It doesn’t fill the whole tank, but on many days it can add 10-20km of range. Given that commute distances in cities (Europe) can be small, and given that in cities most won’t have their own garage to charge, this means you could get away with charging your car once week instead of twice.

In general I’d agree that this solar car thing doesn’t make sense, but Sono did make a compelling case and the whole package kind of made sense. But their execution and timing was off.