I don't think that solar panels will be capable of generating enough power to make much of an impact on an electric car's mileage. Happy to be corrected.
The standard solar panel has an input rate of around 1000 Watts per square meter, however on the solar panels available at present you will only gain roughly 15-20% efficiency at best. Therefore if your solar panel was 1 square meter in size, then it would likely only produce around 150-200W in good sunlight. [1]
For example: 2 square meter panel x 1000 = 2000 x 0.20 (20% efficiency panel) = 400. 400 x 5 hours of sun hours = 2000 Watt hours per day. [ibid]
In 2012, the EPA range for the 60 kWh battery pack model was 208 mi (335 km) and the 85 kWh battery was 265 miles (426 km). [2]
A Tesla gets about 3.5 miles per kWh, so you would be able to get about 7 miles of driving distance per day (5 hours of sunlight) of charging.
The Prius has an optional roof solar panel, which generates about enough power to run the fan in the car when it's parked outside of shade to keep the cabin a little bit cooler.
Probably correct. Tesla had solar roofs stated as one of their second phase of goals but has since concluded it wouldn’t really work. An alternative model is to have fold out roofs that can operate while parked.
I don't think that will get you enough to really change the math. There's only so much additional space you can get with a fold out panel because once you start going over the edge of the car you create clearance requirements for parking and also start taking up space that's generally assumed to be empty for adjacent cars to open doors into. If it becomes too common parking spots will have to get bigger or your roof will have to raise up above the level of 99% of adjacent car doors to avoid being a massive pain in the ass to everyone else.
On top of that it's a lot of mechanisms to add for not adding much space above a roof mounted panel.
