Fill factor

Fill factor (FF) is a characteristic value that describes the shape of a PV source's I-V curve. It tells you how square the curve is. It equals the maximum power divided by the product of open-circuit voltage and short-circuit current, so Pmax equals FF times Isc times Voc. The higher the fill factor, the closer the I-V curve is to a rectangle. Any degradation in the curve shape lowers the fill factor and the power output. Compare a string's fill factor with the module datasheet value and with other strings that use the same module type. A value well below the datasheet at nominal conditions may point to faults that cause yield loss. The sub-factors Imp/Isc and Vmp/Voc show changes in the slope of the horizontal and vertical legs of the I-V curve. This helps when diagnosing underperforming strings. Fill factor works well for comparing strings because it is calculated only from the measured I-V curve. It is also fairly independent of irradiance at high irradiance, so errors in the irradiance or temperature sensors do not affect it. All I-V curve tracers report fill factor. Typical cell values are 0.75 to 0.80 for polycrystalline silicon and 0.80 to 0.85 for monocrystalline silicon.

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• sentence 5: support check: partial - Sub-factors give insight into slope variations of the legs, but the passages do not explicitly say this helps diagnose underperforming strings (only FF is tied to troubleshooting). -->