Average miles driven per year is 13,500 or around 260 miles per week. The better way to look at it is 1864 / 13500 which maximally gives you about 13% of those yearly miles "for free."

With WFH, I wonder how those averages have changed. Some folks are using cars much less often.

However, as someone who has prior experiences with keeping a low mileage car, I'd say parking it out in the sun is a terrible goal. You really want it garaged to slow purely time-based deterioration if you are not hitting high miles per year. A power hookup in the garage is the way to go, and then get house solar if desired.

So then, this car-mounted solar is only useful during an extended outing where you are away from your usual parking. Can it ever become cost effective for the value it provides in such short bursts?

Average is probably not too useful here because there are outliers who drive a lot. Median would be more interesting.