Thanks for the links on ac-coupling. For off-grid, does the reduction of one inverter plus the self-restarting capability of dc-coupled systems make it a better choice than ac-coupled?
The ability to add battery backup to existing grid-tied system without drastically modifying grid-tied system's wiring is definitely a bonus but at $20,000 for 4kw battery backup (ameresco quote) I can see the reluctance to spend the $$.
You have it right. The dc coupled systems use a multimodal inverter which has 2 outputs for ac. One will connect to the grid and non-backed-up loads and the other output will connect to the backed-up loads. You will still need a charge controller, but this is the cost effective solution for now.
The ac coupled system is neat for micro-grids (large systems) and for converting grid-tied systems to battery backup.
I saw a great Ameresco ac coupled system when I was doing a class on an Indian Reservation in the Grand Canyon. Nice work!
It is an easy cheap alternative to get one of these and go to sleep when the sun goes down if the power is out: