An energy converter device includes a reactive hydraulic power transformer (16) for converting periodic and variable hydraulic energy into coherent hydraulic energy. The device may also form a sub-system in energy converter systems for converting kinetic energy, for example from a wave (12), into electricity. The system has a power takeoff (14) including an energy capture device (22) for capturing energy from the wave (12), or other power source, and the power takeoff transfers this captured energy to the transformer (16). An accumulator (18) may be provided for storing and selectively discharging the captured energy to a generator (20) to produce electricity. At least part of the captured energy may be fed back through the transformer (16) to facilitate control over the phase and/or position of the energy capture device (22) relative to the wave (12) or other energy source.