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I think you meant to say 100-200 watts a square meter average output as a 300 watt solar panel isn't much larger than a square meter. Either way, you are right that it is unlikely that solar charging via a car roof will be anything aside from a backup/gimmick. But it is possible to get a good charge from solar panels on a rooftop with not a huge amount of panels.

I meant what I wrote. Sure you get 100W/m^2 when the sun is shining perpendicular to the surface, but divide by night, clouds, angles, dust/snow, latitude, air transmissiviness, line/battery/conversion loss, etc and you’re well under 20W long-term average.

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.

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.

Most of the time the panel is at a sharp angle to the sun, or in shade. Fully half the time it is entirely in the dark.

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