Rather than free, I'd say the marginal cost would be more like what you could otherwise sell the electricity back to the grid for. eg. if you're paying $0.25/kW

Rather than free, I'd say the marginal cost would be more like what you could otherwise sell the electricity back to the grid for. eg. if you're paying $0.25/kWh to buy from the grid, and selling back to the grid at $0.06/kWh, then you're really "paying" $0.06/kWh in opportunity cost to charge your vehicle from your solar panels.