Fill factor is defined as FF = (Vmp × Imp) / (Voc × Isc). Which of the following statements about FF is correct?

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Fill factor measures how closely the I-V curve of a solar cell resembles a perfect rectangle, indicating how much of the potential power at the maximum power point is actually extractable. It is defined as the product of the voltage and current at the maximum power point divided by the product of the open-circuit voltage and the short-circuit current: FF = (Vmp × Imp) / (Voc × Isc). This exact expression shows why the statement is correct: it compares the usable power at the best operating point to the theoretical maximum power available from the cell.

If you flip the ratio, you’d get a reciprocal, which doesn’t describe how efficiently the cell delivers power. Using only Vmp or only Imp ignores half of the power relationship, and using Voc with Isc ignores the maximum-power point altogether. Understanding FF helps you gauge how much of the cell’s potential power is actually realized under practical operating conditions.