A solar irradiance sensor measures irradiance, meaning the solar radiant power received per unit area. The instantaneous value is usually expressed in watts per square meter (W/m²). In PV systems, the sensor enables the available solar irradiance to be compared with the PV power actually generated.

Solar Irradiance Sensors at a Glance

• A solar irradiance sensor provides an important reference value for evaluating the power and energy yield of a PV system.
• Pyranometers and reference cells use different measurement principles and differ in aspects such as spectral range, accuracy, and cost.
• For meaningful measurements, the sensor should ideally have the same orientation and tilt as the PV modules.
• Shading, soiling, installation errors, ageing, and calibration deviations can affect the measurements.

How Is the Solar Irradiance of a PV System Measured?

To evaluate a PV generator, the sensor is usually installed parallel to the modules. It therefore measures the irradiance in the module plane, which is referred to as plane-of-array irradiance or POA irradiance. This measurement accounts for the actual orientation and tilt of the PV modules and is therefore generally more relevant for performance analyses than irradiance measured on a horizontal surface.

Common types of sensors include pyranometers and reference cells. A pyranometer measures solar irradiance across a broad spectral range. Depending on its device class, it can achieve a high level of measurement accuracy.

A reference cell, by contrast, is based on a PV cell and generates an electrical signal that depends on the irradiance. It can provide a practical representation of a PV module’s irradiance response if its cell technology and spectral response are sufficiently comparable with those of the installed PV modules. Differences in cell technology or spectral characteristics can limit this comparability.

The measured irradiance can be combined with module temperature and technical system data to calculate the expected PV power. If the measured power deviates significantly from this expected value, this may indicate soiling, shading, technical faults, curtailment, or other operational influences.

A distinction must be made between irradiance and solar irradiation. The instantaneous irradiance value is expressed in W/m², whereas the solar energy received per unit area over a specific period is expressed in kWh/m².

Where Are Solar Irradiance Sensors Used?

• Performance analysis: Irradiance and actual PV power are compared to evaluate system performance under the prevailing weather conditions.
• Fault detection: Unusually low power despite high irradiance may indicate technical or operational deviations.
• Yield assessment: Cumulative solar irradiation supports the assessment of measured energy yield over days, months, or years.
• System comparison: With suitable sensor equipment, different PV arrays or sections of a system can be compared.

Measurement Quality, Limitations, and Technical Requirements

The quality of a performance analysis depends significantly on the sensor quality, installation position, calibration, and measurement interval. The sensor should be exposed to environmental conditions that are as similar as possible to those affecting the PV generator being assessed. Local shading, soiling, or a different orientation can distort the results.

In addition to suitable calibration, regular inspection and cleaning of the sensor are important. Soiling on the sensor surface can reduce the measured value and may therefore be misinterpreted as low solar irradiance.

Long-term evaluations must also account for ageing, temperature dependence, and possible calibration deviations. Module temperature, ambient temperature, and, where appropriate, wind speed are therefore often measured as additional parameters.

A single solar irradiance sensor does not always provide a representative measurement for large PV fields or arrays with different orientations. Additional sensors may be required where modules have different tilt angles, orientations, or significantly different environmental conditions.

Integrating Solar Irradiance Sensors with EcoPhi

EcoPhi can integrate solar irradiance sensors through available analogue or digital interfaces. The measurements can be combined and visualized together with PV power, energy yield, and module temperature.

This data can support target-to-actual comparisons, performance analyses, and alerts for unusual deviations. The specific integration depends on the sensor type, communication interface, data quality, and requirements of the respective project.

Conclusion: A Basis for Reliable PV Performance Analyses

A solar irradiance sensor provides a weather-related reference for evaluating a PV system. Only the combination of suitable sensor equipment, correct installation, regular inspection, and supplementary measurements enables reliable conclusions about whether the actual PV power corresponds to the available solar irradiance.

Frequently Asked Questions

What Does a Solar Irradiance Sensor Measure?

The sensor measures irradiance, meaning the solar radiant power received per unit area. The instantaneous value is usually expressed in W/m².

What Is the Difference Between a Pyranometer and a Reference Cell?

A pyranometer measures solar radiation across a broad spectral range. A reference cell is based on a PV cell and can provide a practical representation of the irradiance response relevant to PV modules, provided that its cell technology and spectral response are sufficiently comparable.

Why Is Module Temperature Also Measured?

The power output of a PV module depends not only on irradiance but also on its temperature. Measuring the module temperature therefore improves the calculation of expected PV power.

Is One Sensor Sufficient for Every PV System?

Not necessarily. Large systems and installations with different orientations, tilt angles, or environmental conditions may require several sensors.