Serious questions about the solar option available in the Prime:

Not only do black car surfaces (including pv panels) increase cabin heat when left in direct sunlight, but in addition to that - for every degree above 77°f that temperatures increases, your solar panels will lose approximately 0.35% in power production efficiency. Put another way, for every 3°f, you will loose a little over 1% panel efficiency. AND THAT's in the shade. It doesn't even account for DIRECT sunlight on solar panels, which will easily bring your panels (on a 113°f day) up to over 150°f. At that junction, your little roof panel on your car will have lost over 12%.

Solar Panel Temperature Range Explained

Kind of makes one wonder it it'd be better to just get white cars for extreme heat environments.
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I don't know, but I can tell you that in Gen3 the solar roof option powered fans to keep the interior cool when you are parked in a parking lot and not much else. It would be safe to assume that Gen5 won't be very much further advanced than that because you can't do much with the small amount of solar electricity you can generate from the roof of a car.

 

It might be safe, but it's not based on published information from Toyota. Somewhere on this board is a thread there it states the approximate charge rate when parked in a sunny area. At the time, I figured that without plugging in it would cover my weekly errands of 3-5 miles every other day and the weekly trek to the bowling alley one town over.

 

It does not need to be a gimmick. If you consider that the Prime gets around 4 miles per kWh and that many errands consist of a few miles driving interspersed with an hour or so parked, there are circumstances where a lightly used Prime could recharge fully during the day.