A microprocessor type electronic thermostat samples outside temperature and furnace run time at preselected (2 second) time intervals. Data are retained and processed in the microprocessor to eliminate building thermal capacitance effects and transitory weather influences, to provide a real indication of the product of building heat loss coefficient and furnace efficiency. Data are then normalized against a baseline value to provide a universal 'energy index' which indicates real energy efficiency in the same units for all installations, regardless of building or furnace type. In this way, the diagnostic thermostat serves as an energy monitoring device that can warn when the furnace needs servicing, and also demonstrate energy savings from conservation steps taken by the building operator. A convenient energy bar graph and menu-driven LCD display provide a readily understandable display of this information for good consumer understanding.