Pyranometer Calibration

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Soluzione Solare provide accurate, reliable, fully-certified recalibration of photovoltaic and thermopile pyranometers or solarimeters either in the field or in the laboratory for solar farms and photovoltaic plants world-wide.

An accurately calibrated pyranometer is essential when monitoring efficiency and productivity of your solar plant. After several years outdoors in the field and exposed to the elements, pyranometer sensitivity can drift, so regular calibration is important.

Calibration is performed by comparing the measurement values from the test instrument with those given by one or more reference instruments under controlled conditions.

Solarimeter calibration - monitoring efficiency and productivity of your solar plant

Performance Ratio – Pyranometer Calibration

Performance ratio calculation is fundamental for measuring the efficiency and productivity of a photovoltaic system. It is often used in Operation Maintenance (O&M) contracts as a reference for the calculation of premiums or penalties to be added or deducted from the cost of the service.

Note: the CEI EN 61724-1:2017 standard defines performance ratio as the ratio between the actual energy yield and the possible theoretical yield.

The performance ratio neither depends on the orientation of the system nor the radiation to which it is subjected, since the solar radiation actually received is measured on the module level. So the performance ration allows us to:

  1. Calculate the efficiency of the entire photovoltaic system based on the solar radiation received, at any given moment throughout the day
  2. Compare the design quality of photovoltaic systems located in very different areas.
  3. Provide an index of the degradation of the plant by collecting data for the same plant over several years. Of course degradation can also be caused simply by the accumulation of dirt and debris on the surface of the photovoltaic modules, which can simply be removed by cleaning. Or the causes must be sought in partial failures of the photovoltaic system or degradation of the solar panels beyond the specifications.

To obtain the precise performance ratio it is important that the measurement of the amount of irradiation (or irradiance) received is as correct as possible. To do this, a photovoltaic or thermopile pyranometer is most commonly used.

For more in-depth information, click the link:

Should I use a pyranometer or a solarimeter?

Note: the terms solarimeter, irradiance sensor, pyranometer and calibrated cell are interchangeable. On this site we will use irradiance sensor. To learn more click here.

IRRADIANCE = Power of the sunlight at a given moment (W/m2)
RADIATION = Energy of sunlight over a given period of time (Wh/m2)

The calculation of the performance ration can be obtained both by relating instantaneous power with irradiance, and by relating the energy produced in a given day with the irradiation of that same day. The latter gives more accurate results.

For the calculation of the P.R, see also the standard EN: 61724-1: 2017.

Regardless of the irradiation sensor used, to obtain precise results, it is fundamental the sensors are calibrated with an appropriate reference sensor according to the regulations governing the calibration for that specific sensor technology, taking into account that:

  • Being optical sensors, they are not free from instrumental drift caused by aging.
  • It is important that the first re-calibration of the sensors is carried out within the first 12-18 months following your purchase. If not included in the contractual clauses, re-calibration must be done at least once every two years, as prescribed by ISO 9001 for reference measuring instruments.

Recommended calibration intervals should be written clearly in the sensor user manual or calibration report of any good pyranometer.

Indoor or outdoor recalibration?

Recalibration can be done either in a lab or in the field.

Pyranometer recalibration in the lab Pyranometer re-calibration in the lab

Recalibration in the laboratory

Although more economical, lab recalibration takes more time as equipment needs to be dismantled, shipped and reassembled. The solar plant will also be several days without an irradiation sensor, forfeiting valuable data.

In some cases, it is possible to request a replacement solarimeter which can be delivered before disassembling the sensor due for recalibration.

Calibrate indoors or outdoors - which is best?

Recalibration in the field

Field recalibration is performed by a qualified technician with his own field instrumentation. They will typically use a primary sensor to carry out surveys and pyranometer calibration outdoors in the field. This type of calibration has the added costs of travel and (potentially) accommodation. With a view to complete transparency, at Soluzione Solare we state all travel and accommodation costs clearly upfront.

Calibrating a Photovoltaic Pyronometer

How long does PV pyranometer recalibration take?

Actual field recalibrate of a photovoltaic cell pyranometer will usually take under 1 hour, excluding preparation (travel, reaching the sensors in the field, disassembly, wiring-up for measurements and reassembly).

Soluzione Solare photovoltaic pyranometer calibration is EN CEI60904-4 compliant.

The images of some sensors we calibrate in the field and laboratory are shown below.

Tritek Spectron 320 Sunmeter Pro SIS 13TC Atersa SI 13TC T

Calibrating a Thermopile Pyronometer

How long does thermopile pyranometer recalibration take?

Following ISO9847, the shortest thermopile pyranometer calibration requires measurements for needed for about 2.5 hours and the right weather conditions, whilst all other procedures require measurements being taken for at least 2 days (though this standard dates back to 1992, when to read voltage values ​​of a few mV undisturbed, desktop multimeters were necessary and reading the data was extremely cumbersome and time-consuming).

Fortunately, there are now excellent field instruments with accuracy and resolution suitable to help.

However, it should also be considered that the adaptation time of a thermopile pyranometer during sudden variations in irradiation, is 10-20 times longer than that of the most photovoltaic pyranometers. Consequently, more time is needed to calibrate a thermopile pyranometer than a photovoltaic cell pyranometer.

ISO9847 also prescribes a mathematical procedure to determine the correct “calibration factor” from the measurements made. Finally, the newly adjusted sensor is subjected to a recording of the measurements to verify its correct behavior and the alignment of the measurements returned with the reference sensor.

Calibration of a thermopile pyranometer

Calibration report

For all sensor recalibrations, a calibration report is provided with an annotation of the instrumental drift found.

For regulatory compliant calibration contact us for a quote.

IEC 60904 are a set of 9 standards, from IEC60904-1 to IEC60904-10 (excluding 60904-6) implemented by the Italian Electrotechnical Committee (CEI).

The standards relating to production and calibration requirements of photovoltaic sensors are:

  • IEC 60904-2 – the requirements for reference solar devices.
  • IEC60904-9 – solar simulator performance requirements.

Those related to standards are:

  • IEC60904-3 – measurement standards for photovoltaic solar devices, with a reference solar spectrum.
  • IEC60904-4 – a procedure for establishing the calibration traceability of reference solar devices. IEC60904-4 is fundamental for the traceability to pyranometers, pyroheliometers and thus World Radiometric Reference (WRR).
  • IEC60904-8 – spectral response measurement of a photovoltaic device

The latter concern research institutes and manufacturers of solar simulators.


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