I think there was another discussion about putting solar panels on a Model S a month or so back. Solar panels vary output based on angle of the sun, how cloudy it is, etc. The further north (or south in the Southern Hemisphere) you go, the less efficient a panel is going to be, even in direct sunlight.
On a sunny day you can probably get enough panels on a Model S to beat vampire drain, but you might not be able to in a place like Seattle in the winter.