A portable monitoring system that monitors various aspects of the operation of a refrigerant-cycle system is described. In one embodiment, the system includes a processor that measures power provided to the refrigerant-cycle system and gathers data from one or more sensors and uses the sensor data to calculate a figure of merit related to the efficiency of the system. In one embodiment, the measurements performed by the monitoring system include one or more of: an evaporator input air temperature, an evaporator output air temperature, evaporator air flow, evaporator air humidity, condenser air input temperature, condenser air output temperature sensor, electrical power. In one embodiment, the portable monitoring system receives information about the refrigerant-cycle system from either the system itself or from a computer network.