300hrWannaB...
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.
Ummm, well regardless of the expense perhaps one way around the possible lack of daylight would be to use wind power instead of solar.....
Edit to add: Sorry, on second thoughts even for me that comment's a bit cruel. 300hr, Most commercial twins have "Ram Air Turbines" or "RATs" to help cater with, amongst other things, a double engine out.
https://en.wikipedia.org/wiki/Ram_air_turbine