Fraunhofer ISE and PTB reported less than 0.15% difference in maximum power (Pmpp) measurements in their December 2025 interlaboratory test
An analysis of more than 70,000 module measurements since 2012 shows a 1.4% median negative difference in 2025 between manufacturer nameplate ratings and independently measured values
The difference between the 2 laboratories has decreased over time, indicating consistent calibration alignment
Accurate, traceable measurement is central to evaluating PV module performance. With rising power classes, even small deviations between manufacturer declarations and measured output can influence energy yield and financial assumptions.
CalLab PV Modules, the accredited calibration laboratory for photovoltaic modules operated by the Fraunhofer Institute for Solar Energy Systems ISE in Germany, together with the Physikalisch-Technische Bundesanstalt (PTB), Germany’s national metrology institute, conducted their latest interlaboratory comparison in December 2025.
In this comparison, known as a round-robin, identical photovoltaic modules were measured independently by each institute under its respective measurement conditions. The results are then evaluated against a common reference to assess consistency and calibration alignment. The average deviation between the 2 institutions in determining maximum power (Pmpp) was below 0.15%.
Such interlaboratory comparisons have been conducted regularly for many years at both the solar cell and module levels. According to the institutes, the relative deviation between their measurement results has steadily decreased over time, indicating stable calibration chains and controlled uncertainty budgets. The common reference is the mean of both laboratories’ results, which are considered equivalent within defined uncertainty limits.
While the agreement between the 2 calibration laboratories remains narrow, Fraunhofer ISE reports a widening discrepancy between manufacturer nameplate ratings and independently measured values. Based on an evaluation of more than 70,000 PV module power measurements conducted since 2012, the median negative deviation between manufacturer specifications and laboratory measurements reached approximately 1.4% in 2025. The institute notes that this divergence has increased gradually since 2017.
Martin Kaiser, head of CalLab PV Modules at Fraunhofer ISE, stated that maintaining a stable and traceable calibration level is essential for reliable module quality assessment and for evaluating the power and energy yield of procured modules. He added that the annual comparison with PTB serves as an important indicator of calibration consistency.