Wind direction uncertainty is often overlooked in wind resource assessment, despite its potential to cause significant deviations in annual energy production (AEP). This study quantifies directional uncertainty arising solely from wind data, decomposed into three components: measurement uncertainty, long-term wind rose difference (LTWRD), and interannual variability (IAV). Two offshore sites with contrasting layouts and inflow regimes were analysed using statistical and resampling-based approaches.

The results show total directional uncertainties of 0.71% and 4.39% of AEP, with LTWRD emerging as the dominant contributor at the more directionally sensitive site. No consistent pattern in the composition of directional uncertainty was observed across sites, highlighting the need for site-specific assessments.

Given that typical total AEP uncertainties for wind projects average 8–10%, the findings indicate that directional uncertainty can represent a substantial fraction of the total, especially at sites with strong wake sensitivity and unidirectional inflow. Therefore, wind direction should be explicitly included in uncertainty analyses, and long-term correction procedures should address wind direction as well as wind speed.