I can only give you my experience as follows: We have a 7.8 kW solar array connected with two Tesla Powerwall 2s. During the recent PG&E blackouts here in Sonoma County, and the Kincade fire, our system performed flawlessly. It covered everything we needed including some AC load, entire 1600 sq ft home needs, and everything but charging our two Teslas. [NOTE: We heeded Tesla's warning and charged our cars to 90% prior to each blackout.] We probably could've partially charged our Model S and Model 3 by timing it correctly during the day. If the battery is mostly charged, the system modulates how much electricity that comes from the solar array by partially shutting down one or two of the inverters to prevent overcharging of the Powerwalls. As a result, there are times in the morning to mid-day when excess power is available, although we chose not to charge our cars. We could have driven about 10 miles to the nearest supercharger, if necessary, when needing a charge. The SpC remained in operation throughout.
Attached is a screenshot from the Tesla app showing the backup performance for the last few months. As far as I am concerned, getting the Powerwalls was a great decision for use as a backup alone, much less what it does to lower our use of grid power as well.
