## 2 Methodology  
This scoping review was conducted in accordance with the PRISMA-ScR guidelines, which are particularly suited for capturing emerging and interdisciplinary domains such as digital twins in the energy sector. The primary aim was to systematically map the current landscape of regulatory, technological, and economic factors influencing the development of a pan-European electricity grid digital twin. A predefined protocol was followed that included the definition of review scope, identification and selection of sources, data charting, and synthesis of results into thematic categories. The review was guided by three research questions, derived from the conceptual focus established in the Introduction Section:  
1. What are the main regulatory and policy barriers that hinder the development and deployment of a pan-European digital twin of the electricity grid, and how can these be addressed? 2. What are the key technological challenges in building and operating a secure, real-time, and interoperable digital twin of the EU electricity grid? 3. What are the economic opportunities and investment rationales associated with the grid digital twin, and how can funding mechanisms and incentives support its implementation? Eligibility Criteria: To capture the multidisciplinary scope of the topic, the review included a broad range of sources. Eligible materials comprised peer-reviewed academic publications such as journal articles and conference proceedings, as well as official documents from the European Commission, ENTSO-E, the EU DSO Entity, and other institutional bodies. Grey literature, including regulatory proposals, strategic roadmaps, technical reports, industry white papers, and relevant standards, was also included. Only sources explicitly addressing digital twins in the context of the European electricity grid were selected. Documents focused on AI, IoT, or energy policy without relevance to digital twin applications were excluded. The inclusion period spanned from 2013 to 2025 in order to reflect the regulatory and technological evolution since the introduction of GDPR and the Digital Europe Program. Only English-language publications were considered. Information Sources and Search Strategy: A multi-pronged search strategy was adopted to ensure coverage of relevant academic, institutional, and policy sources. Academic databases including Scopus, Web of Science, and IEEE Xplore were queried using combinations of the following keywords: \"digital twin\" AND \"electricity grid\" OR \"power system\" AND \"Europe\" OR \"EU\" AND \"regulation\" OR \"policy\" OR \"data governance\" OR \"interoperability\" OR \"investment.\" In parallel, targeted searches of EUR-Lex, the European Commission's energy and digital policy portals, ENTSO-E, CEER, and the EU DSO Entity websites were conducted to retrieve official legislative texts, action plans, and strategy papers. Additionally, Horizon Europe and CORDIS project databases were reviewed to extract insights from funded initiatives such as TwinEU and InterConnect. Google-based searches refined with site-specific filters, e.g., site:europa.eu, were used to identify grey literature and industry white papers not indexed in academic databases. Source Selection: After removing duplicates, all sources were screened by title and abstract or executive summary according to the eligibility criteria. Records that were irrelevant, including those without a European focus or lacking relevance to digital twin technology in power grids, were excluded. The remaining full texts were reviewed for final inclusion. A PRISMA-style flow diagram (Figure 1) documents the selection process and summarizes the number of sources identified, screened, excluded, and included. Data Extraction and Synthesis: A structured data extraction form was used to chart key attributes from each included source.