As a result of the urgent climate problem, ambitious plans have been set up in recent decades to support the development of renewable energy sources and the financing of green technologies, including hydrogen production. However, the increasing integration of such variable energy sources into the energy system causes stress on conventional production units. To absorb this stress, the energy system requires an increased amount of flexibility. The dissertation examines the techno-economic feasibility of balancing the energy grid through power-to-hydrogen systems, taking into account the impact on the operation of flexibility offered by consumers and their rationality on the energy system. Considering these goals, various optimisation algorithms and models were developed. The first two methodologies optimise the flexible operation of power-to-hydrogen plants to provide various frequency-related system services, taking into account uncertain hydrogen demand and energy prices. The third topic examines the role of different demand response strategies on energy system reliability while keeping operational costs to a minimum. The proposed methodology introduces demand-driven models taking into account consumer comfort and the level of flexibility. The proposed methods make it possible to contribute to the power grid of the future with high shares of renewable energy sources.