Comparison of Pyranometers and PV Reference Cell Sensors

What Are the Main devices Used in PV Monitoring?

The two most common devices are:

• Thermopile Pyranometers
• Calibrated PV Reference Cells or Irradiance sensors

Both are used to measure solar irradiance, but their working principles and applications differ significantly.

What Is a Pyranometer?

Thermopile pyranometers (which we refer to in this work simply as “pyranometers”) are devices containing junctions of different metals in contact with a black-coated surface that absorbs solar radiation (the ‘hot’ junction) and a separate surface that does not absorb solar radiation (the “cold” junction). They measure radiation indirectly by measuring the temperature difference between these two surfaces.

An alternative measurement principle uses a diffuser-based pyranometer. In this design, solar radiation passes through an optical diffuser that evenly spreads incoming light over a photodiode detector. The diffuser minimizes the effects of the incident angle, allowing the sensor to measure irradiance more uniformly across the sky dome.

Diffuser-based pyranometers are often valued for their compactness, fast response, and lower cost, though they may be less accurate in absolute terms.

Pyranometers are essentially characterised by a flat spectral response from ~280 to ~3000 nm, which is the transmittance of quartz. Due to this flat spectral response over a wide range of incident wavelengths, pyranometers are well suited for measuring total incident shortwave radiation, regardless of the spectral composition of this radiation. Uncertainties in pyranometers are typically reported in relation to the measurement of total incident solar radiation.

What Is a PV Reference Cell?

PV Reference Cell or Solar Irradiance Sensor is a miniature of a PV Panel. It converts incident photons to electrons through the photovoltaic effect, and then these electrons generate a measurable electric current, same as PV Panels. The efficiency of this photon-to-electron conversion is a wavelength-dependent function that is specific to various PV device technologies, and to some extent, to individual PV devices.

the current of the reference cell is measured by measuring the voltage across a small resistor that is included in the reference cell package. This voltage is calibrated under the reference spectrum [IEC 60904-3 spectrum] at 1000W/m2, 25 °C using standard techniques [IEC 60904-1].

When Should You Use a Reference Cell Sensor?

A Reference Cell Solar Irradiance Sensor is the preferred choice in several PV-specific applications:

PV Performance Monitoring

Because its spectral, angular, and temperature responses closely match PV modules, it provides the most relevant irradiance values for evaluating plant performance and yield.

Degradation and Fault Detection

By comparing actual module output to the irradiance measured by the reference cell, operators can quickly identify degradation, soiling losses, or unexpected deviations from expected performance.

Real-Time System Control

The sensor’s millisecond response time aligns with PV behavior, making it ideal for real-time monitoring, control loops, and inverter adjustments.

Integrated Environmental Monitoring

Modern reference cells can connect additional sensors (module temperature, ambient temperature, wind speed, wind direction, humidity) and send all data via a single Modbus RTU output, simplifying system design.

Cost-Effective PV Monitoring

Compared to pyranometers, reference cells are more economical while still providing robust and reliable data, making them suitable for both utility-scale plants and smaller PV systems.

When Should You Use a Pyranometer?

A pyranometer is still essential in certain applications where a broader measurement of solar radiation is required:

Solar Resource Assessment

Pyranometers measure the full shortwave spectrum (400–2700 nm), including wavelengths outside the PV conversion range. This makes them ideal for meteorological studies, solar mapping, and site resource characterization.

Comparison with Historical Datasets

Most long-term irradiance records worldwide are based on pyranometer data. Using a pyranometer allows operators to compare present-day conditions with historical solar resource measurements.

Diffuse and Direct Irradiance Measurement

With its hemispherical glass dome, a pyranometer captures both diffuse sky radiation and direct sunlight across a wide range of angles. This capability is valuable when analyzing shading, atmospheric effects, or solar energy availability beyond PV applications.

Independent Energy Studies

For applications not limited to PV (e.g., building energy balance, agriculture, or climate studies), pyranometers provide standardized and comparable irradiance values.

Standards and Certification Compliance

In many cases, pyranometers are required by ISO 9060 standards for official solar radiation measurements, particularly in research or compliance-based projects.

Comparison Chart of Pyranometer Vs. Reference Cell Solar Irradiance Sensor features

Feature	Reference Cell Sensor	Pyranometer
Spectral Response	Matches PV module (only usable spectrum)	Wideband (280–3000 nm)
Cost	Economical but robust and reliable solution for measuring solar irradiance levels. typically ranging from 300 to 600 €.	Expensive solution for PV monitoring system. Pyranometers usually priced between 1,500 and 2,500 €,
Standards	IEC 60904, IEC 61724-1 (PV monitoring section)	ISO 9060, IEC 61724-1
PV System Control Suitability	Same physical behavior as PV modules (spectral, angular, and thermal response are comparable)	Different behavior from PV: measures total solar radiation across all wavelengths, including those PV modules cannot convert, and reacts more slowly to changes in irradiance
Response Time	Standard reference cells typically have 1 second response time, but SEVEN reference can reduce the response time to 500 ms.	About 1 second (slower than PV but still fast enough for most applications)
Temperature Response	Similar to PV modules: current output decreases linearly with rising cell temperature. This effect can be corrected mathematically, making the measurement representative of actual PV performance.	Designed to minimize sensitivity to ambient temperature, but still affected by thermal offsets (e.g., zero offsets from longwave radiation or sensor body heating). Pyranometers don’t mimic PV temperature behavior, so corrections are not directly related to PV output.
Incident Angle	Has a flat glass cover similar to PV modules, so reflection losses increase at higher incidence angles (especially in the morning and evening). This makes it more representative of PV performance but can introduce additional reflection losses compared to pyranometers.	Uses a hemispherical glass dome that minimizes reflection across a wide range of angles. This design ensures accurate measurement of diffuse and direct irradiance from almost all directions.
Application	For the characterization of the PV performance, the Reference Cell Solar Irradiance Sensor allows better detection of changes in PV system.	For measuring the global horizontal irradiance, the pyranometer is recommended to be used.
Connectivity	A Reference Cell or Solar Irradiance Sensor can support up to five inputs, allowing the connection of a module temperature sensor, ambient temperature sensor, wind speed sensor, wind direction sensor, and relative humidity sensor. All data are delivered through a single Modbus RTU output directly to the datalogger.	The pyranometer can only be connected to the datalogger without combining any other sensor Data.

FAQ

• Q: Can a Reference Cell replace a Pyranometer in PV plants?
  Yes, especially for monitoring PV performance. Pyranometers are still recommended if global horizontal measurement is needed.
• Q: Do Reference Cells require recalibration?
  Yes, typically every 2 years, similar to pyranometers, to ensure accuracy.
• Q: Which sensor is more accurate?
  Accuracy depends on the goal: pyranometers are precise for total solar radiation (Global Horizontal measurements), while reference cells are accurate for PV-specific monitoring (Plane of array measurements).
• Q: Can both sensors be used together?
  Many large-scale PV plants use both for a complete picture: pyranometers for global irradiance, reference cells for PV performance tracking.