Due to losses in the ICE and system in general, I would think that power produced on the solar panel directly to the 12 v battery would be worth the double or more as the power would be produced without any losses.
So it seems, that the system and controllers ECU etc. CAN handle power beeing fed "backwards in to the system? No problems with that experinced?
Not overcharging the 12 volt batrery would be taken care of, by putting in a chargecontroller between PV panel and battery. This also prevents power going back from battery to PV panel.
As the leght the Prius can cover purely in EV mode depends among other things on how much power is at hand in the EV battery and also what amount of power goes to drive systems fed from the 12 volt battery, it seems to me, that feeding power to the 12 v battery from PV will stretch the amount of pure EV driving I can do.
For instance if I can cover the power drawn from the driving lights more or less via PV, the ICE stays off longer?
A controller can be had for about 10 bucks and small flexible solar modules kan be bought for 10 buck's each, and I would need 56 of them to make a 28 watt's roof mounted thinn laminated module bringing 20 volts to the controller.
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If we assume, that it can produce 28 kWh a year, and that it replaces a fuel amount of twice that, (50% effeciency in ICE to 12v battery) it is 56 kW, with 7 kW in a liter of gas that amounts to 8 liters a year....
If I assume 15.000 km's driving a year at avarage 22 km/l => 682 liters.
It would give me a 1,17% improvement in milage ?
If 25 km/l it would give me 1,33% better milage
Maybee time to look at a bellypan for more bang for the bucks?