It would probably be better to use a low amperage 240V circuit if you want to charge your car from the PowerWall when the grid is down. Ideally you would be taking the power directly from the solar and they normally rely on the utility transformer to provide the 120V current because they don't connect to the neutral. The DC PowerWall with SolarEdge inverter requires an external auto-transformer to balance the neutral and supply 120V loads when the system is islanding.
My PV system uses micro-inverters which nominally output 240V, so I'd be going with an AC PowerWall.
Apart from possibly EV charging, all of our backed up loads would be at 120V. I admit that I'm not sure how this works - perhaps micro-inverters and the AC PowerWall connect to the neutral. For the EVs, it seems that it would be most convenient to install a 20A 120V outlet in the driveway by our EVSE, as this could have other, non-backup uses such as running a vacuum cleaner, an electric snowblower, etc.
I'm corresponding with Pick My Solar (
Compare Solar Companies | Solar Marketplace | Pick My Solar) regarding California's SGIP and installing a PowerWall, as they're the company that collected bids for our recent PV install and they were the first to tip me off regarding SGIP (before I started reading this thread). But they're now waiting on the state to provide more details as to the SGIP application process.
By the way, I originally tried going with SolarCity for PV, but after wasting several weeks, they eventually "disqualified" us because we have a lot of shading. SolarCity seemed to not grasp the fact that we were paying cash and would thus be in a position to tolerate lower production and a longer system payback time. So our money went elsewhere, and we ended up with better, more efficient panels. After that odd experience, SolarCity is not my default choice to install a PowerWall 2. Hopefully Tesla will shake things up for the better!