Semiconductor or optoelectronic manufacturing is a high-energy consumption industry in Taiwan. Detailed energy consumption data for a manufacturing plant (called fab) is important to calculate energy savings and for energy management. The objective of this study is to integrate the fab energy simulation (FES) tool and energy conversion factor (ECF) to analyze the energy use of hightech fabs and to identify some possible energy savings from the results. We used data from a Taiwanese semiconductor manufacturing fab as an example. In addition to the process equipment, the highest energy consumers in the fab were the compressed (or clean) dry air (22.1%), water chiller (20.6%), process cooling water (5.8%), de-ionized water (3.2%), and exhaust systems (2.0%). When compared to the original data, the results from the studied case showed that the highest energy savings for the compressed (or clean) dry air system were 3,050 MWh (1.81% of the overall energy consumption).