This book outlines the basic principles and methods for developing safe, flexible, reliable, affordable, and energy-efficient electric power systems for residential and commercial buildings. In particular, this book presents an order of simplified but efficient computation and analysis methods for designing and evaluating safe and energy-efficient electrical distribution systems suitable for residential and commercial buildings. These simplified methods are based on known technical principles. In addition, several innovative and already proven energy-efficient technologies and strategies are presented to improve the energy performance of existing electrical systems. The book is designed to be an autonomous textbook and fully covers the entire range of issues on the efficient power supply of residential and public buildings.