This chapter examines energy conservation and efficiency for industrial and commercial sectors with the use of modular approach. For industrial sectors, a detailed examination of the use of modular systems for efficiency improvement in energy use for water and wastewater facilities has been carried out. This includes managing energy costs of wastewater treatment plants, several concrete examples where modular approach has improved energy efficiency of wastewater facilities, and an example where modular design has recovered energy during desalination. The chapter also examines in detail the use of modular approach for a variety of biomass combined heat and power (CHP) technology. The improvement of energy efficiency of small modular nuclear reactor using cogeneration for numerous industrial operations is also evaluated. Finally, several US Department of Energy (DOE) sponsored heat and power projects for various manufacturing industries are articulated. For the commercial sector, the chapter examines energy efficiency of data centers and microgrid projects among others. The use of CHP for several commercial buildings is also examined. Finally, the chapter evaluates in detail the efficient energy use by modular micro-CHP technology and efficiency improvements with the use of nanotechnology.