A direct current (DC) bus voltage from a combined output of a plurality of DC power modules is controlled based on an alternating current (AC) voltage of a power grid. The DC bus voltage tracks the AC grid voltage to provide efficient conversion between the DC power sources and the AC grid, even when the amplitude of the AC grid voltage varies. In one example, a variable reference voltage is generated based on a detected AC grid voltage. The reference voltage increases and decreases in proportion to increases and decreases in the AC grid voltage. In this manner, large differences between the bus voltage and the grid voltage are avoided. By closely tracking the two voltages, efficiency in the modulation index for power conversion can be achieved.