Critical Infrastructures (CIs) have a vital role in the functioning and development of our modern societies. To accurately understand the potential consequences of critical infrastructure disruptions, dependencies and potential cascading effects between CIs have to be considered in risk governance. As a step towards creating a system-ofsystems model of Sweden's national technical CIs, this paper presents a generic modelling approach that extends on current topological approaches by also incorporating capacity flow constraints to capture the salient properties of technical infrastructures. The model is exemplified for Sweden's national electric transmission system, a national backbone information and communication infrastructure, and their interdependencies. The model is populated with real-life data, such as topology, capacities, spatial orientation and the population's dependence on the provided services. The modelling approach is complemented with a discussion on the challenges and opportunities with creating a system-of-system model of national infrastructures and how such a model can guide cross-sector decision making.