And I don't have to buy them from Tesla â" there are plenty of other sellers out there.
Yeah, but if you have a Tesla roof, Powerwall, and Model 3, then it'll handle the credits for you among the reverse net metering, the panel, and your charge-ups at the SuperChargers.
If you put ten Teslets in at home yesterday, you can take ten Teslets out at the SuperCharger today.
Solar City, on its own, had to make up all of the finance costs from net-metering only. As part of Tesla, they can give you flexibility on how to handle the charges. If you consume way more than your production in your cars, and go past the finance costs for your roof, then they can charge that difference to your credit card. But before that, it's much more economically efficient to keep all the charges in-system.
If you have a non-integrated stack then you can do all the same things, but it's necessarily going to cost more because of transaction costs.