Abstract: Real-time monitoring of an energy characteristic of a building such as an energy performance of the building or a carbon offset of the building is performed by first computing a heat transfer coefficient of the building from nighttime steady-state thermal load data of the building and from nighttime steady-state indoor and outdoor temperature data of the building. A thermal inertia of the building is then computed from nighttime transient indoor temperature data of the building and nighttime transient thermal load data of the building. During daytime, a solar radiation gain coefficient is computed from daytime thermal load data, daytime indoor and outdoor temperature data, incident solar radiation data, and the heat transfer coefficient. The energy characteristic of the building is then estimated in real time from the heat transfer coefficient, the thermal inertia, and the solar radiation gain coefficient.