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.
Then 5 years later there was a power outage and the UPS lasted for about 10 seconds before the batteries failed. That's how I learned about UPS battery maintenance schedules and the importance of testing.
I have a calendar alert to test the UPS. I groan whenever it comes up because I know there's a chance I'm going to discover the batteries won't hold up under load any more, which means I not only have to deal with the server losing power but I have to do the next round of guessing which replacement batteries are coming from a good brand this time. Using the same vendor doesn't even guarantee you're going to get the same quality when you only buy every several years.
Backup generators have their own maintenance schedule.
I think the future situation should be better with lithium chemistry UPS, but every time I look the available options are either exorbitantly expensive or they're cobbled together from parts in a way that kind of works but has a lot of limitations and up-front work.