When processing organic waste into biogas, it is necessary to take into account the design features of the equipment, the composition of the raw materials and the technological modes of its fermentation. If there is a lack of heat produced during the chemical fermentation reaction, additional (third-party) heat sources are used to ensure the technological regime. It can be various heat exchangers or electric heaters. The article presents the results of theoretical studies evaluating the power value of additional (third-party) heat sources for heating raw materials in a biogas reactor. The analytical dependences for estimating the power of external heat sources for the considered physical model were obtained on the basis of solving the Fourier heat equation in layered media. The analysis of the influence of design parameters and thermophysical characteristics on the amount of required power of additional (third-party) heat sources, as well as on the distribution of the temperature field in a biogas reactor, is carried out.