Current state of the art
For PV system performance indicators, it is necessary to normalise PV power output from the available solar 
resource. IEC 61724-1 requires the use of POA irradiance sensors. These can either be pyranometers that 
measure hemispherical broadband short-wave irradiance or PV reference devices that are matched to the 
spectral and angular response of PV systems. In principle, PV reference devices are more sensitive to 
performance anomalies than pyranometers because they track PV system power output more closely. The 
ISO 9060 standard provides classification of pyranometers into class A, B or C, but there is no classification 
system for PV reference devices. 
IEC 60904-2 provides a standard for reference cells (a subset of PV reference devices), which is generally 
used for the World Photovoltaic Scale (WPVS) reference cells that are used in calibration laboratories. 
Nevertheless, this does not apply to many good commercial PV system reference devices that are intended 
for a different market. For example, reference devices with digital outputs or integrated temperature correction 
are not compliant with IEC 60904-2. NMIs, research labs and test and measurement lab capabilities can 
provide accurate calibration of WPVS cells (0.4 % uncertainty at PTB). However, accurate calibration and 
characterisation relies on differential response methods, which currently cannot be applied to devices with 
digital outputs. 

Progress beyond state of the art
Following the recommendation of a recent report [9], this project will propose a new definition for the 
European Partnership on Metrology
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measurand of a PV Reference Device. This enables the deviation from the radiometric definition of irradiance 
to be treated as a positive feature, rather than an error in POA measurements. It will include proposed spectral 
and angular response functions and will be the product of extensive consultation with stakeholders. This project
will define a classification scheme for the accuracy of reference devices based on common sources of error 
and overall accuracy according to the new measurand. To support classification and to identify the most 
important sources of error, the consortium will characterise the features of common commercial reference 
devices in indoor and outdoor studies. The classification will become a new proposed work item for 
standardisation in IEC TC82.
In parallel, the consortium will develop and demonstrate methods for the calibration and characterisation of 
reference devices according to the new classification scheme. New methods will be developed for differential 
response measurements of digital devices with an estimation of their associated uncertainties. Features such 
as internal temperature compensation will also be covered. The work will be supported by instrument 
manufacturers who will provide devices, including custom devices containing both analogue and digital 
outputs. A round robin study will validate the new methods, aiming for a target calibration uncertainty of <1.0 %.