The sun hits the earth with a power of 1 360 watts per square meter
However it shatters in the atmosphere, when it reaches the ground it's down to 1 000 watts per square meter
Solar panels have an efficency of 20-30%, so in good conditions you can get 200 - 300 watts per square meter
A typical big-ish car is about 1.5 meters wide by 4 meters long, for a total top area of 6 square meter
6 sqm * 200 - 300 watts = 1200 - 1800 watts
An efficent electric car can get 150 watts per km, so 1200 - 1800 watts becomes 8 - 12 km, hardly motorways speeds
But give it 10 hours of sun and you get about 100 km of range, well above average use (about 60km a day for USA )
Optimal Conditions
That is at a bright sunny day, no clouds, an efficent car with good solar panels. But it does show that the idea of a car you never need to plug-in is possible
Solar panels are closing in (or passing depending on where / what your looking at) 1 dollar for 10 watts. Meaning those 1500 watts would add just 150 dollars for the panels (the extra work for wirering, etc. would almost assuredly at least double that)
For most car buyers an option at about 500 dollars to not have to charge the car ever during streaks of sunny days and then ocassionaly when using it more or during bad weather... it sounds like a done deal
Problems
It mostly looks amazing cause our brains goes to gas cars and what they cost to run. If someone hands you a gas cars and tells you the gas is fucking free when the sun is shining?
GIVE IT TO ME!!!
However this is eletrical cars, it looks cheap cause solar is cheap, but's it's even cheaper to put on buildings and then you don't have to have a lot of "optimal conditions" to run your car on pure sunshine
Summary
Yes you can put solar panels on cars, yes it does kinda makes sense / is not completely stupid
The main problem is that although putting panels on cars is kinda okay, they are amazing to put on buildings and connecting them to the car instead!
The problem is not that solar panels on cars is bad, it's just that there are better options