Lack of accurate methods for assessments of productive capacity of wind power plants is becoming a bottleneck in an increasingly commercialised wind power industry. The inherent components of performance assessment are identified, analysed and ways of minimising uncertainties of the components are investigated. The main components are identified as "site calibration", "wind turbine's sensitivity to flow variables", "plant blockage effects" and "uncertainty analysis". Site calibration is the action of estimating the flow variables at the wind turbine position with measurements of these quantities from another (reference) position. The purpose of the sensitivity analysis is to clarify which and how flow variables influence power output. Plant blockage effects refer to the power plant's influence on the reference measurements of flow variables. And finally, the components' uncertainties and in turn the integrated uncertainty on the average productive capacity of the wind power plant are investigated. It is found that uncertainties can be reduced 1) by including several more flow variables than hub height wind speed, 2) by carrying out site calibration with outmost care and by inclusion of more variables, 3) by taking plant blockage into consideration, 4) by aiming at "plant average" power instead of looking only at individual machines and possibly 5) by introduction of remote sensing anemometer techniques.