Policymakers and regulators increasingly need to balance the requirements of energy demand, security and costs, and to do so they need accurate estimates of performance. This study uses public data to assess United Kingdom offshore wind farm performance to improve policy decisions. We find that average capacity factors have improved from 34.9% in Round 1 (2001) to 41.0% in Round 2 (2002) projects. Drivers and correlations of performance for United Kingdom offshore wind farms are analysed. The research also addresses problems with existing capacity factor estimates and generates new estimates of inter-year variability of capacity factors, providing a more accurate evidence base for policy and regulatory decisions.