Calibration and Measurement Uncertainty Estimation of Radiometric Data

Evaluating the performance of photovoltaic cells, modules, and arrays that form large solar deployments relies on accurate measurements of the available solar resource. Therefore, determining the accuracy of these solar radiation measurements provides a better understanding of investment risks. This becomes especially important as deployment size and investment costs grow to hundreds of millions of dollars. The accuracy of measurements is also important for acceptance testing and operations. Radiometers such as pyranometers are used to measure global horizontal irradiance or plane of array irradiance, whereas pyrheliometers are used to measure direct normal irradiance. Currently, most radiometric data users rely on manufacturers' specifications of calibration uncertainty to quantify the uncertainty of measurements. However, the accuracy of solar radiation measured by radiometers depends not only on the specifications of the instrument but also on the (a) calibration procedure, (b) measurement conditions and maintenance, and (c) environmental conditions. Therefore, statements about the overall measurement uncertainty can be made only on an individual basis, taking all relevant factors into account. This paper provides guidelines and recommended procedures for estimating the uncertainty in calibrations and measurements by radiometers using methods that follow the Guide to the Expression of Uncertainty (GUM). Standardized analysis based on these procedures ensures that the uncertainty quoted is well documented.