Optical properties of multilayer semi-conductor nano-emitters are crucially dependent on the relative energy levels of their different components. For core/shell quantum dots, the relative energy difference between conduction band edge of core and shell materials induces, depending on its value, either a confinement of the electron within the core or a delocalization of its wave function within the whole quantum dot. This results in drastic consequences on the energy and the oscillator strength of the fundamental transition. Surprisingly, the literature currently lacks a definitive value for the energy difference between CdSe and CdS conduction band edges as most of the experimental studies provide values corresponding to specific geometries of quantum dots. Here, we develop a full theoretical model expressing energy levels considering core/shell interface pressure, ligands and enabling the accurate prediction of the bandgap value with the nanocrystal size. It allows to reliably determine the energy difference between the conduction band edge of CdSe and CdS materials, known as the conduction band offset, in such a way that this value can later be used to model quantum dots of any geometry. This value is determined using our model and two different experimental methods: optical spectroscopy and X-ray photoemission (XPS) experiments.