Temperature is an important factor that may affect the performance of anaerobic digestion. Therefore, biogas plants without heating system work only in warmer regions for the whole year. In regions with extreme temperature variations, for instance in Turkey, the biogas plant should be built with heating system. One of the methods is to use solar energy to increase the reactor temperature. In this study, solar heated biogas plants were reviewed. Furthermore, the optimization of insulation thicknesses and solar energy systems for 5 m(3) biogas reactor were carried out for two different cities for three different climatic zones in Turkey. Based on the obtained results, the ratio of annually produced biogas used for reactor heating was calculated for each city, with and without solar heating system. Obtained results indicate that the biogas consumption for reactor heating is decreased by approximately 19% for average of six cities when solar heating system is used. This means that available biogas potential would be increased.