The invention relates to a method of calculating an electrical output of a wind power plant comprising a plurality of wind turbines. Instead of calculating the electrical output of the wind power plant as a simple aggregation of the outputs of each wind turbine, the method takes into account parameters which may vary stochastically throughout the wind power plant, e.g. pitch angle, stiffness in drive train, different possible production with regard to reactive and active effect, mechanical component properties as well as variability in relation to communication times between a power plant controller and the individual wind turbines. The method proposes to make adjustment to a simple aggregation calculation method based on analysis of such stochastic varying parameters.