The Fill Factor of a solar cell/panel is the ratio of the solar cells actual power output (Vpmax x Ipmax) versus its 'dummy' power output (Voc x Isc). This is a key parameter in evaluating the performance of solar cells. Typical commercial solar cells have a fill factor > 0.70. Reject crystalline cells like those found on eBay, or grade B cells, have a fill factor usually between 0.4 to 0.65, and in amorphous solar cells or thin film cells between 0.4 to 0.7.

The fill factor can also be used to determine the series resistance of a solar cell.

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   Fill Factor (FF) = (Vmpp x Impp) / (Voc x Isc)
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Where:

Vmpp = Maximum Power point Voltage
Impp = Maximum Power point Current
Voc  = Open Circuit Voltage
Isc  = Short Circuit Current

More info:

The picture below illustrates the I-V curves of both a high fill factor solar panel and a low fill factor panel.



As can bee seen, both curves (solar panels) hold the same open circuit voltage and short circuit current, however, the lower fill factor panel actually produces less power at its maximum power point compared to the higher fill factor panel. A normal person could easily be fooled in just testing the Voc and Isc for the panels power measurement.

During the manufacture of commercial grade solar panels, each and every single solar cell is tested for its fill factor. If its fill factor is low (below 0.7), the cells are sold off as Grade-B cells, then sliced, and used for hobbyist use.

To make things clearer, look at the picture below.



We can see that the dummy power (or the theoretical power) is the area in red, and the actual power is the area in the blue. The fill factor is the ratio of the blue area to the red area. The higher the fill factor of a solar panel, the closer the blue curve is to the red curve.

One can say that a higher FF solar panel has less parasitic losses, ie, losses due to the series and parallel resistances within the cell itself.

When commercial solar panels are labeled, they take the FF into account, and only measure the real power. When hobby cells are labeled, you will see that most of the time they will just states the short circuit current and open circuit voltage.

Please refer to this document The physics of solar cells for a more detailed explanation.

Nice information. Learn a lot.

Thanks so much,

First thanks for that clear explanation of Fill Factor...
I know fill factor FF is given by the following relationship.

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                      Pmax          Vmp.Imp          v  - Ln(v+0.72)                           q
              FF = ----------  = -------------    =    ------------------    where  v  =    ----------.Voc
                      Pt                Voc.Isc            v  +1                                       nKT                                             
:.    Pmax   =  FF.Voc.Ioc


Unfortunately this doesn't take into consideration the parasitic resistances, varying temperature and irradiance (G). My aim is to calculate Pmax empirically since Voc & Isc can be measured experimentally. Can someone help throw in an expression for FF that accounts for parasitic resistances, changing temperature (T) and irradiance (G). Will appreciate your help.