Modeling exercises on the size-induced blue-shift in the photoluminescence (PL) of nanosolid Si (J. Phys. Chem. B 2002, 106, 11725) have been extended herewith to the size dependence of the Si-2p energy-level-shift measured using X-ray photoelectron spectroscopy (XPS). Results show consistency in both the origin and trend of the core-level shift with that of band-gap expansion upon Si nanosolid formation. Most strikingly, decoding the size dependent XPS peak shift leads to quantitative information about the 2p-level atomic trapping energy of an isolated Si atom (−96.74 eV) and the crystal binding intensity (−2.46 eV) upon bulk solid formation, which is beyond the scope of direct measurement using currently available techniques.