A monitoring system for monitoring operation of a refrigerant-cycle system is disclosed. A differential pressure sensor measures a pressure difference between: (i) air at a first location upstream of an evaporator of the refrigerant-cycle system; and (ii) air at a second location downstream of the evaporator of the refrigerant-cycle system. An electrical sensor measures an electrical quantity indicative of power consumption of the refrigerant-cycle system. A processing system determines whether airflow through the evaporator is restricted based on the pressure difference. The processing system calculates an efficiency of the refrigerant-cycle system based on the power consumption of the refrigerant-cycle system.