A wealth of data on wind plant operation, from lightning strikes to operation and maintenance costs to wind power variability and carbon emission savings, is contained in the latest annual report of wind energy performance published by Germany's institute for solar energy research, ISET. In Germany the risk of wind turbines being struck by lightning has remained constant since 1992 at between four and 13 strikes per 100 operational years, with strikes in mountain regions likely to occur about twice as often as those in coastal or lowland regions. Operation and maintenance costs tend to vary with size and the report suggests that large machines with rated capacities of 1.3-2 MW are the cheapest to run. New data on wind power fluctuations indicate that Germany's wind power fleet provides a more stable supply than generally believed. The maximum recorded power swing over a 15 minute interval in 2005 was 9% of the rated power; the maximum swing over one hour was 20% of the rated power. But these extreme excursions only occur a few times a year. About 30% of the time the intra-hour fluctuations are very small. The accuracy of wind power forecasting is increasing: the average error in the 72 hour forecasts in 2005 was 6% of the installed capacity. Even in a year when winds were low, with 2005 returning just 89% of the energy content of a statistically "normal" year, Germany's wind turbines enabled savings of carbon dioxide emissions of around 23 million tonnes, based on a saving per kWh of 0.86 kilogram because wind replaces the use of "marginal" plant, which in Germany, as in many other places, is coal. Marginal plant are operated to manage the balance between supply and demand.