5
Tesla owner adds 4000 watt solar panel roof to his car after company wouldn't
(www.uniladtech.com)
You'd need to have more moving parts that can break, watch out of aerodynamics, and keep the solar array in a form factor smaller than the car to prevent neighbouring solar cars from deploying and breaking yours.
The concept is nice, but not really worth it IMO.
Edit: not counting that cars are pretty bad in the first place. Put solar on buses? That seems a bigger payoff