When you're ordering Tesla solar it adds $10k to add a power wall. Maybe it's because they include the install and connection to the solar system?
With a warranty of just 8 years a single powerwall wouldn’t have saved me enough to even pay for itself at the rates my power company charges, so I opted out. That's actually true of the so,ar panels themselves as well. Even over 20 years they'll never pay for themselves, but I got those more to help offset my carbon footprint not to save money. But the powerwall just felt like a luxury. My solar panels aren’t going to generate enough to even run the house, so there will be little if any overage. They'll just reduce my usage of grid power. And even if I charged them late at night when power was cheap and used them during the peak of the TOU plan it would have taken almost 30 years to pay for themselves. And where I live power outages are rare and usually only last a few hours, so I don’t really have a need for a full house backup system.
You have said the powerwall warranty is 8 years a few times, but its not. its:
warrantied to maintain a 70% charge for 10 years, unlimited cycles, when charged by solar.
https://www.tesla.com/sites/default/files/pdfs/powerwall/powerwall_2_ac_warranty_us_1-4.pdf