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Short answer: losing the internet and losing electrical power are different failures. If your Zigbee sensor, coordinator or hub, automation engine, and siren remain powered and the alarm is configured for local execution, the sensor can still trigger a local alarm when the internet is down. A mains outage is more disruptive: a battery-powered sensor may stay alive, but an unpowered hub, coordinator, network device, or siren cannot process or announce the event. Phone alerts and remote access usually have additional internet and cloud dependencies.
Follow the alarm path before predicting the result
A Zigbee alarm is a chain, not a single device:
- Sensor: a door, motion, leak, smoke, or other endpoint detects a condition.
- Zigbee radio path: the sensor sends the event to a Zigbee coordinator, hub, or gateway.
- Automation: software decides what the event means. It may run on the hub locally or on a vendor’s cloud service.
- Output: a compatible siren or warning device sounds, flashes, or both.
- Notification: an app, push service, SMS gateway, or remote-access service may inform you.
An internet outage affects the cloud-dependent parts of this chain. A power outage affects every component that is not independently powered. Zigbee describes the radio link; it does not guarantee that the hub, automation, siren, or notification route works offline.
What happens when only the internet goes down?
With mains power still available, a locally controlled alarm can continue to detect and respond. Home Assistant states that local-protocol devices and local automations can keep operating without internet access, while cloud-only devices, remote access, and cloud integrations lose their normal service. See Home Assistant’s offline-operation guidance and its reliability notes at Home Assistant’s reliability FAQ.
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When a local siren can still sound
The sensor must still reach a powered coordinator, and the automation must execute on that local controller. The siren must also be reachable and powered. In that arrangement, the internet is not needed for the sensor-to-controller decision or for the controller-to-siren command.
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Home Assistant’s ZHA integration documents a warning-device action that starts an alert, with sound and an optional strobe for a specified duration. The exact sound is device-dependent. The action is documented at Start an alert on a warning device. This demonstrates why a sensor event alone does not make noise: a separate, supported warning device must receive a command.
When the alarm stops at the cloud boundary
If the automation, device integration, or siren command depends on a remote service, an internet outage can prevent the event from becoming an alarm. A battery sensor may report to a hub successfully, yet a cloud routine may be unable to evaluate the event. Cloud-only features and remote-control paths should therefore be treated as unavailable until connectivity returns.
What happens to phone alerts?
A local siren and a phone notification are separate outcomes. The siren can sound while a cloud push service, remote-access connection, or app feature is unreachable. Conversely, a locally processed event might be logged without producing an audible warning if no compatible siren is configured. The precise phone-alert behavior depends on the platform, notification service, and whether the phone is reachable through the internet or a cellular path.
What happens during a mains power outage?
Power loss must be checked component by component. A battery-powered sensor may continue sensing, but its battery does not power the coordinator, hub, Ethernet switch, router, access point, or mains-powered siren. If any required link is off, the alarm chain can stop.
| Component | Typical power dependency | Effect if that component loses power |
|---|---|---|
| Battery Zigbee sensor | Internal battery | It may still detect locally, but it cannot complete the alarm path if the coordinator is unavailable. |
| Zigbee coordinator or smart-home hub | Usually mains or USB power | No device processing or automation can occur while it is off. |
| Router, access point, or switch | Mains or backup power | Network-connected controllers and services may become unreachable; the Zigbee radio itself may still be present in a hub, but the hub’s other dependencies can fail. |
| Siren or warning device | Mains, USB, or its own battery | No local sound or strobe if it is unpowered, even when the automation runs. |
| Phone-notification service | Internet and the service’s availability | Remote push or app delivery may fail during the outage. |
This is why “the sensor has a battery” is not equivalent to “the alarm works during a blackout.” Battery status covers one endpoint only.
