The solar energy conversion is investigated theoretically. The variation of the temperature of the solar cell subjected to the incident global solar radiation along the local day time is determined. The heat balance equation is solved .The solution revealed that the cell temperature is a function of the maximum value of the daily incident global solar radiation qmax,the cooling coefficient (h), the optical parameters, physical parameters, and the geometrical parameters of the cell . The temperature dependence of the short circuit current Isc ,the dark saturation current I0 , the open circuit voltage Voc , and the energy band gap characterizing a Silicon solar cell is considered in evaluating the cell efficiency . Computations of the efficiency concerning different operating conditions and different astronomical locations (Egypt and Hong Kong) as illustrative examples are given.