The engineering flow models used to estimate annual energy production (AEP) in offshore wind farm layout optimization typically assume inflow homogeneity over the model domain. This assumption lies in contrast with observations of horizontal wind speed gradients in coastal regions where many offshore wind farms are being constructed. Accounting for wind speed gradients in wind farm models may lead to reduced uncertainty in AEP estimates and reduced bias in optimized wind farm layouts. This thesis examines whether accounting for horizontal wind speed gradients with WRF simulated wind resource inputs to engineering wake models impacts AEP prediction for a wind farm cluster in the Irish Sea by comparing results with calculation methods which assume homogeneous inflow. Analysis of a wake free two turbine case under a gradient shows that the assumption of homogeneity leads to errors with the true power which a gradient based method is able to predict. Despite this, results suggest that the overall impact of modelling wind speed gradients on AEP predictions in the Irish Sea cluster is small. Homogeneous and gradient methods using the same wind resource data predicted differences in AEP of between 0.1% and 0.75%, with most cases below 0.75%. Filtering by wind direction reveals AEP differences larger than the assumed wake model uncertainty for two sectors with inflow from land. The AEP contribution from sectors with mean wind speed gradients is limited by low frequencies and mean wind speeds. Additionally, positive and negative power differences predicted by homogeneous and gradient methods were found to balance over the year.