## 4 Conclusions  
This study examined the deviation between day-ahead and intraday PV power generation forecasts compared to the real production in connection with a 1000-MWp PV system. It also analyzed and presented the lithium-ion based and vanadium redox flow battery storage technologies in connection with the photovoltaic grid balancing reduction potential. The aim was to determine the photovoltaic balancing requirement through real data relative to the day-ahead and intraday forecasts. Another goal was also to establish the photovoltaic grid balancing reduction potentials of lithium-ion based and vanadium redox flow battery storage systems. The results made it possible to present the occurrence of the balancing power and energy divergence magnitudes and their percentage distributions in the examined 5-year period. Overall, this method showed that the accuracy of PV power and energy  
Figure 14: The relationship of negative and positive TSO regulation needs in the case of day-ahead and most recent forecast with different VRFB energy storage capacities related to a 1000 MWp PV system for the period of 30/09/2014-30/09/2019. Figure 13: The relationship of negative and positive TSO regulation needs in the case of day-ahead and most recent forecast with different Li-ion energy storage capacities related to a 1000 MWp PV system for the period of 30/09/2014-30/09/2019. forecasting has improved with the usage of the most recent (intraday) forecast method. The best forecast reduced the MW size of regulation compared to the day-ahead data. In summary, the analytical results indicate the probability of balancing power and energy divergence magnitudes and their percentage distributions in the examined 5-year period. The simulation results showed the grid balancing reduction potentials related to the energy storage value of 10 to 10,000 MWh. It was found that the battery technology and its energy storage capacity have a significant impact on the regulation needs of TSOs. Generally, the frequency of need for positive regulation by the TSO was higher than that of the negative ones, thus, ensuring the positive value is a greater challenge. The main reason for this was the energy storage round-trip efficiency phenomenon and to a lesser extent the characteristics of the battery charge status. For example, the exponential relationship between TSO regulation need and energy storage capacity provides information for investment decisions. This paper gives a comprehensive review on the energy issue. The results of this research refer to the Belgian TSO; however, the limitations of PV grid integration are well demonstrated. The developed simulation procedure allows the method to be adapted to any country; therefore, our goal is to develop this concept into an easy-to-use application to help the work of TSOs facing the challenge of PV integration. [PERSON] was mainly responsible for the technical, experimental, and modeling aspects; [PERSON] also conceived and designed the manuscript. All authors contributed equally in the analysis of the data and the writing and revision of the manuscript. All authors have read and agreed to the published version of the manuscript. The publication is supported by the EFOP-3.6.3-VEKOP-16-2017-00008 project. The project is co-financed by the European Union and the European Social Fund. The authors have worked together with the KRAFT Social Innovation Lab of the Institute of Advanced Studies Koszeg (IASK). The authors declare no conflict of interest. The following abbreviations are used in this manuscript: