I had a 20 year old APC start misbehaving on me a few months back, and new batteries didn't fix it. After messing up the repair on the first one, a second one started doing the exact same thing, and I immediately took it apart and, now an expert on how not to fix them and what the root cause was, I swapped every cap on the board that my ESR meter claimed was bad. It's now back in operation, and while I was testing it, I realized it's so inefficient that about 10% of its max rating is just going up in heat. The replacement for the first one, I assumed, would be much more efficient, and it is, but it's still dumping about 7% of its energy into heating the transformer.
OTOH, the older UPS behaves much better with apcupsd, going so far as giving me the firmware versions, battery replacement times, better power monitoring/etc. While the new one works its missing most of what the older UPS could do (although its not a APC).
So, i'm sorta on the fence about just using "backup generators" from Amazon/etc because one of the ones I have has about 5x the capacity of the new UPS and seems to be able to switch without the computer plugging into it having an issue. Plus, its power waste is less than 1%.
The batteries are definitely worth more than the unit is at this point.
A backup generator for me is not an option but this is mostly just to keep internet and a couple machines going in the event of a power failure. One day maybe the used lithium ion one will pop up and I will swap it out.
If you don't actually need the ~2 kw peak power output, there are a bunch of off brand 100-200W power stations with 50-150Wh batteries that aren't much more new than a new battery load for that old UPS (ex: random hit, flashfish 40800mAh Solar Generator - $129, powkey 200W $100). Although the Wh ratings are suspect on may of them, and they aren't online/line-ineractive (which is why they are more efficient) so you lose the power conditioning since they are effectively offline UPS's.
I see people dumping old jackery, ecoflow, models for 1/3 of retail on craigslist/etc somewhat regularly although those tend to be larger models. Its a bit of a mystery to me why some of them aren't advertised as having UPS functions if they have fast switch times, which many of them do since they don't have physical relays. My old line interactive UPSs are all physical relays which click when they switch over, the assumption is that the energy in the transformer is sufficient to cover the switching time, but of course they do brown out a bit when near their current limits.
