Fairly frequently, 6kVA UPSs come up for sale locally to me, for dirt cheap (<$400). Yes, they're used, and yes, they'll need ~$500 worth of batteries immediately, but they will run a "normal" homelab for multiple hours. Mine will keep my 2.5kW rack running for at least 15 minutes - if your load is more like 250W (much more "normal" imo) that'll translate to around 2 hours of runtime.

Is it perfect? No, but it's more than enough to cover most brief outages, and also more than enough to allow you to shut down everything you're running gracefully, after you used it for a couple hours.

Major caveat, you'll need a 240V supply, and these guys are 6U, so not exactly tiny. If you're willing to spend a bit more money though, a smaller UPS with external battery packs is the easy plug-and-play option.

> How bottomless of a pit it becomes depends on a lot of things. It CAN become a bottomless pit if you need perfect uptime.

At the end of the day, it's very hard to argue you need perfect uptime in an extended outage (and I say this as someone with a 10kW generator and said 6kVA UPS). I need power to run my sump pumps, but that's about it - if power's been out for 12-18 hours, you better believe I'm shutting down the rack, because it's costing me a crap ton of money to keep running on fossil fuels. And in the two instances of extended power outages I've dealt with, I haven't missed it - believe it or not, there's usually more important things to worry about than your Nextcloud uptime when your power's been out for 48 hours. Like "huh, that ice-covered tree limb is really starting to get close to my roof."