You're buying power from the utility company, at whatever rate they set, and at the same time you are selling your power to them, at whatever rate they set. The

You're buying power from the utility company, at whatever rate they set, and at the same time you are selling your power to them, at whatever rate they set. They can be thought of as two separate transactions. (You buying power can be thought of as a sunk cost, as this isn't changing it at all (unless you are also storing power), so it can be removed from the question.) At the same time that you are selling power to the utility company, you are also paying Tesla 10-13¢ for every kWh you generate. If you are able to sell at a higher rate than that, you keep the extra.