NOVELTY - Storing energy in the form of methane comprises: (a) generating electric energy using a renewable and/or non-renewable energy source; (b) using at least partially the electric energy of step (a) for the electrolysis of water and/or brine, thus producing hydrogen and/or oxygen; and (c) using at least partially the hydrogen of step (b) and carbon dioxide, and optionally partially the electric energy of step (a), for producing methane by methanogenic microorganisms in a reaction vessel.    USE - The method and system are useful for storing energy in the form of methane (all claimed).    ADVANTAGE - The method and system effectively convert energy from any energy source, such as renewable or non-renewable energy sources, in particular of a renewable energy source, in an inexpensive way and with high process efficiency in a storable and transportable form and thus solves the problem of energy storage, allows for transportation of such energy carrier in current infrastructure and easy delivery to the end-user.    DETAILED DESCRIPTION - Storing energy in the form of methane comprises: (a) generating electric energy using a renewable and/or non-renewable energy source; (b) using at least partially the electric energy of step (a) for the electrolysis of water and/or brine, thus producing hydrogen and/or oxygen; and (c) using at least partially the hydrogen of step (b) and carbon dioxide, and optionally partially the electric energy of step (a), for producing methane by methanogenic microorganisms in a reaction vessel, comprising contacting the methanogenic microorganisms with an in-gas feed comprising at least the hydrogen and carbon dioxide. An INDEPENDENT CLAIM is a system for storing energy in the form of methane comprising at least one device for generating electric energy from a renewable and/or nonrenewable energy source, at least one device for producing hydrogen and/or oxygen by the electrolysis of water and/or brine, and at least one bioreactor comprising a reaction vessel suitable for growing, fermenting and/or culturing methanogenic microorganisms.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of a system.    Renewable and/or non-renewable energy source (1)    Water and/or brine (2)    Bioreactor (3)    Electric energy storage device (4)