I was in another country when there was a power outage at home. My internet went down, the server restart but couldn't reconnect anymore because the optical network router also had some problems after the power outage. I could ask my folks to restart, and turn on off things but nothing more than that. So I couldn't reach my Nextcloud instance and other stuff. Maybe an uninterruptible power supply could have helped but the more I was thinking about it after just didn't really worth the hassle anymore. Add a UPS okay. But why not add a dual WAN failover router for extra security if the internet goes down again? etc. It's a bottomless pit (like most hobbies tbh)
Also (and that's a me problem maybe) I was using Tailscale but I'm more "paranoid" about it nowadays. Single point of failure service, US-only SSO login (MS, Github, Apple, Google), what if my Apple account gets locked if I redeem a gift card and I can't use Tailscale anymore? I still believe in self hosting but probably I want something even more "self" to the extremes.
So now you need to test them regularly. And order new ones when they're not holding a charge any more. Then power down the server, unplug it, pull the UPS out, swap batteries, etc.
Then even when I think I've got the UPS automatic shutdown scripts and drivers finally working just right under linux, a routine version upgrade breaks it all for some reason and I'm spending another 30 minutes reading through obscure docs and running tests until it works again.