PV arrays are commonly wired for the highest voltage that the controller can handle, as this allows the smallest—and cheapest and easiest—wire size that can be used. Maximum power point tracking (MPPT) charge controllers allow the flexibility to configure a PV array for a range of output voltages.
Having fewer parallel strings in a PV array simplifies the installation and raises the voltage, compared to fewer modules in each string. This reduces costs, as fewer overcurrent protection devices—breakers or fuses—are needed, and a smaller combiner box can often be used. Having smaller and fewer wires speeds up the installation process, reducing costs even more.
But there are trade-offs involved with using higher PV array voltages. Higher array voltage results in a slightly lower operating efficiency for the MPPT controller. Controller efficiency decreases as the step-down ratio (PV array voltage to battery voltage) increases. For example, a system with a 4:1 ratio (96 VDC PV array and 24 VDC battery) will operate at a 0.5% lower efficiency than with a MPPT controller operating at a 3:1 ratio (72 VDC PV array and 24 VDC battery).
Some MPPT controller manufacturers provide information, such as graphs that show the impact on efficiency at various PV array voltages and battery voltages.
On systems with longer wire runs between the PV array and the MPPT controller, the savings achieved by being able to use a smaller wire size can be substantial. Sizing wires somewhat larger than required can reduce a typical array-to-controller wire loss of 2% to 1.5%, for instance. This can make up for the slightly lower conversion efficiency of a greater step-down ratio.
However, on systems with short wire runs, the savings from using a smaller wire size may not be enough to offset the lower MPPT controller efficiency. In these cases, having a lower PV array voltage configuration would be a better choice.