This book demonstrates the enormous possibility of energy saving and reduction of CO2 emission achievable through zero-energy buildings, a building not dependent on outside energy sources. It describes technical and economical merits of utilizing renewable energies in building sectors that are not subject to factors such as disruption or price increase due to invasion, wars or sanctions. Three different buildings located in a cold climate in the northeastern USA and warm climate conditions, in California, are examined in this book. The energy balance before retrofitting and afterwards are compared and described for each of the study properties.