In FIRST, a fully integrated reversible solid oxide cell (rSOC) system is being developed and tested in a living lab environment. The system can switch between water electrolysis and fuel cell operation on demand, resulting in a compact, scalable and nearly full-time operable electrochemical storage system. In order to achieve optimal internal thermal management, short switching times and high round-trip efficiencies, dynamic system simulations are carried out and new system control strategies with load prediction models are used. The system design also allows the use of economies of scale of mass production, so that FIRST also addresses the economic potential of a future, decentralised, renewable energy supply.