A budget solution to run for 3 (24 working hrs) days and about 1.5kW

[zanydroid]

I don't understand one of your point. Is California rules not permitting me to set in garage 2 battles, inverter and distribute electricity via power cord to my computers without hookup to home's wires? Is that correct concern?

That probably qualifies as an Energy Storage System under building code and fire code (Those are the two building code books I'm aware of that talk about battery), and that triggers some requirements which requires expensive hardware to satisfy.

And if you want to connect your batteries to a grid-parallel system (output of inverter tied to grid, synchronized, and capable of exchanging energy back and forth of course since they're wired together, even if zero-export algorithm is enabled) then the system needs to pass inspection before the power company will let you do this.

There's not a significant difference in battery fire risk IMO from plugging into your home wiring vs running extension cords. Actually I'd say extension cords is strictly worse than a properly off-grid setup going through your building wiring, since extension cords are not as robust and meant for temporary use.