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This article sheds new light on photovoltaic (PV) module rating according to predicted yield rather than power measured at standard testing conditions (STC). We calculate module performance ratios (MPR) for measured characteristics of eight different module types and compare them with a reference MPR calculated with typical crystalline silicon characteristics. In place of the not yet existing standardized weather data, we use commercially available weather data for three different locations. The reference MPR for the three locations were 95.5%, 94.6%, and 91.0%, respectively, with differences to the other module types of ±8% at maximum. MPR was calculated with reference to nominal power, and-following IEC 61853 -without consideration of potential degradation. The strongest contribution to the initial differences between the module types was due to differences in irradiance dependency. Standard uncertainties for all initial MPR values were calculated and range from 1.8% to 3.0%, including STC power uncertainty. We propose a module rating method that indicates whether a module type's performance is significantly above, below, or essentially equal to the reference. The method evaluates the MPR difference between module type and reference, taking uncertainty into account. Significant differences were only found between modules with obviously different characteristics, but not between the crystalline silicon module types under scrutiny. As the uncertainty analysis did not cover degradation and influences due to the use of not standardized weather data, a sensitivity analysis was performed. Long-term degradation can change the comparative energy rating significantly, whereas the selection of tilt angle and assumptions regarding module operating temperature did not have a strong effect.

This article presents recent progress in reducing the measurement uncertainty for crystalline silicon (c-Si) and thin film PV modules. It describes the measurement procedure and the uncertainty analysis as applied in CalLab PV Modules, Fraunhofer ISE's laboratory for module measurements. The uncertainty analysis covers the complete calibration process in detail, including measurements, correction to STC, and determination of electrical module parameters (I SC , P MPP , V OC etc.) from the I-V curve. Differences between c-Si and thin film modules are addressed, most importantly in terms of spectral mismatch factor and short timescale stability problems. The paper outlines the importance of a comprehensive quality assurance system in a calibration laboratory as a prerequisite for accurate measurements on a daily basis. Particular attention is paid to results from a series of measurements taken every three weeks over a 3 year period, conducted as part of the quality assurance system. In conclusion, this article introduces a bestcase uncertainty for c-Si module calibration of 1.6% for P MPP and 1.3% for I SC . This represents the lowest reported uncertainty for full size module calibration in a laboratory so far. The presented uncertainty in P MPP of cadmium telluride and single junction amorphous silicon modules is 2.9%, and 1.8% respectively. All mentioned uncertainties are expanded uncertainties (k=2).

Measurement results from a worldwide intercomparison of photovoltaic module calibrations are presented. Four photovoltaic reference laboratories in the USA, Japan and Europe with different traceability chains, measurement equipment and procedures, and uncertainty estimation concepts, participated. Seven photovoltaic modules of different technologies were measured (standard and high-efficiency crystalline silicon, cadmium telluride, single and double-junction amorphous and micromorph silicon). The measurement results from all laboratories and for all devices agreed well. Maximum power for the crystalline silicon samples was within ±1.3% for all thin-film modules roughly within ±3%, which is an improvement compared to past intercomparisons. The agreement between the results was evaluated using a weighted mean as a reference value, which considers results-specific uncertainty, instead of the widely used unweighted arithmetic mean. A further statistical analysis of all deviations between results and the corresponding reference mean showed that the uncertainties estimated by the participating laboratories were realistic, with a slight tendency towards being too conservative. The observed deviations of results from the reference mean concerned mainly short-circuit current and fill factor. Module stability was monitored through repeated measurements at Fraunhofer ISE before and after measurements at each of the other participating laboratories. Based on these re-measurements, stability problems that occurred for some thin-film modules and influenced the results were analyzed and explained in detail.

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