High-tech manufacturing consumes huge of energy. For example, a 12 in DRAM (Dynamic Random Access Memory) semiconductor wafer fabrication plant (hereafter referred “fab”) consumes about 400,000 MWh annually [1] in electricity (as much as 25,000 homes use, each consumes about 4,000 kWh/year [2]). The potential for energy savings in fabs is commensurate with the magnitude of their energy consumption. For air-conditioning, fabs are different from those commercial buildings. Fabs need a lot of outside air to keep a positive pressure, while outside air and circulating air often treated separately. The general building energy analysis software handles the air-conditioning system for the outside air mixed with return air through the centralized system, which is different from that of a fab. Plus, the general building energy analysis software does not include electricity consumption items, such as process cooling water system (PCW), compressed dry air (CDA) system, nitrogen system, vacuum system and exhaust system. Process equipment cooling system is not included in the general building energy analysis software modules either. In this study, a Fab Energy Simulation (FES) software was developed. FES with features such as: open source code, multi-language interface, capable of handling different arrangements of components in the make-up air unit (MAU). In this paper, mathematical model is presented in detail. The results are verified by the power consumption data of an operating semiconductor wafer fab, with less than 4% in each output category.