42 The IEC 61400 series of standards addresses wind energy generation technical requirements up to 43 the point of interconnection with the utility grid system. Part 12 of the IEC 61400 series of standards 44 comprises a sub-set of standards which are to be used in the evaluation and measurement of the 45 power performance characteristics of wind turbines. The power performance characterisation of wind 46 turbines of all types and sizes is covered. 47 Wind turbine power performance characteristics are determined from a measured power curve and 48 an associated estimated annual energy production (AEP) and its uncertainty. The measured power 49 curve, defined as the relationship between the wind speed and the wind turbine power output, is 50 determined by collecting simultaneous measurements of meteorological variables (including wind 51 speed), as well as wind turbine signals (including power output) at the test site for a period that is 52 long enough to establish a statistically significant database over a range of wind speeds and under 53 varying wind and atmospheric conditions. The AEP is calculated by applying the measured power 54 curve to reference wind speed frequency distributions, assuming 100 % availability. 55 Part 12-0 provides a general introduction to the available options for power performance 56 measurement and the contributing evaluations which are further detailed in the other parts of the IEC 57 61400-12 series. Although the -12 series also defines the specifications of the meteorological 58 variables (and in particular wind speed) required for the power performance evaluation, the methods 59 and procedures for measuring or otherwise acquiring the wind speed data are defined in the IEC 60 61400-50 wind measurement series of standards. 61 The evaluation of the wind turbine power performance characteristic according to this series of 62 standards requires the measured power curve and derived energy production figures to be 63 supplemented by an assessment of uncertainty sources and their combined effects. The basis of the 64 uncertainty assessment is ISO/IEC Guide 98-3. The wind measurement uncertainty sources shall be 65 identified and quantified from procedures described in the relevant wind measurement standards 66 contained in the IEC 61400-50 series. The wind measurement uncertainties shall be propagated 67 through to and combined with the other sources of uncertainty in the power curve and annual energy 68 production using methods and assumptions described in the IEC 61400-12 series of standards.