A monitor for an HVAC&R system having a compressor, a condenser, and an evaporator, characterized in furthermore comprising: a system temperature processor operable to obtain fluid temperature measurements for the condenser and fluid temperature measurements for the evaporator, the fluid temperature measurements for the condenser and the evaporator being obtained from temperature sensors located near the condenser and the evaporator, respectively, or from proxies of the fluid temperature measurements for the condenser and for the evaporator, respectively; a power parameter processor operable to obtain one or more power parameter measurements for the compressor using one or more current detection devices mounted on the compressor, respectively; and a compressor input processor operable to provide an estimate of a compressor input power parameter for the compressor using the fluid temperature measurements and the one or more power parameter measurements; wherein the compressor input processor is configured to obtain updated fluid temperature measurements for the condenser and updated fluid temperature measurements for the evaporator from the system temperature processor, obtain updated one or more power parameter measurements from the power parameter processor, and provide an updated estimate of the compressor input power parameter for the compressor using the updated fluid temperature measurements and the updated one or more power parameter measurements, the compressor input processor further configured to detect degradation of operational efficiency in the HVAC&R system using the updated estimate of the compressor input power parameter and the updated one or more power parameter measurements and issue a warning and/or signal if degradation of operational efficiency in the HVAC&R system is detected. a system temperature processor operable to obtain fluid temperature measurements for the condenser and fluid temperature measurements for the evaporator, the fluid temperature measurements for the condenser and the evaporator being obtained from temperature sensors located near the condenser and the evaporator, respectively, or from proxies of the fluid temperature measurements for the condenser and for the evaporator, respectively; a power parameter processor operable to obtain one or more power parameter measurements for the compressor using one or more current detection devices mounted on the compressor, respectively; and a compressor input processor operable to provide an estimate of a compressor input power parameter for the compressor using the fluid temperature measurements and the one or more power parameter measurements; wherein the compressor input processor is configured to obtain updated fluid temperature measurements for the condenser and updated fluid temperature measurements for the evaporator from the system temperature processor, obtain updated one or more power parameter measurements from the power parameter processor, and provide an updated estimate of the compressor input power parameter for the compressor using the updated fluid temperature measurements and the updated one or more power parameter measurements, the compressor input processor further configured to detect degradation of operational efficiency in the HVAC&R system using the updated estimate of the compressor input power parameter and the updated one or more power parameter measurements and issue a warning and/or signal if degradation of operational efficiency in the HVAC&R system is detected.