For the purpose of applying the BEST to buildings including cogeneration systems, a module of cogeneration system is being developed. Referring existing cogeneration simulation/assessment tools and mainly considering thermal energy balances, the development work is focusing on modeling the gas engine generator, waste heat use absorption chiller, heat exchanger and controllers. Based on the design concept of the BEST, as the first step, a sample system consisting of one-unit of each equipment has been developed adopting "forward method". Additional requirements to be solved have been recognized such as finding a proper calculation flow and/or iteration algorithm for robust simulation.