Different backup options service different purposes.
UPS: very reliable, along with surge protection. Will begin charging when the lower returns. I have each tank connected to very large (commercial server grade) UPS systems. They’ll run the entire tank for hours, but not days. I also have dedicate UPS for critical systems like aquarium computers and dedicated Aquarium internet connections.
Lithium power station backups: these can hold a LOT of power, I have some large ones that will run orders of magnitude longer than even my larger UPS systems, and I have solar and wind generators that connect to them if need be. I keep enough lithium power backup to run everything (even the lights and heaters) for about 24 hours, excluding solar/wind recharging. However, they are NOT uninterruptible - you must be present to manually connect to them, and disconnect from them again. Do not make the mistake of even thinking they’ll begin charging again when the power returns, they won’t (if power is still being drawn from them).
Individual pump batteries or DC battery/power systems: an awesome way to run small DC pumps for a long time. I even have a DC power system to run my dedicated 4G ‘modem’ that my HYDROS connects to. I use these on at least one flow pump per system to ensure that there are days of water motion available even if I’m not able to get home.
That being said, I’ve never experienced an unplanned power outage, but I take redundancy seriously.
My advice is to use all 3. If you have a house, a Tesla power wall and/or auto start natural gas generator would also be good, if not both. But I always have at least triple redundancy now, and it’s always best to be prepared for 72* hours of power loss because it’s big disasters like typhoons and earthquakes that normally shut off power (or maybe ice storms if you’re in the southern USA).
Why am I so interested in redundancy? Because after decades of keeping aquariums, I’ve learned from losing everything to all kinds of unforeseen circumstances and disasters...