Heterogenization of polyoxometalate clusters by means of adsorption on surfaces is a necessary step prior to their use in heterogeneous catalysis and other applications. Herein, we introduce a general strategy to model the adsorption of polyoxometalates on surfaces, by combining periodic DFT calculations with classical molecular dynamics simulations so as to include the effects of counterions and solvent molecules. The specific case that we discuss here is the spontaneous reduction of the archetypal α-[SiW₁₂O₄₀]^4- Keggin anion adsorbed on silver surfaces. Modelling the electron transfer between the surface and the anion is an excellent test of our approach, as it is necessary to place the Fermi level of the system at the right position. The immediate reduction of the anion, which we observed only when the environment is included, confirms the relevance of both the counterions and solvent to stabilise the adsorbed anion. The strategy used is a breakthrough in the computational modelling of polyoxometalates adsorbed on surfaces and it should be appropriate to study other properties of immobilised metal oxide anions on surfaces.