence is usually an alarm condition. The distinction matters, and mixing them up is one of the most common reasons a working alarm gets removed. Full breakdown in why CO detectors beep.
If your unit chirps only when the power is out, that is the backup cell reporting low — replace it rather than silencing it.
Where Backup Matters Most
Not every room carries the same risk during an outage. Ranked by how often things actually go wrong:
Near where a generator will be running. The exhaust path usually leads back toward the house through a door, a window, or a crawlspace vent. A detector that keeps working while the furnace fan is stopped is the one that catches this.
Sleeping levels. People sleep through the early part of a CO event — it is odourless and the early effects are easy to mistake for tiredness. A unit on backup in a bedroom is doing real work at 3 a.m. Coverage counts per level are in how many detectors you need.
Rooms with a combustion source. Fireplace, wood stove, or a fuel-burning space heater. Also basements, where a furnace or water heater sits and where basement placement has its own logic.
Remote or seasonal properties. A cabin that loses power while unoccupied, or an RV running a generator at a site, is a case where the outage lasts longer than anyone's backup cell. If that is your situation, a battery-only unit as a second layer is worth more than a longer backup runtime. See cabins and RV setups.
Mistakes to Avoid During an Outage
Assuming a plug-in detector is covered. It is only covered if it was specified with backup. Check, don't assume.
Running a generator indoors, in a garage, or under an overhang. Not even with the door open. Not even "for a few minutes." This is the one with no acceptable version.
Using an oven or a gas range as a heater. It is a real and recurring behaviour during winter outages, and it produces CO in a room you are trying to keep warm.
Silencing an alarm to stop the noise. If a detector alarms during an outage, there is usually a reason that did not exist an hour earlier — a generator, a heater, a blocked flue. Ventilating is faster than diagnosing. The false alarm causes guide explains which conditions produce nuisance readings, but treat any alarm during an outage as real until you have a specific alternative explanation.
Waiting for the power to come back to test. A unit that has been sitting on a dead backup cell for two years will not announce it. Test before storm season, not during it. The procedure is in how to test a CO detector.
A Before / During / After Checklist
Before storm season
- Unplug each unit for thirty seconds and confirm it stays alive on backup.
- Confirm every fuel-burning backup heat source has a defined outdoor or vented location.
- Re-check placement height — CO mixes with room air rather than pooling at the ceiling, so ceiling-mounted smoke-alarm logic does not transfer.
- Note the install date on each unit; sensors have a finite life.
- If a unit has no usable backup, either replace it or add a plug-in monitor with standby power on the sleeping level.
During an outage
- Place generators outdoors and away from openings, before you need them.
- Keep at least one detector with usable backup on the sleeping level.
- If a unit displays ppm, glance at it periodically rather than waiting for an alarm.
- If an alarm sounds: ventilate, move to fresh air, then investigate.
Afterward
- Re-test once mains power returns.
- If a unit ran on backup for hours, give it time to recharge before relying on it again.
- If any reading was recorded above normal background, investigate the source before the next outage. A trace event that resolved on its own usually has a cause that will recur.
If a unit is old, has been through many deep backup cycles, or has started chirping intermittently, replacement beats troubleshooting — the lifespan guide has the signals.
FAQ
Does a carbon monoxide detector work during a power outage?
Only if it has battery backup or runs on batteries alone. A plug-in or hardwired unit with no backup — or with a dead backup cell — stops working when the mains does. Unplug it for thirty seconds while the power is on to find out which you have.
How long does the backup battery last?
Varies by model, and it is measured in hours. Rechargeable backup cells in plug-in units commonly deliver a single-digit number of hours of sensing and alarm operation. Treat published figures as an upper bound for a healthy cell and as optimistic for one that is several years old.
Will a hardwired detector work without power?
A hardwired unit with a functioning backup battery will. A hardwired unit with no backup, or with a backup that was never replaced, will not. Many older installations fall into the second category and nobody has checked since installation.
Do I need a detector that works during an outage?
If you use a generator, a fireplace, a wood stove, or a fuel-burning space heater when the power is out — yes, unambiguously. Those are precisely the sources that appear during an outage and were not running the day before.
Should I get a battery-only unit instead?
Not instead — in addition, for the cases where outages run long. Battery-only units are independent of the grid and do not depend on a rechargeable cell that degrades. The trade-off is that you own the battery replacement schedule. This comparison covers when each wins.
Does the display still work on backup?
On most units with a display, yes, though brightness and refresh behaviour may change to conserve the cell. The sensing and alarm functions are prioritised. A dimmer screen is not a malfunction.
Can a power outage cause a false alarm?
It can contribute to conditions that produce a reading — reduced ventilation, a generator near an opening, a cold flue on a fireplace that is suddenly in heavy use. Those are real readings from real sources, not nuisance events, even if the level is low. Genuine false-alarm sources are covered separately in false alarm causes.
Does a CO detector cover a propane or natural gas leak during an outage?
Only if the unit is built for combustible gas as well — a different sensing function under a different standard. If you run a propane generator or heater, confirm the device explicitly covers combustible gas. See gas leak detection and do you need a gas detector.
What should I do if the alarm sounds while the power is out?
Ventilate and move to fresh air first, then investigate. Do not silence it to stop the noise. If anyone has symptoms — headache, dizziness, nausea — get to fresh air and seek medical attention.
The Bottom Line
Three things decide whether you are covered when the lights go out: whether the unit has backup at all, whether that backup is still healthy, and whether you have a second layer for outages that outlast a few hours. The first is a specification question, the second is a maintenance question, and the third is a planning question — and all three are answerable now, in daylight, with the power on.
Spend five minutes on the unplug test this week. It is the only check that tells you the truth.
If you are setting up detection with outages in mind, the HD16 6-in-1 monitor is built around that case: plug-in with an internal rechargeable cell for standby operation, a live ppm readout you can check periodically rather than waiting for a threshold, and sensing that covers CO, combustible gas, smoke, temperature and humidity alongside its own battery. See the full specification for placement and multi-room options.
*This article is general information, not medical advice. Always follow your manufacturer's instructions and your local code requirements. If you suspect carbon monoxide exposure, leave the space and contact emergency services. Symptoms and health effects are described in CO poisoning symptoms. Standards referenced — including UL 2034 for residential CO alarms and UL 1484 for residential gas detectors — are explained in UL 2034 explained.*