Energy efficiency of buildings is of paramount importance due to limited fossil resources, environmental issues, and increasing energy demand. Thus, in recent years building energy analysis using various programs has experienced a rapid growth. However, these programs are expensive and time consuming. In this study, mechanical engineering building of Tabriz University located in northwest of Iran is analyzed to attain a simplified building model that can reduce the computational time and effort. Therefore, 10 models are considered with distinctive design accuracies. EnergyPlus software has been employed to perform energy analysis of the building. The findings show that building materials, internal walls and zoning of the building are determining factors in thermal loads of the building. However, shadings, number of occupants, and electrical appliances have negligible impact. Thus, it can be concluded that the model in which a detailed envelope design is considered and interior walls, shades, electrical appliances, and duration of occupancy are omitted, has the optimum performance in terms of time saving and accuracy of load calculation. Moreover, to gain more reliable results, it is recommended to consider zones in model No.5. The difference between natural gas and electricity consumption of the first and last models is 20,302 m3 (2200$) and 1157 kWh (90$) that indicates the significance of proper HAVC sizing. It is worth mentioning that all monetary units are presented in 2018 U.S. dollar. © 2018 American Institute of Chemical Engineers Environ Prog, 38: 741–749, 2019