I had heard that a common solar panel has about 20 to 30 percent efficiency. The reason was, as I had heard, the electrons only consume the visible part of the electromagnetic spectrum and reflect the rest of the light waves. But why is that so?

Electrons are able to capture radiation energy in discrete quantities of energy defined by the energy level differences in stable and available electron orbital positions. Thus for any given material, there are limited frequencies that can be converted to electrical energy, the remaining frequencies are reflected, refracted or absorbed by other materials in the solar panel that are not electrically active.