The use of photoelectron spectroscopy for the evaluation of the electrochemical potentials in intercalation phases is presented. An in situ prepared thin film electrochemical cell has been used to relate electrochemical quantities as the change of electrochemical potential with electronic properties measured in UHV, as well as with the change of the chemical potential of the electrons, which is equivalent to the position of the Fermi level inside the solid. The changes observed in photoemission data with intercalation are compared to galvanostatic experiments obtained in electrochemical setups using the same sample. From a comparison of the results it is possible to estimate the ionic and electronic contribution to the battery voltage. For Na intercalated into TiS2 the results indicate a contribution of the electronic component to the battery voltage of at least 60%.