Modular electrolysis plants are a promising approach for the efficient production of hydrogen based on renewable energy due to their scalability and adaptability through the flexible addition or removal of modules. However, efficient control of these plants is challenging because of fluctuations in electricity prices and renewable energy generation output, as well as fluctuating hydrogen demand. Based on two application scenarios, this paper derives requirements for control systems to control modular heterogeneous electrolysis plants to realize an efficient operation considering the fluctuating influencing variables.