While I'm EXTREMELY new to the hobby, power outages were one of the big things I researched about and decided to invest in protection against on day 1. I hate the idea that I could have an entire extinction event without me knowing or being able to do anything about (power outage while asleep or away).
I ended up purchasing an Anker 757. It's NOT cheap, but it has 1229whr, LFP battery cells cells, and a 5 year warranty. With the typical coupon on Amazon ($100-150), it ends up being right around $1 per whr. For my Max Nano Peninsula, it will power the pumps and the heater for ~2.25 days.
While researching I found that typical computer UPS systems have substandard battery chemistry (cells often need replacement annually) and they aren't optimized for runtime... they're more optimized for speedy cutover in the single digit milliseconds to provide the user with some number of minutes to cleanly shut down IT equipment rather than experiencing an unclean shutdown.
The Anker unit is really designed to be a "portable power bank" type system, with chunky grabhandles and the like. BUT, it does have a "UPS mode", which will power the AC ports as passthrough from wall power until a power outage, at which time it will perform a cutover to the battery within an advertised 20ms.
There are slightly cheaper per whr "portable power bank" with UPS systems available with the more economical cells (e.g. NCM, like EcoFlow DELTA). The LFP cells, though, offer something like 3k cycles to 80% capacity vs. 500 cycles to 80% capacity with NCM. But, no lithium ion options will thrive being static at a high state of charge... may need to set a reminder to cycle it here and there!