You can turn the Powerwalls off, but all batteries have a discharge rate when they are just sitting around. Turning them off might reduce some of the overhead, but they will still self-discharge.Well, yeah… That would have been a better idea.
Although in my case, the Tesla guy pointing out that I could charge the panels with solar is what set the lightbulb off for me that I could completely go off grid until I got PTO. So lucky mistake.
With a lot of effort, I managed to browbeat the Tesla solar person into connecting me with a Powerwall person who enabled grid charging to protect my Powerwalls when they went below 25%. I am still getting nothing from Tesla but finger pointing at SE and platitudes and have been down since 13 JUNE on my 3rd outage in 20 months of operation. Unhappy here. I was trying to get through to SE and have not yet succeeded.