The invention relates to an energy conversion system having an electrolysis unit (4) for electrochemical generation of hydrogen and oxygen from water, a connection to the public power network (1) for supplying the electrolysis unit (4) with electrical energy, an anaerobic bio-reactor (6) of a clarification plant, wherein the bio-reactor (6) has a discharge device (9) for removal of a methane-containing gas produced in the bio-reactor, a supply device (5) for the supplying of hydrogen into the anaerobic bio-reactor (6) of the clarification plant, a control and regulating unit (11) for controlling and regulating the supply of hydrogen into the anaerobic bio-reactor (6) of the clarification plant and for controlling and regulating the removal of the methane-containing gas produced in the bio-reactor (6), and a device for recovery (18) and/or storage (24) of the methane-containing gas extracted from the bio-reactor.