Using Transfer Matrix method combined with Effective Medium Approximation as well as Finite-Difference Time-Domain calculations we have determined the light absorption in organic solar cells with silver nano particles embedded in the active layer. In calculations, we introduced additional terms in permittivity of the nanoparticles due to their finite size. Our results show, that the optimal nanoparticle concentration in such a system is about 30% greater than when assuming that the permittivity of the Ag nanoparticles equates that of a continuous silver layer. We have found, that although the absorption curves for both models (with and without the finite size contribution) are similar, the dominant mechanism of absorption enhancement is different. When assuming that the permittivity of silver nanoparticles is that of a continuous Ag layer, the light scattering on the surface of the nanoparticles seems to dominate over localized surface plasmon (LSP) excitation, while when considering the finite sizes of the nanoparticles the opposite is true.