[Christ]

Would you mind if your car wore a hat while parked?

I only ask because you could make a solar oven sort of thing that would fit onto your roof, which would help direct more of the light at your panels, making them more efficient, so that in perfect conditions, you could theoretically make as much as 50-55W, when parked in full sunlight.

What this means in reality, is that the actual output of your panels may be ~35W, as opposed to ~20 average.

Also - don't you normally park in the shade? What will parking in direct sunlight do for your commute during the hotter season? It's a Catch 22, there, since parking in the shade does nothing for your solar panels, but parking in the sun means you either open the windows or run a fan to cool you down on the commute home.. this might cancel each other out, no?

You could invest in reflective window treatments, basically cardboard plugs that fit into the window areas with aluminum foil on them.

If you really wanted to get fancy, you could make removable plugs for the windows that are on the same side as the sun will shine, and have solar panels on them as well. (Later project, obviously). If you park in an area that would have the sun on one side in the morning, but the other side in the afternoon/evening, you can switch them around at lunch time.

As for the SUNroof solar oven idea, you could use thermal convection to help cool the panels by raising the solar oven "hat" about an inch off the roof... as the heat rises, it will have to draw in cooler air from the area shaded by the "hat" surrounding the SUNroof.