So, this is probably a matter of buying the right UPS but I am currently using a Pi to control my aquarium via relays. I'd like to hook the whole system to a UPS in case of powerloss. Ideally, I'd have my Pi be able to notify me when my UPS kicks in and safely start shutting down less essential parts of my system (lighting, certain pumps, etc) to preserve battery life. I'm not sure how to detect when a UPS kicks in but, for obvious safety reasons, I have absolutely no interest in trying to DIY build a UPS with such functionality. I suspect it's a matter of finding one that supports monitoring hardware.
To clarify, I'm not looking for a UPS hat for the pi. I'm aware that those exist but the UPS has to be able to power a number of very power hungry pumps.