I might be simple, but I leave the batteries out of mine. When the power goes out, it disconnects, and I get a notification by default. Pretty easy. Yes, if the internet goes out, I get the same notification, but my power and internet go out at the same time pretty much.
There is a thread where someone was monitoring a battery and mains powered device in some way, I’ll try to find it. Obviously it would require your internet to remain up which is iffy and the main reason why I’m lazy and don’t do it.
Here is the first I found: This is awesome! Power is Out based on state of v1 Motion Sensor