NOVELTY - The management unit has a circuit for periodically connecting and disconnecting solar modules (201a-201n) to and from a power bus according to duty cycles. A controller is adapted to receive operating parameters from management units coupled to the power bus. Each management unit is adapted to determine the operating parameters of the respective solar modules and transmit the operating parameters of the respective solar modules to the controller. The controller determines the duty cycles for the management units based on the operating parameters received from the management units.    USE - Management unit such as system management unit and local management unit, for use with a solar panel to control connection of solar cells attached to strings of cells within the solar panel, in a photovoltaic energy production system (claimed) for balancing currents among photovoltaic units connected in series.    ADVANTAGE - The duty cycles can be tuned and/or re-adjusted to shifting shading after an initial set of duty cycles is applied to the solar module so as to improve power performance. The management unit can adjust duty cycle of a solar panel to increase total output power of string, current of the string, current of the solar panel and drive voltage of the solar panel towards target voltage. The duty cycle of the solar module with highest power in the string can be decreased to increase voltage of the solar module, in response to the decrease in voltage of the solar module caused by adjustment to the duty cycles of another solar module on the string since decreasing the duty cycles of the former solar module decreases time period the former solar module is connected to the string, and increases voltage of the former solar module.    DETAILED DESCRIPTION - The power bus is a serial power bus and a string bus (205). The operating parameters are current, voltage, temperature, duty cycles and phase shift. INDEPENDENT CLAIMS are also included for the following:    (1) a photovoltaic energy production system    (2) a method for balancing currents among photovoltaic units connected in series.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic block diagram of a photovoltaic system.    Photovoltaic system (200)    Solar modules (201a-201n)    Inverter (203)    System management unit (204)    String bus (205)