You use sealed lead acid batteries, no hydrogen venting it is all internally re-absorbed.

This is the right track for serious backup. (do you have a basement? does it have a sump pump? Does the pump run during big storms?)

We were building these for field equipment when I was working in oil and gas. We bought lightning arrestors/line filters from one source (I wish that I could remember because they were real good), inverters from another and battery charging/monitoring stuff from a third.
We had a small Li ion battery to back up the electronics and three or four big ass deep draw sealed lead acid batteries for power.
They are big and don't look pretty, but they were 1/4 the price of pre-built units and worked very well.