This chapter describes modular systems for energy conservation and efficiency for residential and transportation sectors. The factors affecting energy conservation and efficiency for residential homes and vehicles (largely automobiles) are examined. For modular approaches for residential homes, passive solar heating, energy saving modules, phase-change-based modular thermal storage in a variety of ways, energy star modules, and various aspects of modular zero-energy buildings are discussed in detail. For transportation sector, the factors affecting efficiency of modular electric and hybrid vehicles are discussed in detail. This includes electrical and mechanical losses and additional sources of energy in electrical and hybrid vehicles. Energy efficiency improvements of fuel cell are also briefly outlined and the role of nanotechnology for energy efficiency in modular vehicles is described in detail. Finally, the chapter briefly outlines integrated modular avionics and advanced modular shipboard energy storage system for fuel saving and energy efficiency.