I’ve now seen multiple reports that as long as you are in North America and have a SmartThings/Aeotec hub with a Z wave radio, you can get one plug-in device, the Zooz zac38 zwave repeater, and use it with their custom edge driver.
[ST EDGE] Zooz Edge Driver Channel
And when it switches itself from mains power to battery, it will send a message to the hub, and you will be able to use that as the trigger in a routine that you create in the SmartThings app.
So that would be the easiest, but I haven’t seen a specific screenshot of what the routine looks like.
Also, if you have only the station or one of the hubs built into another Samsung device, you can’t use it because it’s a Z wave device. (of course, that’s also true of the Aeotec model that I mentioned.) If you don’t have a Zwave hub, you’ll have to look for something using Zigbee or Matter instead, and probably go back to the two device method.
Also, this would only be good for a situation where you have a two socket wall outlet, you only have one device plugged into it, and you want to be notified when that outlet loses power. Then you can use the other socket for the power monitoring device. If you are trying to monitor power for any other kind of device, this method probably won’t work and you’ll need to take a look at @taustin’s method discussed upthread.
If anyone has the Zac38 can you check and see what the routine options are and post a screenshot? Thanks!