Solar Cells (SCs) energy-conversion technologies have been widely studied from their physical fundamentals to potential commercial applications. In particular, SCs Fill Factors (FFs) are a key factor for evaluating the transport efficiency of the photo-generated current and consequently the potential photovoltaic of the device. However, FF dependence on other solar cell relevant electronic properties is not entirely clear, blearing the physical meaning of this factor. In this context, this work reports the derivation of a self-consistent and generalized analytical equation by using simple Shockley diode equation and Lambert W-function that explicitly relates solar cells FFs with simple key electronic parameters. The photo-generated (JL)-to-reverse saturation (J0) current density ratio (JL/J0) was the key parameter considered for this approach because of its considerable limiting impact on FF magnitude. The accuracy of this equation was tested by an exhaustive contrast with a wide variety of experimental data for different solar cells technologies. •A generalized equation to estimate solar cells Fill Factors (FFs) was proposed.•Shockley diode model and Lambert W-function were used as theoretical basis.•Photo-generated and reverse saturation currents effects on FFs were considered.•Theoretical approach was contrasted with different solar cells experimental data.•Theoretical FFs were estimated with errors <3% concerning the experimental ones.