Is there a reason why 2 powerwalls would not back up your AC?
Yes and no... They are older 10 seer models (from 2003) and the LRA is 81A. I actually measured them at about 90A inrush. So the startup current is much too high for the Powerwalls. Tesla told me that soft starter devices would not be enough to bring down the LRA, so denied hooking them up originally. But after some research I found (and installed) some devices that bring the LRA down to about 19A each - well within specs for the 2 Powerwalls. In any case, I have ordered another PW and will have then move the ACs over to the backup load on install (3 PW can backup 56-83 LRA, 2 PW need to stay below 55). I am also going to replace the ACs with much more efficient ones in the winter (budget constraints due to a very expensive roof ).