Let's think about solar panels...
A 500 watt solar panel gets me thinking. How large in square feet, how heavy in pounds, and how expensive would a solar panel have to be to generate 500 watts?
Based on COTS (commercial off the shelf) panels, 80 watts can be generated by a 30 pound panel with a surface area of 6.9 sq ft. That means that 500 watts would require 6.25 panels, a weight of 187.5 pounds, and cover 43.125 sq ft. That's a square with over 6 and a half foot sides! I suppose you could cover the entire top of the car (roof, hood, trunk) will solar panels and get close to that much area. You'd lose one passenger worth of carrying capacity due to the additional weight but that might be acceptable.
However, assuming that you paid around $500 for each panel, you'd be out $3125. If this arrangement did generate 4kWH per day (best case scenario), that would save about 40 cents per day in grid power. That yields an ROI of 7812.5 days or about 217 years. I think we should encourage worksites, malls, grocery stores, etc. to just install metered electrical outlets in their parking lots.