[ResearchAtPlay]

OP is probably correct, maybe even high in their 10 to 20 W/m^2 estimated average output for car mounted PV systems.

Rooftop mounted, optimally oriented PV systems at Seattle latitude have an annual capacity factor of ~ 14%. That means a 1 kW system will average 140 W output over a year. That system has a module area of about 4-7 m^2, equivalent to the area available on a sedan.

On a car that has suboptimal module orientation and solar exposure, "power density" (RE: Smil) is really low.

[robotbikes]

Ok, that makes sense if you average over a year considering night time etc. Yeah, car mounted solar isn't a great idea unless we were in a mars rover type of situation which is not how we use cars.