The global climate change and the rising of energy prices force manufacturing companies to regulate their energy usage. A suitable step to achieve that is the introduction of energy management. This paper presents an ICT based holistic approach to help manufacturing companies in the implementation of energy management system. It consists of methods to support quantitative and qualitative energy efficiency evaluation of their operations. The approach uses an ontological knowledge base containing the structures and rules representing best practices as reference of energy efficiency to support the qualitative evaluation. In the approach, we also develop measurement figures called Energy Performance Indices (EPI) to determine the energy efficiency degrees in different organizational parts of the company. The paper also describes the application of the approach in a small medium sized manufacturer.