A thermostat (10) includes a housing (11) having a control panel (13) with a plurality of manually selectable input elements or input keys (15) and a display panel (17) for displaying various functions of the programmable thermostat. The programmable thermostat also includes a temperature sensor (18), a microprocessor (19) having memory for storing a default program set of default temperature and time values and a desired program set of desired temperature and time values. The microprocessor may compare the desired program set to the default program set in order to calculate the efficiency of a HVAC system. The calculated efficiency is then displayed graphically on the display panel.