Last time I ran the numbers, in ideal situations a truck/trailer could generate enough power from solar alone to drive on the highway. Ideal means the truck is already at speed, it is towing multiple trailers (length is only legal in a few states), they are used the most efficient solar panels made (significantly better than the mass produced panels most people are putting on their house - at a massive increase in cost), it is near noon, and it is near the equator.