Quick Facts
What a pyranometer is
A pyranometer is an instrument that measures solar irradiance, the power of sunlight per unit area falling on a surface. The measurement is in watts per square metre (W per sq m) at any given moment, and integrated over time produces irradiation in Wh per sq m or kWh per sq m.
Pyranometers are essential instruments in solar plant operations because:
Performance Ratio (PR) calculation requires irradiance measurement.
Plant performance comparisons need to be normalised against actual conditions.
Forecasting and reporting use historical irradiance data.
Insurance and warranty claims often require documented irradiance data.
In a typical utility-scale solar plant, pyranometers are mounted on a meteorological station that records irradiance, ambient temperature, module temperature, wind speed, and humidity in real-time. The data feeds the plant’s SCADA system and supports detailed performance analysis.
How pyranometers work
The standard pyranometer uses a thermopile sensor.
A thermopile consists of many thermocouples connected in series. Each thermocouple generates a small voltage proportional to the temperature difference across it.
The pyranometer’s sensor is a blackened thermopile that absorbs incoming sunlight, converting it to heat. The temperature difference between the heated thermopile and the reference body generates a voltage.
The voltage is small (typically microvolts to millivolts) but proportional to the irradiance. Calibration converts the voltage to W per sq m.
The thermopile is housed under a glass dome that protects it while allowing all wavelengths of sunlight to pass through. The dome and sensor design make the pyranometer respond equally to all wavelengths from about 280 to 2800 nm.
Silicon-cell pyranometers use a different sensor: a silicon photodiode similar to a solar cell. The silicon photodiode responds only to visible and near-infrared wavelengths (about 400 to 1100 nm), matching the spectral response of silicon PV cells. Silicon pyranometers are smaller, cheaper, and have faster response than thermopile pyranometers, but with lower accuracy and different spectral coverage.
ISO 9060 classification
ISO 9060 classifies pyranometers by accuracy:
Secondary Standard: Highest accuracy. Annual uncertainty about 2%. Used in research, reference monitoring, and high-accuracy applications. Premium pricing.
First Class: Good accuracy. Annual uncertainty about 5%. Used in utility-scale solar plants, commercial monitoring, and most operational applications.
Second Class: Moderate accuracy. Annual uncertainty about 10%. Used in basic monitoring, lower-budget applications, and short-term measurements.
The classification covers multiple parameters: response time, zero offset, non-linearity, temperature response, directional response, spectral selectivity, and tilt response.
For utility-scale solar plant performance verification, First Class or Secondary Standard is typically required.
GHI versus POA pyranometers
Solar irradiance can be measured on different orientations:
Global Horizontal Irradiance (GHI): Pyranometer mounted horizontally. Measures total (direct plus diffuse) irradiance on a horizontal surface. Standard for resource assessment and regional comparison.
Plane of Array (POA): Pyranometer mounted in the plane of the solar panels (matching their tilt and azimuth). Measures the irradiance the panels actually receive.
For Performance Ratio calculation, POA is the relevant metric. The PR formula compares actual energy output to expected output based on POA irradiance and panel kWp.
For resource assessment and design, GHI is the standard. Designers use long-term GHI data and translate it to POA using tilt and azimuth corrections.
A complete solar plant met station typically includes both GHI and POA pyranometers.
Pyranometer calibration
Pyranometers drift over time. The blackened thermopile slowly degrades; the optical filter ages. Without calibration, measurement errors of 3% to 5% per year are typical.
Annual calibration is recommended for accurate plant performance analysis. The calibration process:
The pyranometer is taken to an accredited calibration laboratory or compared in-situ against a reference instrument.
Multiple measurements are made under various conditions.
The calibration coefficient (V per W per sq m) is calculated.
The calibration certificate documents the result and uncertainty.
For premium installations, the reference calibration chain traces to international standards maintained by metrology institutes.
Without calibration, pyranometer data can drift such that the Performance Ratio appears to change even though the plant is operating normally. This is a common source of confusion in plant performance analysis.
Pyranometer installation
For accurate measurement, pyranometers must be properly installed:
Orientation: GHI pyranometer perfectly horizontal; POA pyranometer matching the panel tilt and azimuth.
Mounting location: Free from shading by buildings, trees, or array structures.
Levelling: Use a bubble level on the pyranometer to verify horizontal mounting.
Cable routing: Protected against UV and mechanical damage.
Earthing: Proper grounding for safety and signal integrity.
Periodic cleaning: Dust and bird droppings reduce measurement accuracy. Monthly or weekly cleaning is standard.
Annual calibration: Maintains measurement accuracy.
Poor installation introduces systematic errors that are difficult to detect. Following best practices is essential.
Pyranometers in solar plant monitoring
For utility-scale solar plants:
Multiple pyranometers distributed across the site characterise spatial variation.
A typical 50 MW plant might have 2 to 5 POA pyranometers and 1 to 2 GHI pyranometers.
The pyranometer data feeds SCADA in real-time.
Daily, weekly, and monthly PR reports use the integrated irradiance data.
Annual PR analysis compares actual to expected performance.
For commercial solar plants:
Typically 1 POA pyranometer at the met station.
Sometimes also 1 GHI pyranometer.
Data feeds plant monitoring portal and SCADA.
For residential solar:
Pyranometers are rare; inverter-side data and online resources typically suffice for residential performance analysis.
Common pyranometer mistakes
Skipping calibration. Drift causes systematic measurement errors.
Using GHI instead of POA for PR calculation. PR should be calculated using POA.
Mounting pyranometer in shaded location. Shadows from arrays, buildings, or structures contaminate measurements.
Not cleaning the dome. Dust and dirt reduce light transmission to the sensor.
Confusing pyranometer with pyrheliometer. Different instruments for different measurements.
Mixing First Class and Second Class data without accounting for uncertainty.
Best practices
For utility-scale plants, install Secondary Standard or First Class pyranometers with both GHI and POA orientations.
Calibrate annually using accredited laboratories or in-situ reference instruments.
Clean pyranometer domes weekly or monthly.
Mount in shadow-free locations with proper levelling and orientation.
Maintain calibration records for each pyranometer.
Integrate pyranometer data with SCADA for real-time monitoring.
For PR calculation, use POA irradiance, not GHI.
Standards and references
Pyranometers are classified per ISO 9060 and tested per related ISO standards. The World Meteorological Organization (WMO) provides reference standards for meteorological measurements. Calibration follows IEC and ISO procedures. Manufacturer datasheets specify accuracy and operating limits.
Related glossary terms
- Solar Irradiance
- Global Horizontal Irradiance
- Direct Normal Irradiance
- Diffuse Horizontal Irradiance
- Met Station
- Performance Ratio
- SCADA in Solar
- Peak Sun Hours
Key takeaways
A pyranometer is an instrument that measures solar irradiance, essential for solar plant performance monitoring and Performance Ratio calculation. ISO 9060 classifies pyranometers by accuracy: Secondary Standard, First Class, and Second Class. Most pyranometers use a thermopile sensor; silicon-cell variants exist for PV-specific applications. POA (Plane of Array) pyranometers measure irradiance on the panels’ surface; GHI (Global Horizontal Irradiance) pyranometers measure on a horizontal surface. Annual calibration is essential to prevent drift; proper installation (shadow-free, correctly levelled, regularly cleaned) is critical for accurate measurement.