The present invention relates to an energy storage device comprising a flexible substrate comprising at least two patterned regions spaced apart from one another along the length of the flexible substrate. Each patterned region comprises at least one groove extending in the longitudinal direction of the substrate (web direction) having a first and a second face, wherein the first and second faces are each coated with a conductor such that there is no direct electrical communication between the conductor on the first and second faces, the at least one groove contains a material for storing electrical potential energy (e.g. capacitive material), the first and the second face of the at least one groove of each patterned region are each in electrical connection with an electrical conductor at opposing edges of the flexible substrate, and the first and the second patterned region are electrically connectable to one another.