This is my understanding as well. It's like walking around all the time with a strawberry plant in a pot just for the free fruit. It's not worth the bother at best, and probably overall a net loss.
So, yeah, if you have no choice but to park your car all day out in the sun, then this might be helpful. Otherwise, it's better to choose covered/enclosed parking if available/viable rather than being seduced by the idea of getting "free power".