You know, one problem with using solar that we haven't mentioned yet is the problem it might cause with your radio. Since the electricity that comes from sunlight moves at the speed of light (instead of the electricity from batteries, which moves much slower) it runs the circuits at a different speed, include the crystal that handles the radio frequency for your R/C unit.
Solar works in other things because there's a person at the controls, but you might have to buy all new crystals of different frequency ranges to be able to use the higher speed solar electrons.
Also, solar electrons are much stickier than battery electrons, so you'd need to put in one of those electrical lint traps (they're the little metal things in plastic you see on phone cords and stuff) and keep it clean by running a magnetized pipe cleaner through it every day.
It's best that you follow Mr. Withey's advice and get an extra battery.