There are conventional solar panels now that are 500 watts apiece. People are setting up large motorhomes with the roof covered in solar panels, and the system actually powers the whole rig for extended camping without shore power. It’s more feasible than you think. Here’s an example from one of the best-known and most accomplished companies doing this, out in AZ:
http://precisionrv.com/2017/08/24/2000-watt-1200-amp-hour-newmar-motorhome-solar-power-system/
https://greentransportation.info/energy-transportation/solar-panel-ev-car.html
1kw per square meter is the theoretical maximum. A car driving down the highway, not accelerating consumes 50kw+, and that goes higher at higher speeds.