The wattage a PV combiner box must handle depends on the total power of the connected solar panel strings, typically calculated by multiplying the number of panels per string by the panel wattage and the number of strings combined.

Determining Combiner Box Wattage

1. Calculate Total System Power: Add up the wattage of all solar panels that will feed into the combiner box. For example, if you have 7 strings of 20 panels each, and each panel is 555W, the total power is 7 × 20 × 555W = 77,700W, which the combiner box must safely handle .
2. Consider Voltage Ratings: The combiner box voltage rating must exceed the maximum open-circuit voltage (Voc) of a string, including a safety factor (typically 1.25× Voc). For instance, if each panel has a Voc of 49.95V and there are 20 panels per string, the maximum string voltage is 49.95 × 20 × 1.25 ≈ 1,248V. The combiner box should be rated at or above this voltage, but not exceeding the inverter's maximum DC input .
3. Account for Current: Each string's short-circuit current (Isc) determines the current rating of the combiner box and its protection devices. The combiner box must include fuses or circuit breakers rated for the maximum expected current, including temperature and irradiance variations .
4. Number of Strings: The number of input circuits in the combiner box corresponds to the number of strings it will combine. For large systems, multiple combiner boxes may be used to distribute the load and simplify maintenance .

Practical Example

In a commercial system with 144 panels of 555W each and an 80kW inverter, designers used four 2-in-1-out combiner boxes, each handling two strings per MPPT input. This setup ensured that each combiner box safely managed the combined wattage and current while staying within voltage limits .

Key Considerations

• Always include a safety margin for voltage and current.
• Ensure overcurrent protection for each string.
• Choose combiner boxes rated for future expansion if additional strings may be added.
• Verify compatibility with the inverter's maximum DC input. By following these steps, you can select a PV combiner box that safely handles the total wattage of your solar array while providing protection and reliability for the system .