A monitoring system for an HVAC system including an evaporator unit device, first and second temperature sensors, and a condenser unit. The evaporator unit device includes a first current sensor that measures current provided to a circulator blower and a differential pressure sensor that measures pressure between a point prior to a filter element and a point after the filter element. The first temperature sensor measures a refrigerant temperature between a condenser and an expansion valve. The second temperature sensor measures a refrigerant temperature between an evaporator and a compressor. The condenser unit device communicates with the evaporator unit device over low-voltage control lines, wherein the condenser unit comprises a second current sensor that measures current provided to the compressor. The evaporator unit device transmits sensor data to a remote monitoring service over a data network. An efficiency of the HVAC system is calculated based on the sensor data.