A wave power generator comprises a buoyant casing (500) intended to float in a body of water. An electric machine (103) located within the casing has an armature and a field source, the electric machine having a fixed part coupled to the casing and a moving part. A counterweight assembly (104) is movable within the casing, comprising the moving part of the electric machine and wherein a relative movement of the counterweight assembly and the fixed part of the electric machine generates electric power. Power storage (400) stores power generated by the electric machine and a control system (200) determines a bi-directional energy flow between the power storage and the armature, wherein energy is returned to the electric machine to drive a motion of the counterweight assembly anti-symmetrically to a motion of the casing.