Energy and resource efficiency have become important objectives for industrial processes. They are taking more and more impact on the competitiveness of companies. Therefore, energy-efficient factory systems are needed in terms of sustainable production. Thus, new or improved models, methods and tools focusing energy efficiency are necessary. In this paper, the “Flow System Theory”, as a tool for modeling technical systems and processes, is presented. It is extended in order to describe the energy flow systems of factories. This is supposed to be the basis for systematic modeling of energy-efficient factory systems.