1. Solar cell efficiency refers to the portion of energy in the form of sunlight that can be converted via photovoltaics into electricity.
2. The efficiency is the most commonly used parameter to compare the performance of one solar cell to another. Efficiency is defined as the ratio of energy output from the solar cell to input energy from the sun. In addition to reflecting the performance of the solar cell itself, the efficiency depends on the spectrum and intensity of the incident sunlight and the temperature of the solar cell.
https://en.wikipedia.org/wiki/Solar_cell_efficiency (1.); http://www.pveducation.org/pvcdrom/solar-cell-efficiency (2.)