District heating (DH) systems with the thermal energy storage (TES) have become widespread and heat accumulation tanks are installed in most cogeneration heating plants. A heat load variation is one of the main parameters that need to be taken into account when planning thermal energy storage. Both seasonal and daily heat load variations need to be analysed in order to determine the most suitable capacity of the accumulation system. The main aim of the article is to determine the necessary capacity of the thermal energy storage in order to level the heat load variation in different seasons. Two different methods have been used to analyse the heat load variation. The relative daily variation of heat supply for the particular DH system ranges from 1.66 to 2.06%. The largest daily heat load variation reaches 29% from the daily average heat supply. Results show that it would be necessary to install TES with the capacity of around 5500 MWh to prevent weekly heat load variations and 634 MWh for daily heat load variations in the winter period. The necessary capacity of TES for the summer period is 80 MWh for daily heat load and 600 MWh for weekly heat load levelling.