I have a 5w panel which I just attached to my dash on a hinge so when I am parked and facing the right way I can tilt it directly into the sun.
Its nice insurance since I took out my alternator altogether - if ever I run too low on battery and am too far from an outlet (I have an onboard 120v charger which I plug in at home) I can let it sit a while and get it started again.
