Imagine then that you put a 1 m^2 panel on the roof and line it flush with the panel so it doesn't add any more resistance. Heck, make that two square meters and put one on the boot lid as well. Then assume that you're getting 130 watts out of it through the sunny day. That's just 65 watts per square meter - quite likely.
13 kWh per hour at 100 km/h is 130 Wh per kilometer, which is incidently the same amount of energy your panels would be generating, in an hour.
When your car sits eight hours on the parking lot of the company you work for, you have generated 8 kilometers of free electricity, or more if you don't need to drive so fast. A car that spends its days outdoors would generate about 16 kilometers of free range each day, assuming it gets dark for 8 hours and the average production is 65 W/m^2.
That means, if you live closer than 8 kilometers from your job, you wouldn't have to recharge the car the whole summer. The only downside is that you can't leave it in the shade.