Accurate preconstruction energy production estimates are vital for maintaining confidence in financial assessments of prospective wind projects. While uncertainty is always present, and it is normal to observe errors at individual projects, it is important that the average error, or bias, of preconstruction estimates for a representative sample of projects remain within an acceptable margin from zero. UL regularly verifies the accuracy of its energy production estimation methods against actual plant performance and assesses the causes of any discrepancies observed; the result is called a back cast study. Interim results from the most recent such study for European projects, which is ongoing, will be presented, with discussion of implications for UL's energyestimation methods.