I agree on this. Using the pvwatts calculator for a very rough estimate of cumulative kWh produced per *month*, a theoretical 380W panel on top of a car that is in perfect sunshine from sunrise to sunset, never shaded or obstructed, on a car in the sunny climate of San Diego CA will produce the following:
61 kWh per month in the best month of the year (August)
39 kWh per month in the worst month of the year (December)
As you can see from this, the kWh per day is quite minuscule, not enough to charge a car to go any appreciable distance.
I believe that solar panels were an option on the Maybach 62S, and they would run the ventilation fan while you were parked so you wouldn't return to a hot car after going to the store.
Like everyone else has said - there just isn't enough area on the top surfaces of a car to do any noticeable charging.
If you were to theoretically have a perfect 400W PV panel on top of a car, and left in direct sunlight, it might be enough to run a medium sized peltier/TEC cooling unit to somewhat cool down the car while you leave it parked. Or a very small heat pump. Would definitely add a lot of extra cost in manufacturing and complexity.
Or just keep the car fan running and use the existing AC system (in ventilation mode, no compressor) to keep the car just as hot as outside (instead of much hotter). If you have some spare power maybe even run the AC when the key gets back in range.
Using "270Wh/mile" from another comment,
(61kWh/month) / (270Wh/mile) / (31day/month) = 7.3mile/day =~ 11.7km/day
(39kWh/month) / (270Wh/mile) / (31day/month) = 4.7mile/day =~ 7.5km/day
My conmute is like 3 or 7 miles (4 or 11 km), depending on where I have to go.
Anyway, I expect that a rooftop installation is much more efficient.
The rough estimate calculation for the theoretical 39 to 61 kWh per month are for a perfectly mounted, south facing, 15 degree tilted PV panel such as might be on the roof of a warehouse, or in a field somewhere. With no buildings or trees or shade obstructions around it. And perfectly exposed to sunlight from the moment of sunrise all the way to sunset. That's the 'default' assumptions built into pvwatts for calculating a fixed installation PV site.
On an actual car that parks under trees, in parking garages, beside buildings in the shade, etc, the actual production would be much less. Not to mention the panel would be 'flat' on the roof and rarely if ever angled facing south, unless you happened to park on a hill with the roof of the car angled south...
It's also not possible to say that a theoretical 39kWh can be turned into so many miles at 270Wh/mile because it's not a perfectly efficient system, I'd guess at least 15-20% would be lost to heat in charging the battery and DC-DC conversion.
60kWh may be enough for occasional short trips.