||The methodology for semi-automatic building energy performance (BEP) simulation eliminates arbitrary decisions from the simulation process. Based on the use of IFC-based BIM, its key feature is the automatic transformation of data from their original source and formatting to what is needed in BEP simulation, per unambiguous data transformation rules that cannot be manually manipulated; it results in semi-automatic generation of input for simulation. When implemented, the methodology reduces preparation of simulation input for a building from days, weeks or months to minutes, eliminates human error, and makes simulation preparation virtually free of cost and resource consumption. GST/IDF Generator tool semi-automatically transforms building geometry properly defined in IFC format into ready-to-execute building geometry defined in input format for “whole building” BEP simulation tools like EnergyPlus. The tool’s performance was publicly demonstrated live in the OGC AECOO-1 Testbed in March and May 2009.