The purpose of this study is to optimize the electrical energy cost (EC) of air conditioning (AC) with consideration of different wall characteristics. The type of wall characteristics is based on the varying composition of the wall. For example: Styrofoam, soil, calcium carbonate and iron. There are three kinds of wall that have been evaluated to design an economical building. To fulfill the goal of this research, a cooling model of building has been designed to optimize the electrical EC of the AC considering to temperature room. Under numerical optimization the electrical EC can be computed by considering thermal conductivity (K) and outside temperature (Tout). Consequently, the electrical EC for Building-1, Building-2, and Building-3 can be reduced to IDR 25,000, IDR 23,000, and IDR 20,000, respectively. In addition, this model is tested considering the value of Tout in Barru South Sulawesi-Indonesia during the entire day of July 17th, 2021 in dry session with geographical coordinates are 4° 24′ 20″ South, 119° 36’ 23” East and Latitude −4.436417.