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How local and cloud architectures compare
| Architecture | Internet outage | Power outage | What to verify |
|---|---|---|---|
| Local sensor, local hub automation, powered local siren | Detection and local sound can continue while those devices remain powered. | Stops when an unbacked-up required device loses power. | Controller, siren, and any network equipment are on power backup if continuity is required. |
| Local sensor with cloud automation | Sensor may reach the hub, but the cloud routine may not run. | Fails if the hub or network equipment is off; cloud access also needs connectivity. | Whether this specific routine has a documented local execution path. |
| Cloud-only sensor or alarm service | Detection, processing, and alerts can be unavailable. | Local equipment may be irrelevant if the service cannot be reached. | Which functions, if any, are documented to run locally. |
| Local automation with a cloud phone notification | Local siren may work; phone delivery may not. | Both depend on which powered components remain available. | Separate local-warning and remote-notification requirements. |
These are architecture patterns, not guarantees for every product. Device support, driver behavior, hub model, and automation configuration determine the actual result.
Platform-specific evidence and limits
Home Assistant and ZHA
Home Assistant documents continued operation for local hardware when internet access is lost, while identifying remote access and cloud integrations as internet-dependent. ZHA connects supported devices through a Zigbee coordinator; its integration distinguishes mains-powered and battery-powered device availability. Those availability settings are software behavior, not a universal Zigbee reporting interval and not proof that a particular sensor will report immediately.
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Zigbee2MQTT
Zigbee2MQTT treats active, non-battery-powered devices differently from passive, battery-powered devices in its availability handling. Its documented defaults are a 10-minute check-in interval for active devices and a 25-hour interval for passive devices before availability handling marks a device differently. These are Zigbee2MQTT defaults, not general Zigbee timing and not a promise that an alarm event waits for, or is detected within, those intervals. Read the current Zigbee2MQTT device-availability guide and check your own configuration.
SmartThings hub-connected devices
SmartThings documents hub execution for supported Zigbee devices using Edge drivers and describes local automations for that supported architecture. The qualification matters: a device, hub, driver, and routine must all be part of the documented local path. Do not infer that every SmartThings routine or third-party integration runs offline. Verify the exact combination in the SmartThings hub-connected device documentation.
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How to audit your own alarm before an outage
Perform this check while the system is healthy. Write down the result for each item instead of relying on a product name or the word “Zigbee.”
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- Identify the coordinator: note whether Zigbee is built into the hub or supplied by a USB/Ethernet coordinator, and what powers it.
- Trace the automation: open the routine or automation and determine whether its trigger, conditions, and action execute on the hub or require a cloud service.
- Verify the warning device: confirm that the siren is supported by the integration, that it is paired, and that its sound or strobe command works.
- Separate outcomes: test local sound, app status, and phone notification as three independent results.
- List network dependencies: include the router, wireless access point, Ethernet switch, DNS or remote-access service, and any notification provider used by the routine.
- Record recovery behavior: check how the hub and sensor behave after reboot, how long they take to rejoin, and whether an alarm event that occurred while the hub was off is lost. Do not assume a missed event is replayed.
Testing internet loss safely
Use a controlled test rather than waiting for an emergency.
- Choose a low-consequence sensor event, such as opening a test contact sensor.
- Disconnect the home’s internet service while leaving mains power on. Do not remove power from the hub.
- Trigger the sensor and observe whether the local automation runs and whether the siren sounds.
- Separately observe the phone notification and remote app access; record if either fails while the local siren works.
- Restore internet service and confirm that the hub, sensor, and notification service recover without manual re-pairing.
Do not use a real smoke, intrusion, or medical emergency as a test stimulus. Follow the platform’s documented test controls and local safety requirements.
Testing power resilience and planning backup power
A UPS can extend operation of the powered local path, but it does not create a universal guarantee. Runtime depends on battery capacity, the electrical load, conversion losses, and the devices connected to it. No single runtime or model applies to every hub, router, coordinator, and siren.
Build the load list
- Smart-home hub or computer running the automation.
