1. A method of monitoring operation of a refrigerant-cycle system, the method comprising: measuring, using a differential pressure sensor, a pressure difference between: air at a first location upstream of an evaporator of the refrigerant-cycle system; and air at a second location downstream of the evaporator of the refrigerant-cycle system; measuring, using an electrical sensor, an electrical quantity indicative of power consumption of the refrigerant-cycle system; and using a processing system: determining whether airflow through the evaporator is restricted based on the pressure difference; and calculating an efficiency of the refrigerant-cycle system based on the power consumption of the refrigerant-cycle system.