The kicker is what happens if you get your kwh at the expense of a one time investment in solar and batteries. The grid or fast charging rate is the worst case in terms of pricing. You can actually charge smarter way cheaper than that. Many fast charging setups essentially only rely on the grid to top off the batteries if solar is not providing enough energy.
Basically, if you charge using solar panels that you own, the miles per dollar is a function of maintenance, and purchase cost of the vehicle and panels. The efficiency of the panels or vehicles are not a factor at all since the variable cost is 0$/mile. You only have variable cost when you charge the vehicle somewhere else. The non variable cost is basically the ownership of the vehicle and panels. Which you might simplify via e.g. leasing constructions.
Basically, if you charge using solar panels that you own, the miles per dollar is a function of maintenance, and purchase cost of the vehicle and panels. The efficiency of the panels or vehicles are not a factor at all since the variable cost is 0$/mile. You only have variable cost when you charge the vehicle somewhere else. The non variable cost is basically the ownership of the vehicle and panels. Which you might simplify via e.g. leasing constructions.