What is the efficiency of a photovoltaic cell?

The efficiency of a photovoltaic (solar) cell is a measure of how effectively it converts sunlight into usable electrical energy. Specifically, it is defined as the relative fraction of incident light energy that the cell successfully converts into electrical current. In practical terms, if a photovoltaic panel receives 1,000 watts of solar light energy and outputs 200 watts of electrical power, its efficiency is 20 percent. The efficiency is strictly a ratio of the converted electrical output to the light input. Other choices are incorrect because they describe different metrics: output RF power to input DC power is amplifier efficiency, and output lumens to input watts is the luminous efficacy of a light bulb.