A few years back Toyota released a Prius with a gimmick solar roof that generated enough power to drive the cabin HVAC system for cabin cooling. Their latest test vehicle appears to do it right. The entire car is covered with very thin and light solar cells which can provide up to 27 miles of driving range per day. Remember, the Prius Prime has a 25 mile AER, so this effectively doubles the electric only range of a Prius Prime. The other big difference is this car is engineered to allow the solar electric to recharge the batteries while driving.
Will this help BEV owners - possibly. Who it will help are the PHEV owners where the limited EV range can cause them to dip into their gas tanks, especially in the winter when that 20% range hit all EVs take comes into play.