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

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.