## 2 Methods  
Electricity generation and power load data was retrieved from the European Network of Transmission System Operators for Electricity (ENTSO-E) database [37], which is also available on the website Energy Charts [38]. The power load and electricity generation data refers to net values. Net electricity generation is the difference between gross electricity generation and self-consumption. Net electricity is the power traded at the electricity exchange and fed to the public grid. Self-consumption includes the electricity that is consumed at the site of production. In a thermal power plant, this is the electricity necessary to run feedwater pumps, mills, cooling towers, and other auxiliary systems. In residential buildings, this is the electricity consumed directly at the site which does not enter the transmission system. Solar PV electricity generated by industry, commercial enterprises, and private owners, which is used for self-consumption, is not included in the net electricity and thus not shown in the ENTSO-E data [39]. In Croatia, the total installed solar PV capacity at the end of 2024 was 878 MW [9, 10]. Out of this total capacity, 776 MW are building-integrated systems generating for self-consumption and feeding surplus to the distribution grid [11], while 102 MW is the cumulative capacity from six utility-scale solar PV power plants feeding electricity directly to the transmission grid [12]. Hence, ENTSO-E data reports all utility-scale solar PV genera tion available to the public grid and the surplus solar electricity from building-integrated systems. On the other hand, all onshore wind power plants are directly connected to the transmission grid, and all are represented in the ENTSO-E data. Power plants generating from renewable energy sources (wind, solar PV, and hydro) achieve marginal costs near zero and are the first to be brought online. The net residual load is met by dispatchable energy sources (nuclear, reservoir hydro, and fossil thermal). If dispatchable sources cannot meet the entire load, electricity is imported. The net residual load (\(P_{\text{residual}}\)) is the difference between the net public load (\(P_{\text{load}}\)) and net VRE capacity (\(P_{\text{VRE}}\)), as follows:  
\[P_{\text{residual}}=P_{\text{load}}-P_{\text{VRE}} \tag{1}\]  
Ideally, the residual load would be perfectly met by dispatchable energy sources, and the exchange would be zero at any moment. In reality, when dispatchable sources are not enough to meet the load demand, electricity must be imported. Oversupply occurs when dispatchable and VRE generation is larger than the load demand. In that case, oversupply electricity is exported or stored. Renewable energy curtailment is used to maintain grid stability and occurs when further energy export or storage is not possible. The net VRE capacity (\(P_{\text{VRE}}\)) includes the net generation capacity from solar PV (\(P_{\text{PV}}\)) and wind power plants (\(P_{\text{wind}}\)), as follows:  
\[P_{\text{VRE}}=P_{\text{PV}}+P_{\text{wind}} \tag{2}\]  
The design of the European electricity market is based on the principle of merit order, which determines the order in which power plants are brought online [40]. Different energy sources have different short-run marginal costs (\(MC_{\text{i}}\) in \(\epsilon\)/MWh), which have to be paid to produce an additional unit of electricity (MWh). Marginal costs include variable running costs and should not be confused with the LCOE, which includes total unit costs over the power plant lifetime. In each trading period, an \(i\)-th power plant offers electricity at the individual \(MC_{\text{i}}\) to cover its production costs, but the final price of electricity will be equal to the most expensive energy source brought online. This price is known as the spot price or system marginal price.