This also influences the decision to take the ITC
We've had our PowerWall for a couple years. I did a calculation about how much grid charging would save me, and it was only $39 for the year. This undoubtably would not justify skipping the tax credit. Our PW was paid for by SGIP, so we did opt not to take the credit.
Our annual true-up runs around $60, so that meager $39 savings from grid charging looms much larger that it really is. But this year, after a year of free supercharging, we started charging our car at home. We beefed up our solar a bit, limited by what our single PW can handle, so all bets are off...
I found my notes, so here is some detail on my calculation. Our daily PW discharge during peak periods is ~50%, i.e. ~7kWh. We are on PG&E EV2-A, NEM1, with an assumed difference between part peak and off peak of $.20. I compared our daily production for a year Enphase data with hourly consumption. (Yes, an 8,760 line Excel sheet) On 51 days, our production was less than our peak consumption, totaling 194 kWh, so that is how much grid charging we would need. Hence $38.80. Precise but not very accurate because I did not simulate the actual state of charge, and a after the first cloudy day, the battery would still have some useable charge.
Far be it from me to advise anyone to cheat on the ITC rules, but Tesla's own Storm Watch does grid charge. And 194 kWh is peanuts. So I'd probably grid charge even if I had taken ITC. I think the ITC should be applied to storage anyway. Storage, with or without solar, lets one stay off the grid during peak times, hence saving everyone money, us, pg&e, and all their other customers too. ITC should be deployed to incentivize this, just like solar, in my opinion.