Sources of uncertainty in measuring direct and global solar radiation are defined in ISO 9060:1990.But what do these mean,and how do they affect the readings obtained in the field?How do these factors impact on the calculations of PV plant efficiency and performance ratios,and how can the uncertainties be minimised in practice?Calculations relating available solar energy to the power generated are becoming increasingly important in determining the return on investment of a solar energy plant.It makes a substantial difference whether the uncertainty in the recorded daily irradiance totals is 2% or 5% Even the best pyranometers cannot provide good quality data if they are not correctly installed,maintained and calibrated.However,often the performance of the radiometer is limited by the characteristics of the plant data acquisition system.This presentation covers the issues affecting pyranometer measurements of Global Horizontal Irradiance (GHI) and Plane Of Array(POA) Irradiance.The topics discussed include:pyranometer classifications,installation and cabling,maintenance,calibration,ventilation units,data acquisition and processing.With the aid of this presentation,and a calculation tool,the user can make a good estimate of the expected generic and site-specific overall measurement uncertainty.The author has over 15 years of practical experience in the fields of solar radiation measurement for meteorology,climatology and solar energy.