Another for Keycloak. Though it is probably overkill for many people’s needs in here - it certainly is for mine! But it is what I have up and running and see no need to change to a simpler option.

I’m no battery expert, so pretty much all of this is supposition. I think it’s reasonable, but if anyone knows this stuff, please correct any errors!

It is likely that the house power backup is run in an ‘offline’ mode where the power load is shifted onto the battery circuitry when needed. UPSs for computers run in an ‘online’ fashion where they are effectively part of the circuit continually. That is why they are so much more responsive. I would imagine that running ‘online’ puts much more wear and tear on the battery resulting in a significantly lower life-span. Regularly replacing the low power UPS batteries would be very much cheaper than having to replace your home power backup every few years.

I wouldn’t class the UPS/Backup power combo as redundant hardware any more than either one of those things are alone. Either one is redundant if the mains power is 100% reliable. Each component is there to help deal with the inherent deficiencies of the overall power system.

Once you have systems that provide meaningful open monitoring protocols, having a single unified view and control plane is just a software problem. I would imagine you could throw data in to something like Grafana to display mains power state, UPS state, backup power state and use the combination of that data to estimate overall run tile and when to trigger a shut down of servers.

I think you have managed to skirt around and discount the answer to your question. UPSs are not intended to be anything but a very short term power source. They are there to provide emergency power long enough to either shut down cleanly or migrate to another long term source of power. In your example with the “home power backup”, you have that and a UPS. The UPS smooths out the power and covers the few milliseconds during the transition. Even large datacenter UPS installations operate in this fashion. Datacenters would have a secondary source of power, typically diesel generators. The UPS batteries will be sized to run the site for a few minutes - enough time to get their generators up and running and the load transferred to them.