While the Sun stays there, energy is free and it doesn't matter if the people still prefer to create pollution and endanger themselves for a while. Sooner or later this will change. Then, for transportation, what about the size of a normal vehicle for installing solar panels? The idea is to use the greatest quantity of area in square meters.
Some would instantly think in the vehicle roof but this is usually limited. If you see a vehicle from the top, all this area should be used for the task. An average parking site, of 15 square meters. An interesting solution is to install a rolling element with two fasteners in both sides of the vehicle. As solar panels could be flexible, the user would roll the solar panel like a paper roll with a spring to return to its position. Like those projector screens.
Then, if all the area is ready the math comes into aid to calculate the power in this space at a certain solar panel efficiency. Will it be enough for avoiding a single attachment to a power receptacle? For sure, not in every month or location. Anyway, a figure for 15 square meters in good conditions is the following. Say 1000 Wh and 20% efficiency and you would ideally get 200 Wh per square meter. With 15 square meters it's 3 KWh per parking lot. With four hours, an impressive 12 KWh input.
An EV battery could have 25-30 KWh, others more. 12KWh means that is near 50%, so the car would have energy for everyday routines in a common parking time to work. Even it would be necessary to turn-off the solar panels. With a 40% efficiency, almost full in four hours. The efficiency must go higher than this in a bit, so in terms of investment, cars that pollute do not seem the best, thinking in years to come.