Not exactly practical (better to have the panels mounted at home), but many people are interested in stuff like this.
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Now lets assume we can get 5 square meters of usable body panels at any one time, and that we get 50W/sqm in america, and say 8 hours of sunlight. That's 2kW per day, or at 200W/mile that's 10 miles free every day. And that's at 15% efficiency, 20% gives 12.5 miles. Now if the Tesla Roadster is a weekend car, then that becomes 70 miles per weekend and no need to ever plug it in.
What do you think?
I think it's not worth keeping a car outside with all the other ancillary damage. I'll plug-in every day over having dashboard cracks, faded leather, UV-damaged paint, etc., thank you.
-Scott