This chapter examines modular systems for energy usage in buildings. While presently the power requirement and heating, ventilation, and cooling (HVAC) for residential buildings largely use fossil fuels, the new trend is to use solar and geothermal energy for these purposes. Modular systems are ideally suited for both solar thermal technologies for buildings and geothermal HVAC systems. The chapter examines Co2nserve for multiple applications, six innovative modular rooftop solar technologies, modular passive solar heating system as well as hybrid solar heating and passive construction, modular Bright Built Home, and modular building integrated solar thermal technologies. For the modular use of geothermal energy for HVAC system, the chapter examines various industrially available geothermal heat pump systems and investigates in detail geothermal heat exchangers and modern modular chillers. Finally, the chapter also briefly examines commercially available modular design of building security system.