REF:
http://mb-soft.com/public2/energyso.html
Jes' #s fer kickin' around. I usually think in terms of a hp per yard2.
Trouble is that production solar modules with even 25% efficiency are 75-300% more expensive than the run-of-the-mill stuff, and getting anywhere near peak efficiency for more than a few minutes a day requires tracking, which is complex, prone to breakage, and darn near impossible on a ground vehicle.
For a fixed installation with plenty of space available, it's drastically cheaper and more reliable to just put in 10-20% more solar panels than to incorporate tracking and high efficiency materials. That doesn't stop a lot of big showpiece projects from using trackers, but it's amusing to look at them on Google Earth and see 5-10% of their panels pointing entirely the wrong direction due to broken trackers.