A typical 1kW panel weighs about 40lbs, of which about 9lbs is the actual silicon producing the energy, and the rest is the protective glass, silicon backing, and structural material around the edges. It seems to me that if the panel is specially made to fit into a car roof as-manufacturered in place of a metal roof, a lot of that wouldn't be needed as it would be built into the roof anyway, and the added panel weight would be pretty negligible.
The DC produced by the panel can also be directly used to charge the battery, no need for inverters.
One trick is to swap out the old 12V lead-acid battery they insist on sticking in electric cars with modern LiFePO4 ones to immediately save 20-25 pounds. Aluminum or alloy wheels might save another 50+ pounds if the car doesn't already have them. So it's conceivable to carry one or two 50 pound solar panels for free. An inverter and additional batteries to buffer the charge will add some weight though, since most electric cars have the artificial limitation that they can't charge while driving.
Nor does it particularly need a "whole assembly" for mounting etc, as it would be built directly into the roof of the car. It may well be that using a solar panel to replace the metal skin of the car will turn out to be equal or less weight than it otherwise would be, though of course much more expensive.
I drive a Nissan Leaf. It gets 5.5 mi per kWh in the summer, 4.8 in the winter. My commute is about 7 miles 1-way. So less than 3kWh used daily assuming I just commute.
They have a 1kW solar panel on their car. If I had a car with the efficiency of my Leaf that charged from a 1kW panel every day, it's conceivable that for my specific use case I would rarely ever charge.
That said I don't think my case is typical and I am not a large enough market segment to cater to. Nor would I ever trade my dirt-cheap Leaf for whatever exorbitant price these things will cost.