The use of terrestrial heat could fulfill certain energy demands. For the evaluation of the possibilities in the energy system of Germany, the available potentials are one of the most important criteria. Within this context, in the past only the available resources and the reserves have been discussed. But these data have hardly practical relevance for the energy-political and energy-economic discussions, since they do not consider the conditions in the corresponding energy system. In this article, therefore, explanations additional to the overall existing amount of heat in underground portions are shown, which could used be also from a technical point of view. In addition, the demand-side restrictions are considered and the share of the available heat is pointed out which can be used within the energy system of Germany. The explanations show that consideration of the demand side is necessary to make relevant statements. The investigations also make clear that terrestrial heat will never be able to cover more than just a part of the energy demand for warmth in Germany.