Buildings are among the largest consumers of electricity with a significant portion of this energy use is wasted in unoccupied areas or improperly installed devices. Identifying such power leaks is hard especially in large office and enterprise buildings. In this paper we present the design and implementation of a system that uses an underlying sensor network to provide accurate real time information about various characteristics like occupancy, lighting, temperature and power consumption at different levels of granularity. All sensor devices require minimal installation and maintenance. Using an experimental installation we evaluate a number of applications and services that achieve energy savings by applying different power conservation policies. Furthermore we provide energy measurements to users and occupants to show how various choices and behaviors affect their personal energy savings.