Will a Wi-Fi freeze alarm work during a power outage?
Usually not. Even a battery-powered sensor needs your router and modem, and those go down with the power. The fix is a service that alerts you when the sensor goes quiet, because silence in winter is itself a warning: the same outage that stopped the readings has probably stopped the heat.
Why the alarm goes quiet when you need it
A freeze alert travels from the sensor, through your Wi-Fi and internet connection, to a server that sends the message. A power cut breaks that chain at the router, and a furnace needs electricity too, so the house starts cooling at the same moment the alarm stops reporting.
Anything that only runs inside the house, including a Home Assistant automation, has the same weak spot.
Three ways to cover it
Alert on silence. A hosted service can notice that readings stopped and tell you. ProbeHarbor has an outage alert that fires when a device has been silent for a set number of hours (two by default).
Keep the network up. A small battery backup (UPS) on the modem, router, and sensor keeps readings flowing through short outages, as long as your internet provider's equipment outside the house stays powered.
Skip the home network. A cellular sensor with a battery reports without Wi-Fi. It costs more and needs a data plan, but it is the right choice for a cabin or a house that loses power often.
Who gets the message
An outage often affects the whole neighborhood, including your own phone's Wi-Fi. Send alerts by SMS or push as well as email, and add a second person who can check on the house.
Frequently asked questions
Do battery-powered Wi-Fi temperature sensors work without power?
The sensor keeps measuring, but it cannot send anything until the router and internet connection are back. Some store readings and upload them later, which helps the record but not the warning.
How long does a house take to get cold after the power goes out?
It depends on insulation and how cold it is outside. A sensor that was reporting until the outage gives you the last known temperature and cooling rate, which is a reasonable basis for deciding how urgently someone needs to go there.
Related
Get a freeze alert for your space
ProbeHarbor watches probes in garages, crawlspaces, and cabins and alerts you before pipes freeze. The free plan includes email alerts; bring an ESP32 probe or connect Home Assistant.