This paper studies an event recognition in a building based on the patterns of power consumption by lighting, ventilation, electronic equipment, etc. It is a big challenge to identify what kinds of events happenedin a building without additional devices such as camera and motion sensors, etc. Instead, we learn when and how the events happened from the historical record of power consumption and apply the lesson into the design of an event recognition system (ERS). The ERS will find out abnormal power usage, which leads to avoid wasting power in a building.