An apparatus comprising: an electrical storage apparatus comprising: first and second electrodes separated by an electrolyte, wherein the first electrode overlays the second electrode; and a flexible substrate of electrically insulating material, and an electrically conductive polymer, wherein the electrically conductive polymer and the flexible substrate form together at least part of the first or second electrode of the electrical storage apparatus such that the electrically conductive polymer provides an electrical path for electrons which are generated and/or stored by the electrical storage apparatus, wherein the flexible substrate comprises a plurality of interwoven fibres, and the electrically conductive polymer is retained by the flexible substrate by being absorbed into the fibres of the flexible substrate, or by forming a surface coating on the fibres of the flexible substrate, or by filling the spaces between the fibres of the flexible substrate; an electricity-generating elastomer layer positioned on one or both sides of the electrical storage apparatus; and an energy harvesting circuit positioned between each electricity-generating elastomer layer and the respective electrode (818, 819), or positioned on the external surface of each electricity-generating elastomer layer, wherein the at least one electricity-generating elastomer layer is configured to generate electricity in response to applied mechanical strain, and wherein the electrical storage apparatus is configured to store the electricity generated by the electricity-generating elastomer layer. an electrical storage apparatus comprising: first and second electrodes separated by an electrolyte, wherein the first electrode overlays the second electrode; and a flexible substrate of electrically insulating material, and an electrically conductive polymer, wherein the electrically conductive polymer and the flexible substrate form together at least part of the first or second electrode of the electrical storage apparatus such that the electrically conductive polymer provides an electrical path for electrons which are generated and/or stored by the electrical storage apparatus, wherein the flexible substrate comprises a plurality of interwoven fibres, and the electrically conductive polymer is retained by the flexible substrate by being absorbed into the fibres of the flexible substrate, or by forming a surface coating on the fibres of the flexible substrate, or by filling the spaces between the fibres of the flexible substrate; first and second electrodes separated by an electrolyte, wherein the first electrode overlays the second electrode; and a flexible substrate of electrically insulating material, and an electrically conductive polymer, wherein the electrically conductive polymer and the flexible substrate form together at least part of the first or second electrode of the electrical storage apparatus such that the electrically conductive polymer provides an electrical path for electrons which are generated and/or stored by the electrical storage apparatus, wherein the flexible substrate comprises a plurality of interwoven fibres, and the electrically conductive polymer is retained by the flexible substrate by being absorbed into the fibres of the flexible substrate, or by forming a surface coating on the fibres of the flexible substrate, or by filling the spaces between the fibres of the flexible substrate; an electricity-generating elastomer layer positioned on one or both sides of the electrical storage apparatus; and an energy harvesting circuit positioned between each electricity-generating elastomer layer and the respective electrode (818, 819), or positioned on the external surface of each electricity-generating elastomer layer, wherein the at least one electricity-generating elastomer layer is configured to generate electricity in response to applied mechanical strain, and wherein the electrical storage apparatus is configured to store the electricity generated by the electricity-generating elastomer layer.