Machine tools (MT), as the key equipment of manufacturing industry, have enormous quantity and consume great amount of energy. In order to achieve consistent improvement of energy efficiency and environmental performance in manufacturing industry, it is significant to acquire the real-time energy efficiency (REE) of MT. However, due to the unavailable parameters and the limitations of using cutting force-measuring instruments, it is difficult for existing methods to be applied to acquire the REE of MT. This paper proposes a novel approach for acquiring the REE without any usage of cutting-force measuring instruments. To develop this approach, a model to characterize the relationship among the REE, input power and spindle speed of MT is established based on the basic data of MT. After obtaining these basic data, this approach only requires obtaining the spindle speed and input power to calculate the REE of MT. It has been tested and validated on a common used machine tool, and the results show its practicability and high level of accuracy for obtaining the REE of MT. This approach can be applied to assess the energy efficiency of MT, to support designers to design high-efficient MT and users to improve the energy efficiency of manufacturing process. (C) 2017 Elsevier Ltd. All rights reserved.