The logs should help. I presume you are only going to use a fraction of the powerwall each day. 5 hours of isolation should generate 25kWhrs of energy. You can control how much of the 39kWhr capacity you plan to use each day.
No, that's not right. If you get the SGIP, then your basis for calculating the ITC is your net cost, and the ITC would be up to 30% of the net cost.
Note that other people have reported that SGIP for "Small Residential" for Tesla Powerwall is already sold out for all steps. However, SGIP for "Large Scale Storage" applies to 3 or more Powerwalls and is currently available for Step 2 with PG&E. Tesla is only about 90% of the way towards exhausting its Step 2 Large Scale Storage developer cap, so they may have room for you still; you'd have to check with them to see how big the queue is. There are some additional requirements to meet for Large Scale Storage than don't apply to Small Residential.
I imagine that if you take the ITC for a solar charged battery, and you get audited, then the auditor may ask you to demonstrate that the percentage of solar power used to charge the battery is in accordance with the ITC taken. Tesla should have the necessary logs to demonstrate this. I forget if this requirement for the ITC remains in force for 5 years or 10 years.
