Substantial losses of power caused by the existence of R_{SH }-which is the shunt resistance- are usually resulting from manufacturing defects, more than bad solar cell design. Low R_{SH} results in solar cells power losses by making an alternate current path for the light generated current. This kind of diversion causes a reduction in the amount of current that flows through the solar cell junction and reduction in the voltage from the solar cell. R_{SH} effect is especially severe at levels of low light, as there would be less light generated current.
The losses in this current to the R_{SH} as a result, has a bigger impact. Also, at lower voltages when the effective resistance of a solar cell is high, the effect of a parallel resistance is high.
The equation for the solar cell in existence of shunt resistance is defined below:
I=I_{L}−I_{0}exp[qV / nkT] – V / R_{SH}
where:
I: the cell output current I_{L:} the light generated current V: voltage across the cell terminalsT: temperatureq & k: constantsn: ideality factor

R_{SH}: shunt resistance of the solar cell.

An estimation for the value of the shunt resistance of a solar cell can be derived from the slope of the IV curve near the point of short circuit current.

Formulas for Shunt Resistance and FF

The effect of the R_{SH} on the fill factor can be calculated in a similar way to the method used in finding the effect of series resistance on the fill factor. The maximum power can be approximated as power in the R_{SH} absence, minus the power which is lost in the shunt resistance. The equation for the maximum power from a solar cell then can be defined as the following:
P’_{MP }≈ V_{MP}I_{MP }– V^{2}_{MP}/R_{SH}= V_{MP}I_{MP }( 1 – ( V_{MP} / I_{MP}) *1/R_{SH}) = P_{MP} (1 –( V_{OC} / I_{SC}) * 1 /R_{SH} )P’_{MP = }P_{MP} ( 1 – R_{CH }/ R_{S})
While a normalized series resistance is defined as:
r_{SH} = R_{SH} / R_{ch}
Using the assumption that open circuit voltage and short circuit current are both unaffected by shunt resistance, allows the shunt resistance impact on FF to be defined:
P’_{MP} = P_{MP} ( 1 – 1 / r_{SH })V’_{OC}I’_{SC}FF’= V_{OC}I_{SC}FF(1– 1 / r_{SH})FF’=FF (1– 1 / r_{SH})
In the equation above the FF that is unaffected by shunt resistance is named FF_{0}and FF' is denoted by FF_{SH}. The equation then is:
FFs_{H}=FF_{0} (1– 1 / r_{SH })
A slightly more accurate empirical equation, for the relating both FF_{0} and FF_{SH} is then:
FF_{SH}=FF_{0} (1– (V_{OC} + 0.7)/ V_{OC }* (FF_{0} / r_{SH} )
This is valid for r_{SH} greater than 0.4
Normal values for the area normalized shunt resistance are in the range of MΩcm^{2}for the type of laboratory solar cells, and for commercial solar cells 1000 Ωcm^{2}.