An interlaboratory comparison between the CalLab PV Modules of the Fraunhofer Institute for Solar Energy Systems ISE and Germany’s national metrology institute, the Physikalisch-Technische Bundesanstalt (PTB), has confirmed the highest level of precision in photovoltaic module power measurements. The latest results show that the two organizations deviate on average by less than 0.15% from the reference value.

Regular comparisons between independent calibration laboratories are considered an essential element of quality assurance, ensuring that the PV industry can rely on independent and traceable measurement results.
At the same time, discrepancies between manufacturers’ nameplate power ratings and independent laboratory measurements have continued to widen in recent years. In 2025, the median deviation reached around 1.4%. Since 2017, the gap between the power specifications of many PV module manufacturers and the measurement results obtained by Fraunhofer ISE has steadily increased, reaching a negative median deviation of 1.4% last year.
The findings are based on an evaluation of more than 70,000 power measurements conducted on photovoltaic modules at Fraunhofer ISE’s calibration laboratory, CalLab PV Modules, since 2012.
“Engineering, procurement and construction (EPC) service providers in particular depend on highly precise, independent power measurements when selecting PV modules, not least to safeguard their investments,” said Martin Kaiser, Head of CalLab PV Modules at Fraunhofer ISE. “A stable, traceable calibration level at our accredited laboratory forms the absolute basis for reliably assessing module quality as well as the performance and energy efficiency of purchased modules. The annual round robin comparisons with PTB are one of the most important indicators for us to ensure we are on the right track.”
In the most recent comparison, conducted in December 2025, PTB and CalLab PV Modules once again demonstrated a deviation of less than 0.15% in the determined maximum power (Pmpp).

Regular round robin tests are regarded as one of the most important proofs of quality assurance for accredited testing and calibration laboratories. For many years, PTB and Fraunhofer ISE have therefore carried out continuous measurement campaigns at both solar cell and PV module level to secure quality standards and verify calibration levels. For these comparisons, both institutions independently measure the same PV modules and solar cells under various test conditions and subsequently align their results.
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