64 This part of IEC 61400 specifies a procedure for measuring the power performance 65 characteristics of a single wind turbine and applies to the testing of wind turbines of all types 66 and sizes connected to the electrical power network. In addition, this standard describes a 67 procedure to be used to determine the power performance characteristics of small wind turbines 68 (as defined in IEC 61400-2) when connected to either the electric power network or a battery 69 bank. The procedure can be used for performance evaluation of specific wind turbines at 70 specific locations, but equally the methodology can be used to make generic comparisons 71 between different wind turbine models or different wind turbine settings when site-specific 72 conditions and data filtering influences are taken into account. 73 The wind turbine power performance characteristics are determined by the measured power 74 curve and the estimated annual energy production (AEP). The measured power curve, defined 75 as the relationship between the wind speed and the wind turbine power output, is determined 76 by collecting simultaneous measurements of meteorological variables (including wind speed), 77 as well as wind turbine signals (including power output) at the test site for a period that is long 78 enough to establish a statistically significant database over a range of wind speeds and under 79 varying wind and atmospheric conditions. The AEP is calculated by applying the measured 80 power curve to reference wind speed frequency distributions, assuming 100 % availability. 81 The standard describes a measurement methodology that requires the measured power curve 82 and derived energy production figures to be supplemented by an assessment of uncertainty 83 sources and their combined effects. Uncertainty sources of wind measurements shall be 84 achieved from procedures described in the relevant wind measurement equipment standards 85 while uncertainty of the power curve and annual energy production shall be achieved by 86 procedures in this standard.