In photovoltaic monitoring, a solar irradiance sensor is a device that measures the solar power (irradiance) falling on a surface. This measurement is essential because the performance of a PV system depends directly on how much sunlight it receives. Depending on their sensing technology, pyranometers are divided mainly into two categories: Silicon irradiance sensor or PV reference cell and Pyranometer types.
⚖️ Comparison: Si Irradiance Sensor VS Thermopile Pyranometers
| Feature | Reference cell | Thermopile Pyranometer |
|---|---|---|
| Technology | Silicon sensor | Thermocouple (heat-based) |
| Spectral Range | 400–1100 nm | 300–2800 nm |
| Response Time | < 1 second | 5–20 seconds |
| Accuracy | Very high for PV monitoring | Very high for meteorology and research |
| Typical Cost (2025) | €150 – €500 | €600 – €2,000+ |
| Best Use Case | PV system performance monitoring | Scientific research, climate studies, and meteorological applications |
A PV reference cell sensor (Silicon irradiance sensor) has the same receiving surface of a PV module a flat glass but his strength lies in possessing the same receiving capacity of a PV module (non-linear response, see red area in Fig. 1)
This means that a high-quality PV cell pyranometer captures the full spectrum detectable by a PV module and only that spectrum. In this way, the sensor serves as a precise reference for all solar irradiance power receivable by a silicon crystalline module.
The relationship between the output power (normalized at 1000) and the corresponding irradiance value enables accurate assessment of a PV system’s financial performance. Consequently, any decline in the photovoltaic system’s efficiency can be detected by a monitoring system equipped with a PV cell pyranometer of sufficient precision.
That said, there is a common “pragmatic assumption” among many professionals in the sector (O&M teams, asset managers, etc.) that deviations between irradiance measurements taken with pyranometer type and reference cell typically range between 3% and 6%.
However, preliminary data from a study conducted with the Department of Physics at the University of Milan show that these deviations can range from 3% to 13%. This does not include exceptional cases such as abrupt variations in irradiance caused by passing clouds, which can increase deviations up to 80%. Such differences are understandable, given the varying response times of the two sensor types. Even at sunset, deviations exceeding 15% between the two measured irradiance values have been observed.
A thermopile pyranometer is capable of detecting the entire solar spectrum that reaches the Earth’s surface, from ultraviolet (UV) to far infrared (FIR) radiation. This is illustrated in Fig. 1, where the yellow area represents the reference solar spectrum.
This characteristic allows the pyranometer to accurately measure solar irradiance on the Earth’s surface (expressed in W/m²), making it an essential instrument for meteorological studies and climate change research.
Another distinctive feature is its very low drift. In a photovoltaic field, the output signal from a pyranometer continuously exposed to sunlight and freezing conditions—remains stable for a very long time. Deviations within 1% of those recorded by our reference thermopile pyranometers (SR20), installed on the system for two years, have been observed. This level of stability has been found in pyranometers from the best manufacturers.
However, the flat spectral response of thermopile-type pyranometers (see Fig. 1, cyan bell curve) can reduce measurement accuracy when evaluating the performance of a photovoltaic system.
The three graphs below show the percentile variation of irradiance detected by a thermopile pyranometer and a monocrystalline photovoltaic cell pyranometer.
Currently, we are analyzing this data with a spectrophotometer with the Department of Physics at the University of Milan.
At Soluzione Solare, we help professionals and companies choose, install, and maintain high-quality solar radiation sensors for reliable energy performance monitoring.
