Electric and gas systems are two complementary, interconnected energy systems. Each of the systems has a different size and coverage. Their networks cover different areas, the structure of their customers varies across regions, and their presence varies in industry and households. The interrelatedness of these systems is both direct and indirect. The direct interrelatedness is visible in the tasks performed by the gas system in its function of a supplier for electricity production plants. Other than the mentioned direct connection, tasks performed by these two systems complement each other while supplying the consumers, for instance in tasks of heating, hot water preparation, and similar. However, there are also some specific features that create an impression that some systems' elements are separated. The indirect connection regards climate conditions. The aim of the paper is to analyse and present the impact which a temperature drop has on electric and gas system, as well as to analyse the interrelatedness of the two systems in extreme weather conditions taking Zagreb, Croatia as an example area. In winter the consumption of electrical energy and gas rises considerably. Any shortage leads to reactions. A threat to one system indirectly endangers the other. The paper tries to clearly present the systems' very important interrelatedness and interdependence in terms of higher demand for energygenerating products using the example of temperature drop. An immediate and considerable temperature drop provokes very similar responses from both systems. This phenomenon has not been recognized clearly enough by the professional community, and its consequences have not been fully considered.