Am I the only one that parks my car in the sun outside my office 5 days a week?
Regardless, a 25 W panel might cost $100 and generate maybe 25kWh (75 miles) of electricity over a year. Even at PG&E's punitive $0.34/kWh, that's still not great.
Panels on my roof are cost effective because you can get them cheaper when you buy big heavy panels, and they produce more power when they're always outside. Also, I can sell electricity to PG&E at the daytime rate and charge my EV at their night time rate (well, assuming Model S will have some kind of charge timer at some point).