The storage of fluctuating energy production is a major challenge on the pathway to a fully renewable electricity supply. This paper investigates the impact of the storage technology Power-to-Gas (PtG) in the implementation of the Energiewende. A detailed cost optimisation model based on data from an existing system in an actual German region describes the optimal composition and application of energy supply technologies. The electricity demand of the region can be covered with 100% renewable energy (RE) at a levelised cost of electricity (LCOE) of 11ct/kWhel. A 100% RE supply is possible with or without PtG. However, long-term energy storage system such as PtG reduce the LCOE of the energy system significantly. We conclude that the capital expenditures (CAPEX) of PtG must at least reach values below 2500€/kWel to compete with short-term solutions such as Li-ion batteries. Beside PtG this investigation identifies several system components with extensive impact on LCOE and demand of long-term energy storage, such as Power to Heat (PtH), wind turbine technology and hydropower. The combination of such influences increases the impact of each of them on the LCOE. Several energy system components can positively influence the implementation of the Energiewende, PtG is one of them.