# Is the fill factor of a photo-voltaic cell constant?

An important factor in specifying the efficiency of photo-voltaic (PV) cells is called the fill factor (FF). It is defined as the ratio of the maximum power of the cell to the product of the open-circuit voltage $V_{oc}$ to the short-circuit current $I_{sc}$. $$P_{max} = V_{oc} * I_{sc} * FF$$

Is $FF$ constant in this equation? I've found online that $$FF = \frac{V_{mp}*I_{mp}}{V_{oc}*I_{sc}}.$$ But it wasn't specified if those values are under STC or not. If they aren't, then how would I get $V_{mp}$ and $I_{mp}$?

• Hello and thank you for the edit, it's correct. Mar 21 '15 at 19:25

Power output of a PV panel is driven by several exogenous factors: the amount and spectrum of light on it; the presence and distribution of any shadows; the voltage applied to the panel; and the temperature of the panels - which is driven partly by the ambient temperature.

If you've got the power rating of the panel then you can calculate the fill factor: it's the power rating divided by the open-circuit voltage and then divided by the short-circuit current.

If you've got the power rating, and you know you're applying maximum-power-point-tracking to the voltage, and you know the ambient temperature, then the PV datasheet will usually give you a coefficient for how quickly max power changes with changes in temperature. The datasheet may also have several curves for various combinations of insolation and temperature. Together, these will give you a decent first estimate.

The whole thing is a non-linear system, and in the end, you'll only know the power out under specific conditions for a specific system by running it through those conditions.

Fill Factor is important for PV (photovoltaic) cell and panel designers but of less direct importance to users. What is usually more important are the practical parameters which result from a given fill factor. Reduced fill-factor cells will also be found to have lower efficiency of conversion, requiring larger panel areas per Watt out. Actual insolation levels are usually variable enough that variations based on fill factor are in many cases liable to be swamped by other variables. Insolation levels plus panel cleanliness, and variations across the day and season and local air conditions all add up to a produce very variable and uncertain output level.

Reputable manufacturers provide Vmp. Imp, Voc and Isc.
They also specify coefficients of change per degree C of Vmp, Imp, Wmp at 25 C.
Given all these parameters one could theoretically calculate panel output in given insolation conditions. Even after allowing for the factors mentioned above typical non MPPT installations often achieve substantial mismatch between panel Wmp operating point and load which further reduce available output.

Parameters are almost always at 25C panel temperature and AM1.5 insolation conditions. Fill Factor is specified by some but often isn't, and can be easily calculated from the supplied parameters.