The usable kwh of the gen1 is 10.9 so maybe target a 12kwh capacity?
I'm kind of taking on a similar project. I have a 5.1kwh 24v pack that I intend to use for various purposes. Last night I dumped some of it into my gen2 volt (14.0kwh usable). I have a 3000w pure sine inverter 24v in, 120v out. It's massive over kill at this output voltage but I also wanted to be able to use it for backup home power.
In order to get the volt charger to not complain about the lack of grounding of the invert I had to tie ground and neutral together but then the charger went green.