Reliability Comparison of National and International Data Sources for Power Sector Transition Assessment: The Case of Türkiye

International datasets are widely used to compare national energy transitions, yet their consistency with official national statistics is rarely tested. This study compares Turkish Electricity Transmission Corporation (TEİAŞ) statistics with IRENA capacity data and Ember generation data for Türkiye over 2000–2025, traces each series to its primary source and examines how discrepancies change indicators. IRENA and IEA reproduce TEİAŞ figures exactly, so their agreement reflects a common origin, not independent confirmation. Ember reproduces market data from EPİAŞ. From 2018 it reports less electricity than TEİAŞ, and the gap reached 13.1 TWh in 2025, concentrated in solar PV, natural gas and biomass. The solar component emerges from 2021, as self-consumption PV spread, because market data record only the grid injection of unlicensed plants. Share-based indicators shift by about one percentage point, whereas the 2025 solar capacity factor is 21.7% with TEİAŞ and 18.6% with Ember data. Metered injection of unlicensed PV and PVGIS yields are consistent with the TEİAŞ level, so the apparent decline in solar performance arises from mismatched boundaries. Türkiye belongs to the leading wind and solar convergence club, and reaching 120 GW by 2035 requires about 8 GW per year, with wind as the binding technology.