Two points. If this worked (which I do not think will), you'd have a bit problem with what to do with the energy when the plane is sitting on the ground. You

** cannot ** turn off solar cells. They will convert solar energy to electric potential whenever there is light. You can, turn the system "off" but you are only turning off the inverters. There will be dangerous potential around the panels whenever there is light. This is a hazard for a lot of people.

As for how useful - lets try some math - based on thumb in the air guesses. I'm sure there are people out there who can and will point out the big holes in this model - so we can improve on it.

Being very generous - let's say we could find a cell that produces 15 watts/sq ft. This is on the high side - and probably much more than the cells use on Solar Impulse (organic cells are more like 3-5 w/sq ft).

The 787 has a bit over 3501 sq ft of wing area - so lets assume 3000 of it is available for use by the cells.

3000 sq ft * 15 w/sq ft means 45

KW at peak - which means "high noon for the a/c"

Now - most models assume 1/2 of that for the "day" average due to various factors like incident angle (an angle of 45 degrees will decrease incident energy to .7 for instance). So - we have 45

KW * .5 = 22.5

KW for the generating day - which is considered to be 8am - 4pm. I think we can assume that most planes will fly outside that window - so lets make a guess (based on nothing) that this represents only 66% of the flight time (on average), so drop it by .66 = 14.5KW average.

So - we arrive at 14.5KW on average. In an hour flight - 14.5 KWH

Now - one

RR T1000 at takeoff thrust generates 52000 hp or 38480

KW (based on

RR statement that each blade generates 800 hp and there are 66 blades - this apparently is only for the bypass). Lets derate this to 40% for average cruise - so about 15,000 KWH per hour flight * 2 (engines) or 30,000 KWH.

So - our solar system, on average, reduces the required load by 0.048% - say .05%.

Assuming the fuel to electricity path of the engines and generators is 20% efficient (I have no idea), we reduced fuel flow by 0.24%

Okay- is a .24% fuel reduction worth the certification difficulty - and what impact will it have on the efficiency of the wing.

And where did I go wrong in this back of the napkin calculation?