The energy decomposition analysis based on block localized wave functions (BLW-EDA) allows to gain physical insight into the nature of chemical bonding, decomposing the interaction energy in (1) a "frozen" term, accounting for the attraction due to elec-trostatic and dispersion interactions, modulated by Pauli repulsion, (2) the variationally assessed polarization energy and (3) the charge-transfer. This method has so far been applied to gas-and condensed-phase molecular systems. However, its standard version is not compatible with fractionally occupied orbitals (i.e. electronic smearing) and, as a consequence, cannot be applied to metallic surfaces. In this work, we propose a 1 simple and practical extension of BLW-EDA to fractionally occupied orbitals, termed Ensemble BLW-EDA. As illustrative examples, we have applied the developed method to analyze the nature of the interaction of various adsorbates on Pt(111), ranging from physisorbed water to strongly chemisorbed ethylene. Our results show that polarization and charge-transfer both contribute significantly at the adsorption minimum for all studied systems. The energy decomposition analysis provides details with respect to competing adsorption sites (e.g., CO on atop, vs. hollow sites) and elucidates the respective importance of polarization and charge transfer for the increased adsorption energy of H 2 S compared to H 2 O. Our development will enable a deeper understanding of the impact of charge transfer on catalytic processes in general.