With increase of ambient temperatue there is a deficiency in electrical energy that solar cells supply than their values under ideal conditions (25 degrees c - 1000 w/m(2)),this sitiuation be of a high affection especially in countries of a hot climate. In this work the cell temperature inside a solar panel is calculated depending on values of ambient temperature, solar radiation and multi other parameters related to climate and solar panel manufacturing. The ideal and actual energy that is supplied by a certain area of solar cells under ideal and actual conditions is measured to determine the defficiency between the two cases, then a new formula is applied to determine the exact substitutive area of solar cells that must be added to the initial area so as the energy supplied by the cells under actual conditions is the same as that under ideal conditions. A comparsion and evaluation of the results are made for multi inclinations according to the climatic data of Baghdad city.Constants are derived for Baghdad city which can be applied in designing PV systems to gel values of actual energy supplied by solar cells at any temperature are exactly the same as that calculated under ideal conditions at Tc=25 degrees c.