[WJW]

It'd need to be lower than the existing maintenance plus fuel burden of course, since sunshine is free but oil and coal are not. In any case, if you can't change the power source (as you say, passengers jets are pretty tricky to electrify) then you don't have to build solar panels for those so the upfront cost would go down.

Thinking about it more it would seem likely that as overall oil consumption falls, the relative cost of oil derivatives like jet fuel would go up because the advantages of scale decrease from what they are now. It's a pretty fascinating subject since solar prices seem to follow a Moore's law type price evolution at the moment while oil will get cheaper as well while demand decreases. Presumably there is a balance point somewhere?