A characteristic general feature of electronic structure of the iron-based superconductors is the strong renormalization of a quasi-particle spectrum, which is associated with strong electron–electron interaction, the components of which remain controversial. We are performed a detailed comparative analysis of the photoemission spectra of iron-based superconductors (FeSe and BKFA) and superconducting cuprates (Bi-2201). The constants of the high-energy and low-energy parts of the self-energy, which determine the renormalization in cuprates and iron-based superconductors, are separated, and the essential role of the electron–phonon interaction in the latter ones is proved.