## 3 Results and Discussion
### Renewable Energy Oversupply  
The results presented in the previous sections have been obtained assuming a power system with a wind-to-solar capacity ratio of \(P_{\text{wind}}/P_{\text{solar}}=1.0\) by 2030, where the capacity of onshore wind and solar PV is 1500 MW each. Renewable energy targets for Croatia indicate a wind-to-solar capacity ratio of approximately 1.0 by 2030, with a declining trend thereafter as solar PV capacity is expected to outpace wind power. This capacity roughly matches the overnight baseload demand. In 2030, the baseload is estimated at 1465 MW, a slight increase over the baseload of 1446 MW recorded in 2024. In Croatia, sun and wind energy availability is such that solar PV achieves 1250 full load hours while onshore wind achieves 2500 full load hours on average per year. Thus, the annual electricity generation ratio would be \(E_{\text{wind}}/E_{\text{solar}}=2.0\) for a capacity ratio of \(P_{\text{wind}}/P_{\text{solar}}=1.0\).  
Generally, power systems can be designed with different wind-to-solar PV ratios. In cooling dominant regions, higher solar PV capacity supplies more electricity in the summer period. In heating-dominant climates, increased wind capacity generates more electricity in the winter months [61; 62]. The most recent ENTSO-E data for 2024 reveals that the \(E_{\text{wind}}/E_{\text{solar}}\) ratio is 2.9 in Portugal, 1.6 in Austria, 1.5 in Greece, 1.2 in Spain, and 0.8 in Italy. The corresponding wind-to-solar PV capacity ratios (\(P_{\text{wind}}/P_{\text{solar}}\)) are 2.0 in Portugal, 0.8 in Austria, 0.6 in Greece, 0.9 in Spain, and 0.3 in Italy. These ratios will continue to decrease in the next years as the installation rate of solar PV exceeds that of wind power.  
Renewable energy oversupply occurs when the power generation from renewable sources exceeds demand, leading to the need for export, storage, or curtailment to maintain grid stability. Renewable energy curtailment is the intentional reduction or shutdown of renewable electricity generation, often performed to maintain grid stability (technical curtailment) or to alleviate negative price scenarios (economic curtailment) [63]. In solar-heavy power systems, oversupply may occur during daytime hours of summer months, while in wind-heavy systems, oversupply may occur during nighttime hours of shoulder months when power demand is low.  
Redispatch and curtailment are not directly modeled in the present study; rather, renewable energy oversupply is evaluated. Data about technical and economic constraints which affect redispatch and curtailment are often proprietary and not publicly disclosed by Transmission System Operators.  
Figure 8 shows the percentage of renewable energy oversupply as function of the wind-to-solar PV capacity ratio. In a power system with 1500 MW of solar PV and 1500 MW of onshore wind, the renewable energy oversupply is 5.3% at the annual level. Typically, between 50 and 80% of renewable energy oversupply may be curtailed if no energy storage or export options are available [64; 65], which would return a curtailment rate between 2.7% and 4.2% in Croatia. Renewable oversupply is lowest in the summer (3.3%) and highest  
in the spring and autumn (7.0%). Renewable energy oversupply becomes particularly pronounced when solar PV capacity exceeds wind capacity (\(P_{\rm wind}/P_{\rm solar}<1.0\)) and when the wind capacity exceeds the baseload demand (\(P_{\rm wind}>1500\) MW).  
Countries with large shares of variable renewable generation face high percentages of renewable curtailment. Renewable curtailment can reach up to 12% in island systems such as Cyprus and Ireland, while it typically remains below 5% across the European continent [66; 67]. In 2023, Germany curtailed 19 TWh of renewable energy due to grid congestions and redispatch, representing 4% of the total annual electricity generation (482 TWh) [68]. Spain curtailed about 2% of the annual electricity generation in 2024, mostly in areas where the growth of solar PV outpaced the transmission capacity.