Accurate production planning in both the short and long term is very important in cogeneration plants. Especially if the cogeneration unit operates under free electricity market conditions, which complicates the decision-making process as an additional planning condition with variable heat, fuel and CO2 costs. On the other hand, when a cogeneration plant uses a heat accumulation system, it is impossible to make a production decision without using a computer system; the human factor in decision-making can lead to erroneous decisions without traceability. The role of modern computer systems is growing and greatly influences the optimal production planning process in cogeneration plants, regardless of the installed capacity and in the operation with heat accumulation. One of the problems solved by the research is the integration of real operating modes and conditions (applied thermal insulation solution) into the algorithm. The developed methodology allows not only to plan the operating modes of the cogeneration plant, but also to evaluate the efficiency of the battery solution. This study shows the developed methodology for calculating heat loss for a heat accumulator depending on the operating mode and on the need to introduce a correction coefficient. When determining the total influencing expenses of the cost model of the heat accumulator operation mode, their mutual influence is shown and integrated into the decision-making algorithm for the next day's free market conditions in the electricity market computer program tool. The aim of the algorithm is maximally increase the total gross revenue threshold for the planning of cogeneration operations and to exclude operating modes that may cause losses.