(1) The document describes using ARX models to identify physical parameters for building energy performance assessment based on in-situ measurements. (2) It finds that the best model uses nighttime indoor temperature, outdoor temperature, and heating power data to estimate the building's heat loss coefficient. (3) Applying the model to different occupancy periods, it estimates the heat loss coefficient as 63.2 W/K with a standard deviation of 5.1 W/K.