One way to answer this question is just to consider "energy".

What is the effective crusing power of (say) the pair of engines in a Boeing 737 (in kilowatts)?

(Wikipedia says the crusing **thrust** is about 21kN per engine... but how from that to power?)

What is the available solar energy if you covered the top-side of the plane in solar cells?

Solar energy being 700-1000watts per square metre, and solar cells being 12-18% efficient typically (research cells now approaching 40% efficiency but not commercially viable). What's the top-surface area of a 737 ?

Can anyone else fill in some figures?

I think the answer will show that airline travel in a form similar to what we are accustomed to is simply not possible (by a very large margin) with solar power alone.

(If only customers weighing 70kg and under were allowed, and no baggage ... then that might help, but even so I think it's a long way off, and the plane would have to look very different.)