Sorry, I don't know the answer to the question. For aftermarket retrofit, it seems very impactical to me. I think it would be very hard to fit to the car and would cost a fortune or a fair amount of time and expertise to do it well.
I do like the idea of solar panels on electric cars though. I just think it needs to be built into the car in the first place for economies and scale and design, and marketed honestly, i.e. it will only realistically provide a small proportion of the car's energy unless you use it very occassionally.
I see that Sono Motors "up to" 34 kilometres refers to a fully sun day in June.
Our innovative manufacturing process enables the seamless integration of solar cells into the body of the Sion or other surfaces.
sonomotors.com
They estimate 3km from the solar panel for a cloudy day in December.
One thing that would be nice about a solar panel on a car is that if you ever ran out of juice and couldn't make it back to your house you could just park it and come back and get it a while later. Having that as a backup option might actually be a little extra peace of mind. In some cases, it would make more sense to do that than get it towed.
I wonder how much it would cost to build a solar panel into a car if done at OEM level with economies of scale. I think the actual solar cells are going to be worth a couple of hundred dollars, and then you have got to factor in other components in the system and what is means to the engineering process. Maybe $300-$1000 per car?
However, the car does need to be parked in the sun most of the time it's parked for it to be worth it. The energy gained while driving will never pay off the cost of the panel, and the avoided grid electricity used when driving may not even cover the carbon emissions in the panel's construction.
And so it only works if both you
and others using the car including passengers are happy to leave it parked in the sun and come back to a hot car. If it's mostly going to be parked in an underground car park or a garage it's not going to work.