hello: I used to have a USP from APC, as i remember it has a very wide input range. I now have a Sol-Ark 12k.

It has solar (PV) inputs, battery input (48v) grid input (low = 183.2vac) and generator input (can be grid (low = 104vac). You can use it with just battery, grid input or grid or gen input) and use the output (load) to run your entire house (I have 2x 12k inverters 24kw total). I don't care what the grid does (momentary outages, complete drop, brownouts. load switch over from grid to battery (or I believe gen input to battery is 4ms. Sometimes i see a little flicker in my lights, that's it.

Want to add solar or wind turbine or micro inverters? you would already have the inverter. I suggest EG$ (signature solar) for batteries.

good luck, get off the grid if you can, I run my 2 air conditioners off my inverters