The design of energy systems has become an issue all over the world. A single optimal system cannot be suggested because the availability of infrastructure and resources and the acceptability of the system should be discussed locally, involving all related stakeholders in the energy system. In particular, researchers and engineers of technologies related to energy systems should be able to perform the forecasting and roadmapping of future energy systems and indicate quantitative results of scenario analyses. We report an energy flow model developed for analysing scenarios of future Japanese energy systems implementing a variety of feasible technology options. The model was modularized and represented as functionals of appropriate technology options, which enables the aggregation and disaggregation of energy systems by defining functionals for single technologies, packages integrating multi-technologies, and mini-systems such as regions implementing industrial symbiosis. Based on the model, the combinations of technologies on both energy supply and demand sides can be addressed considering not only the societal scenarios such as resource prices, economic growth and population change but also the technical scenarios including the development and penetration of energy-related technologies such as distributed solid oxide fuel cells in residential sectors and new-generation vehicles, and the replacement and shift of current technologies such as heat pumps for air conditioning and centralized power generation. The developed model consists of two main modules; namely, a power generation dispatching module for the Japanese electricity grid and a demand-side energy flow module based on a sectorial energy balance table. Both modules are divided and implemented as submodules represented as functionals of supply- and demand-side technology options. Using the developed model, three case studies were performed. Required data were collected through workshops involving researchers and engineers in the energy technology field in Japan. The functionals of technologies were defined on the basis of the availability of data and understanding of the current and future energy systems. Through case studies, it was demonstrated that the potential of energy technologies can be analysed by the developed model considering the mutual relationships of technologies. The contribution of technologies to, e.g., the reduction in greenhouse gas emissions should be carefully examined by quantitative analyses of interdependencies of the technology options. (c) 2014 Elsevier Ltd. All rights reserved.