Variable frequency drives (VFDs) are widely applied on induction motors that drive fans and pumps in heating, ventilation and air conditioning systems. Under partial loads, VFDs not only adjust frequency to reduce motor speed and mechanical output power (load) but also adjust voltage to reduce motor electrical input power. The ratio of voltage to frequency affects the motor efficiency. On the other hand, VFDs not only consume energy, but also create harmonics, which can cause additional motor energy losses and interfere with sensitive electronic equipment. So far, not enough studies have been done to identify the motor losses and the effects of harmonics on system energy consumption. The purpose of this paper is to experimentally investigate the additional energy loss resulted by VFDs and the optimal ratio of voltage to frequency of VFDs. First the energy consumption with different voltage-frequency ratio settings under different loads is measured on the same system to identify the optimal voltage-frequency ratio. Then the additional energy loss is identified by comparing power difference of a chilled water system with and without a VFD. The measurement results show that the VFD results in 2.5% energy loss and the optimal voltage-frequency ratio is not always the flux optimization mode.