Assuming that only water molecules penetrate in low-pore diameter zeosils, we determined how the chemical potential of water evolves in the presence of electrolytes. By performing a series of Monte Carlo simulations using our adapted NCMC move, we showed that chemical potential of water in electrolyte solutions displays the same dependency in pressure as chemical potential of pure water. Our model was able to reproduce experimental intrusion pressure in various zeosils (silicalite-1, chabazite en *BEA-type zeosil) for different electrolytes at relatively high concentration (molality of ∼4.62 mol/kg), showing that our model accounts of both concentration and nature of the electrolyte solution. On the contrary, the van 't Hoff law of osmotic pressure assumes electrolyte solutions to be ideal and does not reproduce not only the small variation of intrusion pressure existing in the presence of monovalent electrolytes but clearly understimates intrusion pressure for divalent-cation electrolytes. Hence, our model suggests that a high performance electrolyte for energy storage in nanoporous materials should have a low water activity to guarantee the highest intrusion pressure. We note, however, that the presence of silanol defects in zeosils increases their hydrophilicity and therefore decreases the intrusion pressure. In order to decipher the role of the silanol defects from the pore diameter