The ability of certain materials to convert sunlight to electricity was first discovered by Becquerel in 1839, when he discovered the photogalvanic effect. A number of other significant discoveries ultimately paved the way for the fabrication of the first solar cell in 1954 by Chapin, Fuller, and Pearson [1]. This cell had a conversion efficiency of 6%. Within 4 years, solar cells were used on the Vanguard I orbiting satellite. The high cost of boosting a payload into space readily justified the use of these cells, even though they were quite expensive. Space applications eventually led to. | 18 Photovoltaic Cells and Systems Roger Messenger Introduction Florida Atlantic University Solar Cell Fundamentals Conversion of Sunlight to Electricity Cell Performance Utility Interactive PV Applications The PCU Simple UI PV System UI PV System with Battery Backup Stand-Alone PV Systems Systems with No Storage Systems with Storage Introduction The ability of certain materials to convert sunlight to electricity was first discovered by Becquerel in 1839 when he discovered the photogalvanic effect. A number of other significant discoveries ultimately paved the way for the fabrication of the first solar cell in 1954 by Chapin Fuller and Pearson 1 . This cell had a conversion efficiency of 6 . Within 4 years solar cells were used on the Vanguard I orbiting satellite. The high cost of boosting a payload into space readily justified the use of these cells even though they were quite expensive. Space applications eventually led to improved production efficiencies higher conversion efficiencies higher reliability and lower cost for photovoltaic PV cells. By the 1980s PV cells had been introduced to terrestrial applications where conventional electrical sources were expensive and by the turn of the millennium PV cells have become cost-effective in a wide range of utility interactive and stand-alone applications. Conversion efficiencies at the turn of the millennium for large-scale PV modules ranged from just under 10 for thin-film modules to over 30 for gallium arsenide GaAs concentrating cells. This chapter presents a thumbnail sketch of the basic theory of solar cells and then focuses on several examples of PV applications in utility interactive systems and stand-alone systems. Each type of system generally requires a power electronics interface unit to enable the PV system to transfer solar electricity optimally to the desired load or storage system. The intent of this chapter is to present the what of PV systems. For the why and how of PV .