Problem I perceive with this is that you need the power cut to start when the PowerWalls are fully charged ...
... most of our power cuts here are either momentary (lightning nearby and something upstream trips and almost immediately power is restored, but all electronic gear and all the clocks in the house "die" during that interval).
The other type is some twit in a mechanical digger going through a cable, or an elderly tree just decided today is the day to keel over ... that is all sorted out within a couple of hours, so even a largely discharged Powerwall would usually do me for that situation.
Beyond that is a once or twice a decade storm, and because we are rural it might be a day or two before we are back on. For that I have a generator, only large enough to power essential circuits, but if we had some sunshine (storms normally in Winter and not much PV power at this latitude in Winter), and a powerwall which I could recharge from generator, then running the generator 24/7 would mean that PowerWall gained some during the night when we had everything turned off, gained a bit more from PV during the day, and then I would have more than just generator on its own for evening lighting and TV etc.
But Powerwall not yet easily recharged from Generator, and not sure about from PV if the Grid is off. We do have isolator switch for the grid (so I can run the Generator without back-feeding the grid), and I believe that once the generator is running the PV will also work ... but not certain that everything will play well together.
I keep meaning to buy a bigger generator ... and we keep not having a once-a-decade storm ... although when we do it will catch me out of course
The one thing I wish I had done when we rewired the house was to put all the essential circuits on a single board, so we could easily isolate all the other power-hungry junk, and only consume survival electron-rations without all sorts of mucking about trying to figure out what ELSE was soaking up power and needed turning off.
Past storms have been "trail an extension lead through an open window to run the TV and a couple of lights" which is a long way short of ideal!