Across a 72-hour test with the WAN cable pulled, nineteen of twenty-three devices behaved identically. The four that failed were the four requiring a vendor account. “Works offline” has three separate meanings: local control still responds, automations still fire, and the app still reaches the device on the LAN.
- Three meanings, not one. A device can pass local control and fail both the others.
- Voice assistants fail completely, because speech recognition is a cloud service.
- Cloud plugs freeze in their last state rather than failing safe.
- Weather-conditioned automations stop silently. Give every one a fallback branch.
The test
The local network stayed up: switch, access point, hub, all powered and talking. Only the WAN link was pulled, which is what a real outage looks like. Seventy-two hours, with a scripted check every fifteen minutes on every entity in the house.
One clarification, because it is the source of most confusion online. "Works offline" has three distinct meanings and vendors use whichever flatters them: the device responds to local control, the automations still fire, and the phone app still works. A device can do the first and fail the other two.
Results by device
| Device | Local control | Automations | App on LAN | Notes |
|---|---|---|---|---|
| Zigbee sensors | yes | yes | yes | No behaviour change at all |
| Thread lock | yes | yes | yes | Guest codes could not be issued |
| Local hub | yes | yes | yes | Lost weather-based conditions |
| Wi-Fi camera (RTSP) | yes | yes | yes | Recording continued locally |
| Voice assistant | no | no | no | Refused every command |
| Cloud plug | no | no | no | Relay stuck in last state |
| Robot vacuum | partial | no | no | Button on the unit only |
| Smart TV apps | no | n/a | no | Panel and HDMI fine |
Nineteen of twenty-three devices behaved identically with the internet gone. The four that failed were the four that require a vendor account to operate, which is the same list as the four that fail the buying test in our longevity guide.
The failures worth knowing about
Voice assistants fail completely
Speech recognition happens in the cloud, so an offline smart speaker cannot turn on a lamp sitting in the same room, connected to the same hub. This surprises people more than anything else in the test. Local voice control exists, and it is a deliberate build, not a default.
Cloud plugs freeze rather than fail safe
The cloud plug did not turn off, it stayed in whatever state it held when the connection dropped. If that is a heater, the failure mode matters. Any device controlling something with a safety dimension should be local, for this reason alone.
Weather-conditioned automations silently stop
This was the subtle one. Our blind automation used a sunset time and a cloud-cover forecast. With no forecast, the condition never evaluated true, so the blinds simply never moved and nothing logged an error. Any automation depending on external data needs a fallback branch, not just a condition.
Building for the outage
- Keep the automation engine on-site. A hub that evaluates rules locally is the single decision that determines the outcome of this test.
- Prefer Zigbee, Z-Wave and Thread for anything that matters. Locks, leak sensors, heating. Their transports do not involve the internet at any point.
- Give every external-data automation a fallback. Sun elevation, local time and last-known-state are all available offline.
- Put a physical control on every critical path. A battery wall remote bound directly to a Zigbee group works with the hub itself switched off.
- Test it deliberately. Pull the WAN cable on a Saturday morning, not during a storm. Twenty minutes tells you most of what a 72-hour test does.
Your phone's Wi-Fi. If the phone drops to mobile data when the Wi-Fi has no internet, the local app stops reaching the local hub, and you conclude the hub is down. Turn off "switch to mobile data automatically" before drawing any conclusions.
Frequently asked
Will my smart lights work without internet?
Zigbee, Z-Wave and Thread bulbs will, provided the hub is local. Wi-Fi bulbs from a cloud-only vendor generally will not, even though the bulb and your phone are on the same network.
Can I use voice control offline?
Not with a standard Alexa or Google speaker. Local voice pipelines running on the hub do work offline, at the cost of a smaller vocabulary and more setup.
What happens to my smart lock in an outage?
A Zigbee, Z-Wave or Thread lock keeps working on the keypad and on any local schedule. What stops is remote unlocking and issuing new guest codes, both of which need the internet.
Does 4G backup on the router solve this?
It solves it entirely, for cloud devices, and it is worth having. But it is a monthly cost that exists to work around a design choice. Local devices need no backup link at all.
Keep reading
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The error almost never means what it says. Nine times out of ten the border router exists and IPv6 is the thing that is broken.
→RadioZigbee and Wi-Fi on 2.4 GHz: pick the channel that stops the drops
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→LongevityThe great bricking: how to buy gear that outlives its manufacturer
Wemo's cloud went dark in January 2026. Here is the four-question test we now run before any purchase.
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