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.