Devices and methods for assessing the performance and efficiency of an HVAC system using a tuple including a percentage of runtime over a lookback window coupled with a ΔT value for the difference between the HVAC setpoint and the outdoor temperature. The performance monitoring may be done using a device that is independent of the HVAC thermostat or may be incorporated within a smart thermostat. Trends in measured tuples over time can warn of system degradation.