· Senior Member R.I.P.
Debris that normally strikes a vehicle that you think little about would seriously damage the solar panels. Even dirt or sand kicked up by wind will degrade the panels.
Even then it may not be 100% because of the longitude of the sun changing from month to month.The panels need to be point towards the direct sunlight. So if they are on top of the car they would only be working at 100% at noon. Suppose you could open or close the hood and trunk every couple hours.