Because the energy of an X-ray with particular wavelength is known (for Al K X-rays, E = 1486.7 eV), and because the emitted electrons' kinetic energies are measured, the electron binding energy of each of the emitted electrons can be determined by using the photoelectric effect equation,
where E is the binding energy (BE) of the electron measured relative to the chemical potential, E is the energy of the X-ray photons being used, E is the kinetic energy of the electron as measured by the instrument and  is a work function-like term for the specific surface of the material, which in real measurements includes a small correction by the instrument's work function because of the contact potential. This equation is essentially a conservation of energy equation. The work function-like term  can be thought of as an adjustable instrumental correction factor that accounts for the few eV of kinetic energy given up by the photoelectron as it gets emitted from the bulk and absorbed by the detector. It is a constant that rarely needs to be adjusted in practice.