A mathematical model based on MATLAB is first used to solve the coupled non-linear I-V equation of a single junction (SJ) solar cell using GaAsNSb quaternary alloy lattice matched to GaAs substrate. The response of the Photovoltaic (PV) Cell using series or parallel connection of two individual cells is investigated and the effect of variable irradiance and temperature conditions on the I-V and P-V characteristics of the cell is studied. The influence of series and parallel resistances R-s and R-p on the single diode model is calculated using an iterative computer simulation model. The experimental values of the essential parameters of the solar cell - short circuit current I-sc, open-circuit voltage V-oc, Fill factor (FF) and the efficiency of the SJ solar cell are used for calculating the optimized values of R-s and R-p to match the theoretically calculated output power with the experimental Maximum Power Point (MPP). The values of five different parameters I-sc, V-oc P-max FF and eta are compared using four different mathematical models under variable temperature and irradiance by including the effects of R-s and R-p.