200w is an instantaneous measure, not a daily measure. I would guess just over 1kw/h generated over the course of a day - between the panels not being at the correct angle, clouds, and just how long the sun is up that seems reasonable. You could get a large SUV up to maybe 3 with the most efficient panels in the desert.

I pointed out a semi because for long distance shipping they don't use much power (they get as much as 10mpg - but I don't know how to turn that into watts) and because of that long trailer they could potentially be close to energy neutral around mid-day (though only for a couple hours during ideal conditions).

200 watts instantaneous peak is one square meter, given panels are in the 20% efficiency range.

Most cars are around 2m wide, 4-6m long, less windows. Off noon, the side pannels become relevant.

Capacity factor for sensible systems are around 10-15%, which encompasses both weather and night.

>200 W is what I expect as a 24h average; that's 4.8 kWh/day

I guess we don't know where you live, but on the surface of planet Earth, you only get 12 hours of daylight, averaged over the course of a year. Plus dusk and dawn aren't really good for solar power. And then clouds are also a factor for many locations.

I'm already accounting for all that. Footprint of a car is close to 8-10m^2, that area of 20% cells with a 10-15% capacity factor (covering night and clouds) is 160-300 watts.

http://www.wolframalpha.com/input/?i=1kW%2Fm%5E2%2A8m%5E2%2A...

http://www.wolframalpha.com/input/?i=1kW%2Fm%5E2%2A10m%5E2%2...