Personally, I rather like the idea that the A320 or 777 with 2 engine failure and a loss of electrical power might be able to rely on alternate PV power for the instruments/radios/computers/actuators enabling a controlled glide descent to an airfield. I don't imagine that you'd need too many m2 of pv to power the electronics.
Solar panels only work with daylight, so need to store energy for rest of time , means batteries, additional weight and problems ( not only 787 but also Solar impluse /Hawai )
However great potential for GA / daylight ops. Many Gilders today already use this to keep battery power at max.
Last bit on RAT and Airbii : visit at Air Transat A330 /Azores and Air Asia A320 /Java final reports to see the limits of RAT power.