[MetroMPG]

If you were going to put a 12 volt nominal solar panel on a vehicle, you might as well wire it into the existing 12v system, rather than trying to run an accessory directly from it (with the exception possibly of small ventilation fans).

It'll keep the battery topped up to counter small losses from computer memories (radio etc.) and the battery's natural discharge rate.

The effect of a sunroof size panel on fuel consumption would be real, but very small - essentially undetectable at the pump.

Make sure to also use a charge controller so you don't over charge the battery!