Electric engines doesn't necessarily mean batteries. Consider micro-turbogenerators.
http://dukespace.lib.duke.edu/dspace/bitstream/handle/10161/...
As others mentioned, fuel has the advantage of getting lighter over travel time.
Also, electric motor planes should be wrapped with solar cells.
Solar cells add weight. As an energy source its hard to quantify them since they never run out, and weight the same the whole trip. Maybe more like batteries. Anyway energy/kilo is probably pretty bad. So for any finite trip, leave off the solar cells and use gasoline.
It's easy to quantify solar cells on an airplane, just look at power consumed by propulsion and compare to power generated by the cells. The result is that the power generated is pretty much useless and doesn't even justify the weight of the solar cells, unless you design a plane to be extremely light and with nearly no payload, a la Solar Impulse.
Solar could be good for drones that stay almost permanently airborne and just need to carry some electronics, but it's pretty much useless for transportation. If you must use electricity there, you'd want to use batteries on the plane, and keep your solar panels on the ground.
Solar is bad for any aircraft that needs to change altitude rapidly, or contend with weather. The best way to transport passengers on a solar-powered aircraft would probably be to have a very large solar powered plane that remains airborne for months at a time, flying above the clouds, with smaller airplanes that shuttle passengers between it and airports.
Of course with that you have to deal with the issues of rendezvous of two flying aircraft, energy storage through the night, and pressurization of a large space. But it would avoid many of the inherent issues in solar powered aircraft.
I'm sure it'll help, but the fundamental problem with solar powered aircraft is that one pound of solar panel can lift barely more than one pound. That means saving weight on the motor can cut out a lot of solar panels, of course, but it also means you still need a ton of solar panels just to lift the structure of the aircraft and the payload.
I don't think we'll see cheap solar drones anytime soon. Current solar aircraft need to be gigantic and fly extremely slowly. Small drones typically want to be quadcopters, which are inherently inefficient, but have other nice properties. If you want a solar-powered small drone, make a stationary solar array, give the drone a swappable battery pack, and charge batteries with the stationary array while different batteries are powering the drone.
I'm starting to think good VR might do more to kill hydrocarbon consumption than electric vehicles. So many examples where moving meatbags around could be avoided altogether.
