Renewable Expansion and the Transformation of European Electricity Systems: Hourly Evidence and a 2030 Outlook

European electricity systems are transforming as renewable generation expands across national markets with different generation portfolios and operating conditions. This study assesses 25 European electricity systems over 2015–2025 using harmonised hourly ENTSO-E generation and load data, day-ahead electricity prices, and population indicators. The framework integrates hourly fixed-effects models, temporal renewable-penetration measures, population-normalised indicators, structural clustering, and a conditional outlook to 2030. Higher solar and wind generation is associated with lower contemporaneous fossil generation, but the magnitude varies across countries and should not be interpreted as a physical substitution rate or causal effect. Renewable penetration increased substantially, while higher variable-renewable shares were associated with a greater incidence of very-low and negative day-ahead prices. Population-normalised and clustering results indicate a broad transition toward higher renewable and lower fossil generation without convergence to a single European system configuration. A backtested multi-model ensemble extends the historical evidence to 2030 with uncertainty intervals and external scenario benchmarking. The contribution is integrative rather than methodological: operational, temporal, market, and structural dimensions are evaluated jointly to assess the consistency and heterogeneity of the European electricity transition.