An energy meter apparatus includes a sensing unit including a first temperature sensor configured to measure temperature of the fluid at a first boundary of the system and a processing-reporting unit comprising a second temperature sensor configured to measure temperature of the fluid at a second boundary of the system. One or both of a flow sensor and a pressure sensor is included in one of the sensing unit and the processing-reporting unit, and the pressure sensor is included in one of the sensing units and the processing-reporting unit. The processing-reporting unit is configured to establish and maintain a first wireless communication channel for communication with the sensing unit. The sensing unit is configured to report sensor measurements to the processing-reporting unit via the wireless communication channel.