Book excerpt: Solar Energy Systems: Progress and Future Directions presents some new concepts and ideas regarding future steps in the development and progress of solar thermal energy. Preliminary results for advanced control of solar plants are presented using effective defocusing mechanisms.The authors discuss the sizing ratio of grid-connected photovoltaic systems and the relationship of this parameter with the maximum available power. Additionally, the optimum value of the sizing ratio of a grid-connected photovoltaic system is defined as one that maximizes the yearly energy efficiency in the photovoltaic installation considered.The concluding study makes a comparative econometric analysis on the relationship between the solar energy consumption that is produced by solar energy systems and the economic growth for the group of G-7 & G-20 countries. The econometric analysis includes panel data techniques with a chosen appropriate production function.