Let's run the numbers for say a Boeing 767. It has a wing area of 290 m^2 and the main fuselage of 5 meter width and 60 meter length, so lets assume another 200m^2 of surface area. 490m^2 of solar panel can generate 0.2 * 490m^2 * 500W / m^2 = 49 kW.
Compare that to the 800kW that the APU in the tail supplies to keep electric and hydraulics running if the main engines fail or the well above 200 MW that the main engines put out.
Compare that to the 800kW that the APU in the tail supplies to keep electric and hydraulics running if the main engines fail or the well above 200 MW that the main engines put out.