Solar rooftops on cars don't provide enough juice for it to amount to much. Let's say for the sake of argument you could fit a square meter of a car's roof as

Solar rooftops on cars don't provide enough juice for it to amount to much. Let's say for the sake of argument you could fit a square meter of a car's roof as a 20% efficient solar panel (a ludicrously high efficiency). Solar irradience is roughly 1 kW per square meter giving you roughly 200W. That will provide roughly 1.6 kWhr of energy. This is enough to drive roughly 8 miles on an average EV (20 kWhr/100 miles). That's about half the average commute one-way. Not nothing, but still invokes the question if it's worth the effort.