48V Offgrid Solar Power System EG4 6000XP, one or two inverters?

Buy one 6000xp, but plan install for two. If one isn't sufficient, you can easily parallel a second. Or add batteries to the single.

BTW: max AC passthrough for one unit is 37.5a. Make sure that is enough for on grid usage.
OP... glad to see a very similar reason and configuration for the path I started on this past week. So I will be watching with interest.