Power to gas is a technology that can significantly support the integration of renewable energies into existing energy infrastructures. Beyond this the implementation of power to gas will lead to a further merging of the power and natural gas grids, which marks an important step forward on the way to a future energy system. To evaluate the potential of the technology, as a basis for future political measures and as a precondition for successful pilot plants, it is important to develop realistic system concepts. Moreover, the expected costs for the technologies and their products (hydrogen and methane) need to be estimated. Furthermore the tolerance of the existing natural gas grid over hydrogen needs to be investigated in order to path the way for the implementation of this integration route for green power. Answers to this topics will be provided by the R&D-project "Energy Storage Concepts" initiated by the DVGW (German Association of Gas and Water). First results show that the wide parts of the existing gas grid are able to host at least up to 10%-Vol. of hydrogen with minor adoptions. On the other hand it was also found that some elements (e.g. underground storages, gas turbines and CNG-tanks) are more sensitive over hydrogen and needs to be investigated in detail. An other finding is that plant sizes requests careful case by case optimisation according to the expected green power generation, the estimated capacity of the power grid to accommodate this energy as well as the capacity of the gas grid. It becomes also clear that the injection of green gases (hydrogen or renewable methane) generally needs to be evaluated case by case. First economical considerations show that power to gas facilites, if operated as peak shaver, need subsidy. Due to this situation as well as based on the experiences with biogas injection facilities it is recommended to initiate standardisation activities soon in order to achieve engineering and construction practises that leads to high efficient plants.