Right, so if you think of it as a grid tied system it will generate power when ever it can, and people will charge on it. I've got 5.2kW of panels on my roof and can verify attest to the fact that even though I might use 28kWh in a day I can generate 34kWhr (during the summer, during the winter my best days are about 20kWhr) I would guess that as long as the duty cycle on the fill up is small they will consume net zero power.

The SunTech panels [1] produce a nominal 295W and are 2 x 1M so in your 6 x 4M square you can put 12 panels, nominal output 3.5Kw, my system is 90% "efficient" (5.2 kW Panel rating, 4.7kW delivered to the grid) assuming the same that is 3.15kW delivered to the grid. over a nominal 6 hr solar 'day' that is about 18kWhr/day. So the duty cycle on a power station would need to be one fill up every 3 days. If the travelling is mostly on weekends I could see the stations getting a couple of fillups on the weekend and having 5 days of no visitors. Time will tell. It is handy that Elon's the primary stockholder of Solar City :-) Good discount on panels.

[1] http://www.solarsystemsusa.net/solar-panels/by-brand/suntech...