How to optimally control the Powerwall with solar is a fairly complicated question. It's unclear if the future TOU software will allow us enough control, but here is what I think would be best:
If you have solar, NEM, and didn't take the ITC, then ideally you'd like to buy all your energy for consumption at the cheapest rate, and push all your solar production out to the grid at the highest rate. That's my understanding of what is allowed by the California utility tariffs--time shifting of production and consumption, but no straight rate arbitrage.
So as an example with a TOU plan with low, medium, and high rates, and consumption of 20 kWh/day during the medium and high periods, and solar production of 10 kWh/day during the low and medium periods, you'd like 30 kWh of usable storage. That would take 33 kWh to charge up, which you get from 23 kWh of grid energy during the low period and the 10 kWh/day of non-high solar. You'd discharge 10 kWh of that during the high period (time shifted solar production) and the other 20 kWh during the medium and high periods (time shifted consumption). Except you might not need the full 30 kWh of usable storage, as during the medium period you'd want to be both charging from the solar and discharging for consumption, so some of that would net out.
With solar, NEM, and taking the ITC, the situation is simpler, as all you can do is time shift production. So you charge from solar during the low and medium periods, and discharge during the high period (whether for self consumption or not, doesn't matter economically with full NEM). In this situation I'd be tempted to take only 90% of the ITC, so I could charge the Powerwall 10% from the grid (for the round trip losses) and push out my full solar production during the high period.
Cheers, Wayne
