As a result of the lack of flexible capacities and an increase in the fraction of nuclear power plants in European Russia, the combined heat and power (CHP) plants currently need offloading during the off-peak hours of electric power demand. The adjustment of electric power demand necessitates additionally increasing the electric power of the power-generating units in the on-peak hours and decreasing it at nighttime. The existing methods for increasing the power of combined heat and power plants suffer from a number of serious drawbacks. The reduction in the electric power in the heating period is restricted, as a rule, by high heat demand. In this work, we show the possibility of extending the control range of the electric power output at CHP plants through the use of the heat-accumulation properties of the heat-supply networks and buildings and, accordingly, through the changeover of the operating modes of the cogeneration plant. Several operating-mode variants of the cogeneration plant are considered. The calculations conducted show that the consideration of the heat-accumulation properties of the heat-supply networks and buildings allows, having reduced the electric output of the CHP plant at nighttime, for an increase in the thermal output to increase the electric output in the day reducing the thermal output. An important requirement for the heat supply to buildings is the maintenance of comfortable indoor conditions. The optimal permissible temperature range for living space in the cold period of the year is 20–22°C. The possibility of ensuring the comfortable conditions indoors is confirmed by the calculations of the indoor air temperature by the method previously proposed by the authors that considers the heat-accumulation properties of the heat-supply networks and buildings. The indoor air temperature decreases by the end of the discharge period, i.e., the time when the accumulated heat has been consumed, to approximately 19°C; consequently, it does not go down below the permissible range. It should be noted that this is predominantly due to the heat-accumulation properties of the buildings. The material of the main heating system pipelines has a low accumulation capacity, i.e., the pipe walls transfer heat rapidly to the water that enters the pipeline at a temperature lower than that of the pipes. However, due to the considerable length of the heat system’s pipelines, the temperature of the heating-system water that arrives at the consumer remains constant at the time moment after the temperature perturbance on the heat source.