The Energy Return on Energy Invested (EROEI) is simply the amount of usable energy acquired from a particular resource divided by the energy expended in obtaining the usable energy. However, it is not so simple to determine all of the energy expenditures that should be included, or not, in the denominator of this critical measure of the viability of an energy source or process. Discrepancies in estimated EROEI levels are particularly significant for renewable energy sources and for solar photovoltaic (PV) systems in particular. This book presents the first bulk analysis of a large-scale, real-world deployment of 3.5 GW of installed, grid-connected plants in Spain, including all of the factors that limited and adjusted the real electricity output through one full-year cycle. The results will be compared and contrasted with publicly released data from official Spanish sources. Spain represents an ideal case study as the country has the highest penetration of solar PV energy at 2.3 percent of total national demand as well as the greatest expertise in grid connection and grid management of intermittent, modern renewable systems. This book will be a key to understanding the net energy available to society from energy sources in general and from functioning solar PV systems under real-world conditions in particular, while providing critical insight into the capacity of renewable energy sources to fill the foreseeable gap between world energy demand and depletion rates for fossil fuels.