For two decades, the capacity factor of wind power measuring the mean energy delivered by wind turbines has been assumed at 35% of the name plate capacity. Yet, the mean realized value for Europe over the last five years is closer to 21% thus making levelized cost 66% higher than previously thought. We document this discrepancy and offer rationalizations, emphasizing the long term variations of wind speeds. We conclude with the consequences of the capacity factor miscalculation and some policy recommendations. Click on the link below to download the full document.