The spec say 1.8x3-3.5m

Are you talking about the entire width and length of the vehicle? The roof is smaller than the total footprint. Especially because the width of the vehicle includes the mirrors sticking out.

10% seems rather low.

No, it's pretty high for the use case you describe. Utility scale solar with panels pointed toward the sun tends to achieve about 20-25%, with some American desert installations at 30%.

Home balcony solar tends to get 10% in places like Germany, with the higher latitude and higher likelihood of overcast skies.

Putting it on a vehicle roof would be lower than that.

So 2 kwh per day is optimistic.

i don't really need more than 30km a week.

So why buy a vehicle at all? Seems like the resources that go into an underutilized vehicle would be better used for things like paying fares on taxis.

You're better off just charging with 100% utility solar/wind from the grid and paying money for it, rather than trying to combine a mediocre solar array in a costly way that kills your vehicle efficiency.