Recent energy and economic analysis of a cogeneration system has been implemented by a manual calculation that is based on monthly thermal loads of buildings. In this study, a cogeneration system modeling validation with a detail building energy simulation, eQUEST, for a building energy and cost prediction has been implemented. By analyzing the hourly building electricity and thermal loads, it enables designers to decide proper cogeneration system capacity and to estimate more reliable building energy consumption. eQUEST also verified economical and environmental benefits when the heat pump system is integrated with the cogeneration system because the mechanical system configuration benefits from the high efficiency heat pump system while avoiding the building electricity demand increase. Economic analysis such as LCC (Life Cycle Cost) method is carried out to verify economical benefits of the system by applying actual utility rates of KEPCO (Korea Electricity Power COmpany) and KOGAS (KOrea GAS company). As results, the proposed system consumed approximately 40% less energy than the Alt-2 in terms of source energy. LCC analysis results also show that the proposed system could save about 10-14% of energy cost during the life cycle compared to the Alt-1 and Alt-2. It could save 6 7% of the total life cycle cost and it is equivalent to around 1-1.3 billion Won in cost.