[MetroMPG]

On a car which isn't driven regularly, a solar panel save more fuel than on a daily driver.

An infrequently used battery will self-discharge over time (approx 1% per day is a figure I've seen thrown around), and has to be recharged via the alternator each time it's started up. So a panel that can keep that battery topped up will help a little bit.

But on a daily driver, you're right in thinking the small panels will amount to a drop in the ocean.