The solar cells collecting grids present a serious problem and more particularly under solar concentration. Our contribution in this article is to seek the best compromise between shadow effect and series resistance effect. The cell considered here is of Si (silicon) type, n(+)p with circular geometry (radius a = 4.9 cm), a silver metallization (rho(M) = 1.6 x 10(-6) Omega cm), and a contact resistivity of rho(C) = 10(-5) Omega cm. Our calculations are made under the condition of AM 1.5 with 1 sun concentration. The various power losses caused by this grid are: losses due to the grid shadow, losses in grain boundaries due to the metal/semiconductor contact, power dissipated in the resistance of layer between bars, and losses in the grid metallization. We summarize the results of our model by: For a sheet resistance rho(S) = 10 Omega/square:tau(oop)(%) (optimum shadow fraction) = tau(cop)(%) (optimum conduction fraction) = 5.086% leading to theta(optimum) = 0.0652 rad.