Abstract: An assessment system and method are described that capture indoor temperature measurements and corresponding outdoor temperature measurements in order to determine a thermal efficiency of a structure. The assessment system identifies quiescent periods and trims these periods to eliminate undesirable influences such as auxiliary heating or solar gain. The quiescent periods are then compared to outdoor temperature differences to determine the thermal efficiency of the structure. The system can model the structure's performance metrics, through inferred qualitative and quantitative characterizations including, but not limited to, the building's rate of temperature change as a function of internal and external temperatures, the building's heating, cooling, and other energy needs as they relate to the building envelope, appliances, and other products used at the site and occupant behavior.