Modeling Photovoltaic Devices
Voc is the open cct voltage with a small capacitance. This is shown as UL(E) for solar energy E at the far right 
The maximum point point starts around 82%Voc and drops with rising (-ve) current. and is the intersection of the knee and optimal load line
Since this generates current proportional to photo power input, it is negative.
The load line equivalent resistance at the Max Pwr Point or MPP is related to W=V^2/R where V is ~80% average and R is the sloped load line.  This is near the Rs value of the model.
critical parameter determined by PV max power [W] spec. 


Rs = ( Voc*82%)^2 / W approximation  
tolerance (+4% for low E, -4% high high E from sun) is my rule of thumb