You might want to check out
Grid Tie and Grid Interactive Systems
Those guys should be able to give you some high quality information.
I've looked at how GTIs work in the past, and using them off-grid can be problematic.
With 3 or 4 kw of PV connected to a GTI, that power will normally be used in your house and the excess will be absorbed into the grid. (Your neighborhood load).
But, if you don't have the grid, and your loads are only 2 kw, where does the rest of that PV power go?
If your GTI isn't real smart, it might crank up it's voltage to try to dump all it's PV power to the "grid".
But since there is no grid, it needs to ignore some of the PV power and take just what you need..
I know there are new Outback GT boxes that will run off grid.
But they aren't real cheap...