Performance Ratio (PR) is widely used to assess PV plant performance, construction quality, degradation and financial value. It can also form part of contractual guarantees. But the reliability of PR depends directly on the accuracy of the measurements behind it.
In research carried out with Delft University of Technology, Hukx examined how pyranometer measurement uncertainty affects PR calculations. Lead researcher Matthijs Ates argues that seemingly small differences in measurement accuracy can translate into significant operational and financial consequences.
Why PR uncertainty creates financial risk
PR calculations depend on measured solar irradiance, power output and module temperature. Uncertainty in any of these measurements feeds into the final result.
Ates gives a simple example: if a plant records a PR of 90 per cent with uncertainty of ±5 per cent, its actual PR could realistically lie between 85 per cent and 95 per cent.
This matters when engineering, procurement and construction (EPC) contracts include PR guarantees. Hukx’s research found that the Value at Risk associated with performance-related liquidated damages increases linearly with PR measurement uncertainty.
Lower accuracy can therefore increase the likelihood of both ‘false acceptance’ and ‘false rejection’ during performance testing.
Going beyond the Class A baseline
The study compared annual PR measurement uncertainty under IEC 61724 monitoring requirements using a typical ISO 9060 Class A pyranometer and Hukx’s SR300-D1.
According to the research:
These figures are based on Hukx’s analysis and should be considered company-specific research findings.
The difference between the two systems comes from several sources that contribute to PR measurement uncertainty.
Irradiance is the largest source of uncertainty
The research found that solar irradiance measurement accounted for approximately 75 per cent of PR uncertainty in a monitoring system with nameplate Class A accuracy specifications under otherwise perfect conditions.
For asset owners and operators, Ates recommends several practical measures:
Soiling is particularly important because it can increase measurement uncertainty in an uncontrolled way and cause a structural overestimation of plant performance.
Ates also identifies zero offset and directional response errors as major contributors to uncertainty in nameplate Class A pyranometers.
Reliability depends on more than initial accuracy
Long-term measurement also requires consideration of instrument stability and physical resilience.
Electrical spikes caused by switching or lightning can damage pyranometer circuit boards, creating gaps in the data stream. Hukx says its industrial Class A instruments include integrated surge protection, with additional external protection available.
The SR300-D1 includes remote diagnostics, an accelerometer, surge protection and features for dew and frost mitigation.
Regular recalibration remains important because even stable instruments can suffer physical damage or develop measurement drift.
Better measurements support better decisions
Reducing PR uncertainty has implications beyond contractual acceptance tests. Ates says more accurate measurements can improve performance loss rate calculations, plant valuations, fault detection and assessment of operations and maintenance quality.
The industry is becoming more aware of these issues as margins tighten and operators seek to understand why assets underperform.
For Ates, ISO Class A should therefore be treated as a baseline rather than the final measure of pyranometer performance. The important question is how accurately an instrument performs in the measurements that ultimately influence technical and financial decisions.
As PV portfolios mature, knowing how much uncertainty sits behind a performance figure could become almost as important as the figure itself.
Read the full interview with Hukx on PV Performance Ratio, pyranometer accuracy and measurement uncertainty in PES Solar: https://pes.eu.com/exclusive-articles/pv-system-performance-ratio-measurements-going-beyond-class-a-standards