NOVELTY - The energy transfer device (108) has a second interface unit which is adapted to transfer energy between a second port and an energy storage capacitor (138). A first interface unit and the second interface unit are adapted to transfer energy from the first port to the capacitor and from the capacitor to the second port when energy available at the first port is greater than the energy available at the second port. A first terminal (140) and a second terminal (142) of the capacitor is coupled to the common return node.    USE - Energy transfer device for solar power systems.    ADVANTAGE - Provides uniform current among the series-connected photo-voltaic devices by controlling the energy exchanger to maintain the series-connected photovoltaic devices at a maximum power point by drawing power from high-performing devices and applying power across low-performing devices. Optimizes the power output from the multijunction photovoltaic device by monitoring the voltages at the high-current-producing and low-current-producing junction to determine switching at the switching device of the converter. Minimizes power losses and enables potentially lower ratings for the power circuits to reduce size and costs of the energy transfer device. Improves energy exchange by eliminating the false peak power points.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are also included for the following:    (1) a system comprising an energy transfer device;    (2) a solar photovoltaic array;    (3) a system for driving electrical load; and    (4) a subsystem.    DESCRIPTION OF DRAWING(S) - The drawing shows a block diagram of the solar system having the energy exchanger having bi-directional direct current to direct current converters for balancing output current of photovoltaic devices.    Energy transfer device (108)    Bi-directional direct current to direct current level-shifting converters (130,132,134)    Energy storage capacitor (138)    First terminal (140)    Second terminal (142)