Futurology

Yeah this is total BS.

The sun delivers about 1.3kW of energy per square meter and an average car takes up about 8 square meters of space. So that's about 11kW to work with, but even very efficient solar panels only harvest about 20% of that. So that leaves us with about 2.2kW to work with. Now in order to convert that energy into something usable and charge the battery, more losses are added, depending on the situation this would be around 20%, leaving 1.76kW. This means charging a 50kWh battery would take around 28 hours. Obviously we can only expect around 6 hours of really good sun on an average day, with the rest of the day having much less energy available. So charging the car would take several days in perfect conditions. If it's cloudy, if it's raining, if you've got shade over the car, if you drive the car etc. it will take more time to charge it.

In reality however, this is with modern commercial grade equipment, oriented perfectly to the sun at the proper angle. Putting that in a car isn't possible. Stuff is going to get hot, which leads to reduced efficiency, active cooling costs energy so that's probably not a good solution. The car isn't going to have all of it's surface facing the sun, by definition a part of it is going to be in it's own shade. If the sun is perfectly overhead for max efficiency on the roof, the sides aren't going to get anything. Angles are going to be wrong and people prefer to park their cars in the shade or under cover. Cars also tend to be used, so they get pretty dirty driving around, that reduces efficiency further. So if they get half of what I described, they would be doing real good. Just takes a week to charge the car, but still, doing good.

But then there's the real kicker. It isn't possible to get anywhere near 20% out of a paint or surface finish. Modern solar panels have many tricks to get their efficiency as high as 20% (and even nearing 30% with the newest techniques, but that isn't commercially viable at the moment). Solar paints are way worse and do badly even in perfect lab settings. One of the issues is the energy generation isn't as optimal to begin with, but another issue is getting the energy transported out of the material is problematic. This leads to efficiency numbers in the 1% range. This means going from charging in a week, it will take months, if it charges at all.

Except for niche use cases, putting solar on anything except roofs usually makes no sense at all. It's handy for camping, on top of the RV or for example on a boat, where having a little bit of DC is handy when no other sources is available. But when there is a roof available, just put it on the roof and use the power from there.