[revanx_]

How do you achieve the X <= 10 ms switching capacity? You need this if you actually want your equipment to survive the power outage.

Also, I'm extremely sensitive to humm noise output by inverters so I can only tolerate UPSs that have that "Green mode" option which (I guess?) powers directly from AC when under quality power but still provides protection.

[jotm]

Capacitors or using a circuit where the battery is always supplying power (can't remember what it's called).

Could make a post-PSU UPS, then you don't need an inverter at all (also the most efficient design, no conversion losses)

[rootusrootus]

My preferred inverter/charger manufacturer, Victron, has "UPS Mode" for their Multiplus units that will switch in 12ms. That's similar to many UPS's, though some of the higher end ones are sub-10.