Fill factor is a way to compare the maximum power of a module (Imp X Vmp = Pmax) to short circuit current (Isc) and open circuit voltage (Voc). It tells us how square a module's I-V curve is, which relates to the efficiency and performance of the module.

The formula for fill factor is: (Vmp * Imp) / (Voc * Isc)

The blue square in the illustration above is Imp x Vmp and represents the maximum amount of power this module can produce. At short circuit current (Isc), the module produces the maximum amount of current, but there is no voltage or power. At open circuit voltage (Voc), the module produces the maximum amount of voltage, but there is no current or power. Even though you will never see short circuit current and open circuit voltage simultaneously in the real world, the product of these two numbers, represented by the green box, gives us a good number to compare to maximum power.

Fill factor is always less than one and typically between .7 to .8. Comparing the fill factor of different modules or strings of modules gives us an easy way to quickly identify issues with the PV array.

Calculating Fill Factor for use with the SMFT-1000 and PVA-1500

When entering module data manually into the Fluke Solmetric PVA-1500?software, Fluke SMFT-1000, TruTest?, or the mobile app, you will need to include fill factor. This value is rarely provided by the module manufacturer. However, calculating it is straightforward. Simply multiply the maximum power current (Imp) by the maximum power voltage (Vmp) and divide by short circuit current (Isc) times open circuit voltage (Voc) or (Vmp * Imp) / (Voc * Isc). This information (Imp, Vmp, Isc, and Voc) is all listed on the module label and in the module datasheet. It’s worth noting that, occasionally, module manufacturers might use different abbreviations for maximum power current and voltage, such as Impp and Vmpp.

Here's an example of a fill factor calculation for a 425-watt module:

Electrical Specification For Fill Factor

Table from Mission Solar Energy, MSE PERC 72, 430W

(Vmp * Imp) / (Voc * Isc) = Fill Factor
(10.78 X 39.44) ÷ (11.42 X 49.04)
425.1632 ÷ 560.0368 = 0.759