A system for maximizing the measured efficiency of an HVAC&R system including two pressure sensors, two temperature sensors, a flow sensor, a power voltage sensor, a power current sensor, and a controller. Each pressure sensor may be adapted to measure different refrigerant pressures and generate respective pressure signals. Each temperature sensor may be adapted to measure different refrigerant temperatures and generate respective temperature signals. The flow sensor may be adapted to measure a refrigerant flow rate and to generate a flow signal. The power voltage sensor may be configured to measure an electrical voltage input and generate a power voltage signal. The power current sensor configured to measure an electrical current input and to generate a power current signal. The controller may be adapted to receive the signals, calculate a measured efficiency, and output a first voltage output signal having a value dependent upon the measured efficiency.