The essential aim of this study is to estimate the daily utilizability and useful energy of a flat-plate solar collector in Algeria for different climatic zones. For this purpose, a program based on Klein’s method was developed to estimate the daily utilizability and the useful energy characterized by the optical efficiency and the global loss coefficient that are equal to 0.72 and 7.9 W/m2K, respectively. The seasonal potentials of the whole studied zones are represented by geographical maps; then, a parametric study is performed to see the influence of the inlet temperature of the heat transfer fluid, the optical efficiency, and the global coefficient losses on the average utilizability per month for each site. Subsequently, the utilizability variations considering different sites and during various months are analyzed in terms of the average temperature of the heat transfer fluid, the optical efficiency, and the coefficient of global losses. The results show that the usable potential produced by this type of solar collector is 1500–1700 MJ/m2 in climatic zones E5 (Tanegrouft), E4 (Sahara), and E3 (Pre-Sahara) and is 1300 MJ/m2 in climatic zones E2 (Highlands) and E1 (Coastal) during the summer season. While for the winter season, the usable potential is 500–700 MJ/m2 in climatic zones E1 and E2, and 700–900 MJ/m2 in E3, E4, and E5, which can be considered the most favorable zones for the conversion of the available solar energy into thermal one through the flat-plate solar collector. The study of the effect on the system performance considering usability has shown that the effects of the overall loss coefficient are more significant compared to optical efficiency.