Referring primarily to FIGS. 1 and 2, examples of structures and methods of preferred embodiments of the invention are described. A power source, in this example a photovoltaic cell or array 10, produces an output voltage PV. A regulator circuit 12, in this case a buck regulator shown in simplified form in the schematic, is operably coupled to the power source 10 and to a current limit adjustment circuit 14. The current limit adjustment circuit 14 preferably includes an amplifier configured as an integrator 15, as shown. The output of the buck regulator 12 is coupled with a load 16, preferably an energy storage medium, such as a rechargeable battery or capacitor. In operation, the circuitry shown in the example of FIG. 1 is adapted to function in at least two modes. Operation in a first mode occurs when the output power PV of the power source 10 is lower than the power PL required by the load 16. Operation in a second mode occurs when the output power PV of the power source 10, in this case a photovoltaic array, is greater than the power PL required by the load 16. Preferably, by changing the current and/or voltage level at the power source, power delivered to the load can be maximized for the prevailing conditions at the source. For example, under circumstances which cause a reduction of voltage at the power source, the current level can be adjusted to maintain the maximum power point, sometimes referred to in the art as MPPT (Maximum Power Point Tracking), at the most advantageous level for the conditions. This relationship is shown in the voltage/current graph inset of FIG. 1. This is preferably accomplished by monitoring one or more parameters reflective of the actual real-time performance of the circuit. In this way, the performance of the circuit is enhanced as conditions change, regardless of the cause of the change. For example, if a voltage drop occurs in a photovoltaic cell, the current level delivered to the load is adjusted, preferably by adjusting the reference level of a regulator module. The maximum power point is maintained irrespective of whether the voltage drop is caused by a shading of the cell, or increased operating temperature, or some other cause or combination of causes. Thus, there is no need for the use of independent sensors dedicated to monitoring lighting conditions, temperature, and so forth. The efficiency and performance of the system are enhanced, and the need to anticipate the specifics of potential performance problems is reduced.