The main transmission system, the key component for transferring energy in machine tools, is essential for studying the energy efficiency of machine tools, highlighting the need for manufacturers to measure the input power of the main transmission system to develop energy-saving technology. In response to such a need, this article proposes a new approach that can be applied to calculate the input power of the main transmission system fed by a converter. To develop this approach, the influences of the converter and frequency variations on motor loss are analyzed to build the idle and loaded loss motor models for manufacturers. Based on these two models, the relationships between the input power of the main transmission system, the motor, and its frequency are investigated to establish a calculation approach that is convenient because only the motor input power measurement is required. Finally, 20 group experiments are conducted to validate the proposed approach, and its applications are discussed. The applications demonstrate that this method provides a high level of accuracy and validity for calculating the input power of the main transmission system driven by a converter and could be widely used to study high-efficiency machine tools and monitor the energy efficiency of machine tools.