I do not have first hand experience/info on leasing Solar Panels or Powerwalls, just want to clarify and maybe point out somethings.
If you lease, I think you can NOT claim the 30% Income Tax Credit or the SGIP (for California, about $3-4k Each PW). That is a big chunk of the cost.
It's possible to lease or buy/finance Solar Panel system and/or Powerwalls together or separately, ask your Financing company via your installer.
$40k for 20kW solar system is really good price, @ $2/watt is unheard of recently...pls check to see if this price is because of 30% ITC off. 1 Powerwall now is usually $8K-$10K, installed.