- Zigbee coordinator if it is separate from the hub.
- Router, access point, and any switch required for the controller.
- Siren or warning device, if it is mains-powered.
- Any modem or fiber terminal needed for remote notifications.
Then check the actual wattage or VA rating of each device, the UPS output waveform and plug compatibility, and the expected runtime at the combined load. Keep the local siren on backup only if its power input and the UPS output are compatible. A UPS cannot keep a battery sensor’s radio path useful when the coordinator itself is not powered.
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Understand what backup power cannot solve
- It cannot restore an internet connection that is down at the provider.
- It cannot make a cloud-only automation local.
- It cannot guarantee phone delivery when the mobile or notification service is unavailable.
- It cannot make an unsupported siren respond to a Zigbee warning command.
Common failure cases and fixes
“The sensor still shows a battery level, but no alarm sounds.”
The sensor may be alive while the hub or siren is off. Check power to the coordinator, the automation host, and the warning device before replacing the sensor battery.
“The app says the sensor is unavailable immediately.”
Availability is platform-specific. Battery devices often sleep and check in on their own schedule. In Zigbee2MQTT, the documented 25-hour passive-device default illustrates that availability status is not an instant alarm indicator. Check your configured availability settings and the sensor’s own event log.
“The siren works manually but not from the sensor.”
Manual control proves that the warning device can respond; it does not prove that the sensor automation is triggering or that the automation has permission to call the warning action. Inspect the trigger, conditions, action target, and event log. Confirm the siren remains powered and paired.
“The siren sounds, but I receive no phone alert.”
This is consistent with a local alarm and a failed cloud notification path. Check internet connectivity, notification-service status, phone permissions, and whether the routine deliberately separates local and remote actions.
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“Everything returns after power is restored, but the event was missed.”
Most sensor alarms are event-driven; a controller that was off may not know that a door opened or water was detected during the outage. Verify the platform’s restart and rejoin behavior, and use independent safety devices where regulations or risk require coverage during controller downtime.
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What to expect in each outage scenario
| Scenario | Sensor detection | Local siren | Phone alert or remote access |
|---|---|---|---|
| Internet down; all local equipment powered; local automation | Usually available through the local Zigbee path. | Can work if the siren is supported and powered. | May fail if it requires cloud connectivity. |
| Internet down; cloud automation | May reach the hub, but the decision can fail at the cloud step. | Usually unavailable through that cloud-dependent routine. | Unavailable until the service reconnects. |
| Mains down; sensor battery remains, hub and siren unpowered | The endpoint may detect, but no complete alarm path exists. | Unavailable. | Unavailable from the local system. |
| Mains down; required local equipment on a suitable UPS | Can continue if the sensor and coordinator remain joined and powered as required. | Can continue for the UPS’s load-dependent runtime. | Still depends on powered network equipment, upstream internet, and notification services. |
Bottom line for a reliable design
Design for the outcome you actually need. If the priority is an audible warning inside the property, use a documented local automation, a compatible Zigbee warning device, and backup power for every powered link that the local path requires. If the priority is a phone alert, treat it as a separate service with internet and provider dependencies. Before trusting the system, test internet loss and controlled power loss with your exact sensor, hub, siren, and automation; Zigbee alone cannot answer the question.
Frequently Asked Questions
Will a Zigbee door sensor work without Wi-Fi?
It can, provided the sensor communicates with a powered Zigbee coordinator and the automation is executed locally. Wi-Fi or internet may still be needed by the hub’s cloud routines, remote access, or phone notifications.
Does a battery-powered Zigbee sensor keep the alarm running during a blackout?
No. Its battery can keep the endpoint sensing, but the coordinator, automation host, network equipment, and siren need their own power. Backup those required components if you need coverage during an outage.
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Not reliably through a cloud-dependent notification route. A local siren may operate while remote push or app access fails; exact behavior depends on the platform and configuration.
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