[zzz61831]

Yep, and once you start using custom DC UPSes the biggest issue will be battery chargers that kill lead-acid batteries. Things are better with LiFePO4 these days, but for lead-acid I haven't found a solution except for literally hacking a pulse charger and battery management on top of arduino.

[dreamcompiler]

I've had good luck with solar charge controllers for RVs, e.g. [0]. You have to drive them with DC of course but they're designed to be driven by very poorly-regulated DC like that coming from a PV panel. So a cheap AC/DC converter (either switched or linear) driving one of these things should preserve your batteries. Most of them also provide a DC load output which gets automatically switched over to the battery if the input DC (or in the case of a UPS, the input AC) fails.

[0] https://www.rvweb.net/best-solar-charge-controllers-for-rv/