The observation of visible luminescence with finite quantum efficiency at room temperature from nanoporous silicon (nP-Si) prompted renewed hope for the possible integration of Silicon (Si) based light emitting devices. Since then, nP-Si quantum wires research has attracted considerable attention. However, the origin of photoluminescence (PL) and the blue shift in PL peak is still a debatable issue. We present a phenomenological model formulation to explain PL spectra from np- Si with diameters of wires ranging from 1-6 nm based on the quantum confinement model. We explicitly calculate the expression for PL spectra as a function of energy upshift. And we take the size distribution of nanoporous Si quantum wires as Gaussian. To validate our model we compare our simulated PL spectra with experiments. To observe the nature of blue shift further we use the energy gap vs. diameter as an input parameter from pseudopotential simulation and tight binding fit to our model and generate PL that shows the same nature as experimental one. Our results clearly show the blue shift of PL as a function of energy upshift and wavelength by varying the diameter of wires. We believe that Gaussian model looking at PL of nP-Si quantum wires is new observation that provide very good qualitative agreement with other observations