February 8, 2005

Annual average output only one third of the time

This graph is from "Wind Report 2004," by Eon Netz, the grid operator for a third of Germany. It shows how many quarter-hours of the year the total infeed from the 5,900 MW of installed wind capacity on their system was at or above a certain amount.

For example, the highlighted horizontal rule indicates the average infeed over the whole year. It intersects the curve at about 12,500 quarter-hours, meaning that the total infeed from wind was equal to or more than the annual average for only 12,500 out of 35,040 quarter-hours, or about one third of the time.

Because wind turbine generation falls off logarithmically when the wind speed is below the ideal 30 mph or so, and the turbines have to be shut down when the wind is too fast, I would guess that this experience would also apply in regions showing better performance than the Eon Netz region's annual average of one sixth of capacity. That is, even as the average annual infeed approaches a third of capacity, and much of the curve shifts upward, it would be at or above the annual average still only a third of the time.