The goal of energy monitoring and eco-feedback systems is to induce energy consumers to change their behaviors to achieve a more sustainable way of life. Comprehensive research and commercial solutions provide energy consumers with information about overall energy costs or appliance-level energy usage. However, in order to better promote spontaneous energy saving with an eco-feedback system, personalized information is required. The conventional solutions cannot identify the energy usage of an individual user in a shared residential environment. In this paper, we propose the Personalized Energy Auditor, which estimates personal energy usage at home. Our system monitors and analyzes appliance usage, as well as the energy cost of the daily activities of residents. The system then estimates personal energy usage automatically, by linking appliance usage data with the individual user. Our system was installed in residential homes, and the experimental results indicate that it accurately estimates personal energy usage.