Our energy system is rapidly evolving. There are big challenges ahead of us, including mitigating climate change, securing our long-term energy supply, and providing access to sustainable energy for everyone. Energy collection, conversion, transport, storage, distribution, and end-use application are all steps in providing energy services to society. All these elements form an energy system, which is complex and interacts with other systems in a multitude of ways. Transitions in energy systems and the merits of innovation for the individual components of each can only be analysed in the context of the complete system.