This book addresses the need of the Kingdom of Saudi Arabia to diversify its power, as well as tap the strong potentials of the Renewable Energy Sources in the western mountains. In KSA households, water-heating requirements are typically met by Electrical Water Heaters (EWHs). In this book, a study considering the replacement of EWH by SWH in the highlands of Al-Baha region is presented. Analyses demonstrate that heating domestic water via solar thermal energy is simple, reliable, and cost-effective. The results indicate that the payback period of the proposed suitable size of SWH for each housing unit category (thermosiphon system) is approximately five years, and the cost of fuel to produce energy of EWH that can be saved using SWH is calculated. In addition,the environmental impact of using SWH instead of EWH is assessed and the annual energy produced by SWH is evaluated. The emissions reduction from power plants due to SWH use is estimated, demonstrating the positive environmental impact of SWH.