I should be based on the local sales tax for the installation site and it was for mine, but with one weird twist for one line item;
Solar Panels - Taxed
Inverters & Balance of System - Taxed
Mounting Hardware - Not Taxed
Installation, Permitting and Other Fees - Not Taxed
Powerwalls - Taxed
Backup Gateway - Taxed
Powerwall Installation - Taxed
Powerwall+Solar Discount - Taxed (deducted from amount)
Labor isn't taxed in California, so that makes sense, but I would have expected that the "Mounting Hardware" would be taxed and it was omitted. If I add up the rest and multiply by my local tax rate of 9.000% it exactly equals the Taxes line item.