This lecture explores the role of hydrogen in the future energy mix, focusing on its generation from renewable sources like wind power and photovoltaics. It discusses the key drivers of the energy transition and the research on hydrogen at the University of Bremen. The presentation delves into the challenges and modeling of domestic energy demand, emphasizing the importance of understanding human behavior and indirect costs in energy systems. The lecture also covers the development of energy models, addressing issues like variance stability, sensitivity analysis, and spatial-temporal analysis. It concludes with a discussion on the integration of energy demand models into larger-scale energy system modeling tools.