## 6 Discussion  
The results presented in this research paper confirm that the increasing integration of renewable energy sources (RES) into European electricity systems is fundamentally changing the dynamics of wholesale electricity prices. The verification of Hypothesis H1 showed that higher hourly generation from solar and wind sources leads to a systematic decrease in the market price, while at the same time increasing short-term volatility. This relationship was most pronounced in countries with high levels of variable generation, such as Germany and Estonia, while it was less pronounced in countries with lower RES penetration. The year-on-year price spread analysis (Hypothesis H2) revealed a significant increase in volatility after 2021, with a spike in peak prices and extreme daily differences in 2022. These changes cannot be explained only by the increase in RES, but also by the effect of external fundamentals, particularly geopolitical crises and gas market tensions. The results show that while variable RES generation contributes to increased price volatility, the occurrence of extreme price extremes (year 2022) is primarily influenced by a combination of external factors such as geopolitical events and the like. Regression analyses performed separately for each year (Hypothesis H3) showed that the predictive power of the relationship between RES generation and price is not stable. In years marked by crises, the relationship between RES generation and electricity price has been shown to be stronger and more predictable, as confirmed by the higher values of the R\({}^{2}\) coefficient. On the contrary, during stable years, the predictive accuracy of the regression models was lower, indicating that this relationship changes over time, depending on market conditions. The correlation analysis (Hypothesis H4) revealed significant regional differences in the interaction between RES, residual load, and electricity price. Stronger negative correlations between RES generation and price were observed, especially in Germany and France, while in countries with less market flexibility, these links were weaker. The observed differences point to the importance of the regulatory environment, the availability of storage capacity, and the ability of the system to absorb variable generation. An analysis of the incidence of negative electricity prices as a function of the level of generation from solar and wind resources confirmed the existence of repeatable generation thresholds, beyond which the probability of negative market prices increases significantly. This relationship was most pronounced in countries such as Germany and France, where the proportion of negative values increased sharply at high combined RES production volumes. The results show that the occurrence of extremely low prices is not random but related to a clearly quantifiable dominance of renewable generation over consumption. This empirical dependence allows the formulation of practically applicable decision rules for market operators and producers. The results therefore support the validity of Hypothesis H5. The observed non-linear relationship between renewable energy generation and the occurrence of negative electricity prices is the result of a combination of technical, market, and regulatory factors. In lower output ranges, the system typically has sufficient tools to absorb the generated energy--whether through dispatchable generation, exports to neighbouring grids, or limited demand-side flexibility. However, once a certain production threshold is exceeded (e.g., approximately 65,000 MW in Germany), these balancing mechanisms gradually become saturated. From a technical perspective, conventional power plants (e.g., nuclear or coal-fired) often have limited ramping capabilities and high costs associated with shutdown and restart operations. As a result, they remain in operation even during periods of electricity surplus.