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Internet loss and mains-power loss are different failures. If your Zigbee sensor, coordinator or hub, automation engine, and siren are powered and connected through a local control path, an alarm can still detect an event and sound without internet access. A phone alert or remote app connection may fail because those commonly depend on cloud services.
During a power cut, a battery-powered sensor may remain alive, but it cannot keep a powered-off hub, coordinator, router, switch, or siren operating. Continued protection therefore depends on which parts of the alarm chain have backup power and whether your platform documents local execution.
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The outcome depends on which link fails
A Zigbee alarm is a chain, not a single device:
- The sensor detects an opening, motion event, leak, temperature change, or other condition.
- Its Zigbee radio sends the event to a coordinator or hub.
- An automation evaluates the event locally or through a cloud service.
- The automation commands a warning device, sends a phone notification, or both.
Removing internet access affects cloud links. Removing mains power can stop every powered link, even when the sensor itself uses a fresh battery.
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|---|---|---|---|---|
| Internet unavailable; all local equipment powered | Usually can detect and report over Zigbee | Can continue when the platform and automation run locally | Can sound if it is a supported, powered warning device | Cloud notifications, remote access, and app features may be unavailable |
| Internet unavailable; cloud-dependent setup | May detect locally, but the event can stop at the hub or cloud boundary | Cloud automation may not execute | Usually will not sound unless an independent local path exists | Usually unavailable |
| Mains outage; no backup power | Battery endpoint may remain powered | Powered-off coordinator, hub, router, or network equipment cannot process the event | Powered-off siren cannot sound | Unavailable if the controller or internet equipment is down |
| Mains outage; backup power on required equipment | Can report while its battery lasts | May keep running for the backup system’s load-dependent runtime | Can sound if it also has power | Depends on the controller, network, internet connection, and notification service |
These are conditional outcomes, not a guarantee for every Zigbee product. Zigbee defines the radio connection; the location of the automation and the power arrangement determine what survives an outage.
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What an internet outage does to a Zigbee alarm
Local Home Assistant path
Home Assistant states that local-protocol devices and local automations can continue responding when internet access is lost. In a typical arrangement, a Zigbee sensor talks to a coordinator attached to Home Assistant, and a local automation commands a siren. That sensor-to-coordinator-to-siren path does not need a cloud server, provided the Home Assistant host, coordinator, siren, and any required local network equipment remain powered. See Home Assistant’s internet-connectivity guidance.
The same documentation distinguishes cloud-only devices and integrations. A cloud-managed sensor may appear in the same dashboard but still require an internet round trip before an automation can act. Remote access to Home Assistant and cloud integrations are separate internet-dependent functions, so their failure does not by itself prove that a local Zigbee rule has stopped.
Supported SmartThings hub execution
SmartThings documents local execution for supported hub-connected devices using Edge drivers. When the specific Zigbee device, hub, driver, and automation are in that supported architecture, the hub can evaluate the rule without sending it to a remote service. SmartThings’ hub-connected device documentation is the relevant reference; do not assume that every routine, device type, or custom integration is local.
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Cloud-dependent alarms
If the sensor event must reach a vendor’s cloud before a rule is evaluated, loss of internet interrupts that step. The Zigbee radio can remain healthy and the sensor can still be powered, yet no command reaches the siren. A cloud-only siren or notification service has the same dependency. Check the product’s execution mode rather than inferring it from the word Zigbee.
What a power outage does
A battery sensor is only one powered component
Door, motion, leak, and temperature sensors commonly use batteries. Their electronics may continue detecting an event during a blackout, but the battery does not supply the coordinator, hub, Ethernet switch, Wi-Fi access point, router, or mains-powered siren. If any required link is off, the event cannot complete the alarm chain.
Some warning devices are battery-powered and others are plugged in. A battery-backed siren can remain available longer than a plug-in model, but its compatibility, battery state, and behavior must be confirmed for the exact platform. A sensor’s own battery life says nothing about how long the complete alarm system will operate.
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Network equipment may still matter
A Zigbee radio does not need internet to exchange frames with its coordinator, but the coordinator may connect to the automation host through a powered Ethernet or Wi-Fi network. If your hub and coordinator are separate devices, both need power. If the siren is connected directly to the coordinator, a home router may not be needed for the local radio path; if the coordinator is a network appliance or the automation host depends on a switch, those devices do need power. Map the actual topology instead of relying on a generic wiring diagram.
UPS planning
A UPS can extend operation for a local controller and network equipment during a mains outage. Runtime is determined by the UPS battery capacity and the wattage of every connected device; there is no universal runtime for a Zigbee alarm. Inventory the hub or Home Assistant host, coordinator, router, switch, access point, and siren, then check each device’s input requirements and the UPS’s supported load. The UPS idea follows from the documented dependency on powered local hardware, not from a measured result for a particular model.
Why the sensor does not automatically make a sound
A sensor reports an event; it is not necessarily an alarm sounder. The automation must call a compatible warning device, and that warning device must be powered and reachable.
