RCinFLA that has the visual appeal of watching grass grow for a science project. If you want people intersted, and accurate results, doing WORK is a good method. I don't think Garyw's daughter is going for six sigma accuracy.
Your short circuit current would only measure an 'ideal' power output not a real one. The work done by a motor will show real world loss, like what occurs in the real world. In theory good power conversion circuitry can get within 80-95% of full power utilization out of a solar cell, but again this is a school science project, gotta keep it real.