Effect of geometrical and structural parameters on the efficiency of the tandem solar cell based on the III-V nanowire array on silicon is studied by the means of coupled opto-electrical simulations. A close to realistic structure, consisting of A1GaAs core-shell nanowire array, connected through a tunnel diode to a Si subcell is modelled, revealing the impact of top contact layer, growth mask and tunnel junction. Optical simulation of the tandem structure under current matching condition determine optimal geometrical parameters of the nanowire array. They are then used in the extensive electrical optimization of the radial junction in the nanowire subcell. Device simulations show the necessity of high doping of the junction in order to avoid full shell depletion. The influence of bulk and surface recombination on the performance of the top subcell is studied, exposing the importance of the good surface passivation near the depleted region of the radial p-n junction. Finally, simulations of the fully optimized tandem structure show that a promising efficiency of eta = 27.6% with the shortcircuit current of J(sc) = 17.1 mA/cm(2) can be achieved with reasonable bulk and surface carrier lifetime.