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In Home Assistant’s ZHA integration, the ZHA warning-device action can start an alert on a supported Zigbee siren or other warning device. The action can request sound and an optional strobe for a specified duration. The documentation notes that the exact sound depends on the warning device, so a successful command does not imply identical tones across models.
This creates three separate checks:
- Is the siren paired to the same coordinator or otherwise reachable by the automation platform?
- Does the platform expose it as a supported warning device with an action or command your rule can use?
- Does the siren have power during the failure you are planning for?
Phone alerts are a separate result
A local siren and a phone notification follow different paths. The siren can sound while a cloud push service, remote-access tunnel, or mobile app session is unreachable. Conversely, a notification might arrive after an internet connection returns even though nobody heard a local sound.
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How device availability settings affect what you see
Home Assistant ZHA
ZHA distinguishes mains-powered and battery-powered Zigbee devices in its integration settings. The displayed availability values are software settings for that integration, not a universal Zigbee timing standard and not proof that a particular sensor reports immediately after an outage. See the Zigbee Home Automation integration documentation when interpreting an unavailable device.
Zigbee2MQTT
Zigbee2MQTT treats active, non-battery-powered devices differently from passive, battery-powered devices. 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 them according to its configuration. These are Zigbee2MQTT defaults, not population statistics and not a promise that every sensor will report an alarm within either period. The exact behavior is described in the Device Availability guide.
Availability status is also different from event delivery. A sensor may be marked available while idle, or remain shown as available for a period after its coordinator disappears. Treat the status timer as an operational hint, not confirmation that an alarm command reached a siren.
Compare alarm designs before relying on them
| Design question | What to verify | Why it matters |
|---|---|---|
| Sensor-to-hub communication | Is the path direct Zigbee to a local coordinator, or does a cloud service sit between the device and automation? | Only the local path can normally continue when internet access fails. |
| Automation location | Does the rule execute on the hub or host, or in a remote service? | A powered local engine can evaluate an event offline; a cloud engine cannot if its connection is down. |
| Warning-device support | Is the siren a supported Zigbee warning device for this coordinator and integration? | Pairing a sensor does not automatically create an audible output. |
| Power arrangement | Which devices are battery-powered, plugged in, or connected to a UPS? | The weakest powered link determines whether an outage breaks the chain. |
| Desired outcome | Do you require a local sound, a phone alert, remote status, or all three? | Each outcome has different hardware and internet dependencies. |
Build and verify a more resilient setup
- Draw the real chain. List every sensor, coordinator, hub, automation host, router or switch, access point, siren, and notification service. Mark each as battery-powered, mains-powered, or UPS-backed.
- Confirm local execution. Read the platform documentation for your exact hub, driver, device, and automation. Home Assistant’s local-protocol guidance and SmartThings’ Edge documentation describe supported architectures, not every possible configuration.
- Confirm the siren action. Trigger the warning device from the platform’s own action or service interface, then verify the requested duration, tone, and strobe behavior. Do this while the system is healthy.
- Plan backup power by load. Measure or obtain the input wattage for the local controller and networking equipment you intend to keep alive. Select backup capacity only after checking total load, connector compatibility, and the required runtime.
- Separate local and remote objectives. Decide whether an audible alarm must work without internet, whether a cellular or other independent notification route is required, and whether remote dashboard access can be sacrificed during an outage.
- Test an internet failure. With household power on, disconnect the internet connection without turning off the local network. Trigger the sensor and observe the event log and siren. Then check which phone or remote functions fail.
- Test a power failure safely. Simulate loss of mains power only after confirming that doing so will not disable a real safety system. Check whether the sensor, controller, network path, and siren remain powered, and record the time until any component stops.
- Restore and inspect. After each test, confirm that devices rejoin, automations resume, batteries are not reporting low voltage, and no duplicate or delayed alerts were created.
Troubleshooting by symptom
| Symptom | Likely break | What to check |
|---|---|---|
| Sensor event appears in the log but no sound occurs | Automation target, warning-device support, or siren power | Run the siren action directly, confirm the device is reachable, and verify the rule’s condition and duration. |
| No event appears during an internet outage | Cloud-dependent automation or an offline host/network component | Check whether the rule is documented as local and whether the coordinator and automation host still have power. |
| Sensor is shown as unavailable after a blackout | Coordinator or network equipment was off, or availability timers have not completed | Restore power, allow the device to rejoin, and interpret the platform’s availability settings for that device type. |
| Local siren sounds but no phone alert arrives | Notification service or internet path | Inspect the notification integration separately; local alarm execution does not prove cloud delivery. |
| Phone alert arrives late after service returns | Queued, retried, or newly processed cloud notification | Review event timestamps and avoid treating delayed delivery as proof of real-time outage coverage. |
What you can and cannot conclude
You can conclude that internet loss is survivable when the complete sensor-to-siren automation is documented to run locally and every required device stays powered. You cannot conclude that from Zigbee branding alone. You can also conclude that a battery sensor may remain powered during a blackout, but that does not establish that an alarm was processed or heard. Backup power improves the odds only for the equipment it actually supports, for as long as its load and battery capacity allow.
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