Photovoltaic module low irradiance performance testing evaluates how a solar module converts weak light into electrical power, typically at 200 W/m², compared with its rated output at Standard Test Conditions (STC). Because field-mounted modules spend a large share of their operating life under cloudy skies, early mornings, late evenings, and low-sun-angle conditions, low-light behavior has a direct and measurable impact on annual energy yield. This article explains the technical basis, applicable standards such as IEC 61215-2 and IEC 61853, sample requirements, test procedures, and practical applications of low irradiance performance testing for PV modules.
Technical Overview
Low irradiance performance testing measures the electrical output characteristics of a photovoltaic module under reduced irradiance levels, most commonly 200 W/m², while maintaining the cell temperature and spectrum defined by Standard Test Conditions (1000 W/m², 25 °C, AM 1.5G spectral distribution). The principal purpose of the test is to determine how much of the module's rated power is retained when the incident light intensity falls to a fraction of the standard level, since real outdoor operating conditions rarely match the STC rating point.
The dominant international references for this test are IEC 61215-2 (Terrestrial photovoltaic (PV) modules – Design qualification and type approval – Part 2: Test procedures), which designates the low irradiance measurement as MQT 09, and the IEC 61853 series (Photovoltaic (PV) module performance testing and energy rating), which characterizes module efficiency across a full matrix of irradiance and temperature conditions. Supporting measurement standards include IEC 60904-1 for I-V curve measurement, IEC 60904-2 and IEC 60904-6 for reference devices, IEC 60904-7 for spectral mismatch correction, IEC 60904-9 for solar simulator classification, and IEC 60891 for temperature and irradiance corrections.
From a device physics perspective, low irradiance performance is governed primarily by the shunt (parallel) resistance of the solar cells, recombination losses that reduce open-circuit voltage, and spectral response characteristics. At low light intensity, the photogenerated current is small, so even modest leakage currents through low shunt resistance paths consume a proportionally larger share of the generated power, degrading fill factor and efficiency. Modules with high shunt resistance and low recombination therefore tend to retain a higher percentage of their STC efficiency under weak light, which is a key quality and bankability indicator.
Testing Samples
Crystalline Silicon Modules
Monocrystalline and polycrystalline silicon modules, including mainstream technologies such as PERC, TOPCon, heterojunction (HJT), passivated emitter rear locally diffused (PERL), and conventional Al-BSF designs, represent the majority of samples submitted for low irradiance testing. These modules are typically tested in their production configuration with the front glass, encapsulant, and backsheet intact, so that all optical and electrical losses of the finished product are captured.
Thin-Film Modules
Thin-film technologies such as cadmium telluride (CdTe), copper indium gallium selenide (CIGS), and amorphous or microcrystalline silicon are also tested for low irradiance behavior. Because some thin-film products exhibit metastability, preconditioning such as light soaking in accordance with the relevant technology-specific stabilization procedures is required before the measurement so that the sample is in a representative stabilized state.
Bifacial and Emerging-Technology Modules
Bifacial modules are evaluated primarily on the front side for low irradiance performance, with rear-side characterization performed separately using bifacial measurement methods. Emerging technologies, including perovskite and tandem cell architectures, may be assessed for research and development purposes, although the interpretation of results must account for their specific spectral sensitivity and stability behavior.
Sample Preparation Requirements
- Samples must be representative production units or qualification test specimens without visible defects such as cracks, delamination, or discoloration.
- Thin-film and other metastable samples must be stabilized through preconditioning prior to measurement.
- A temperature sensor is attached to the rear surface of the module to verify that the cell temperature remains within the permitted tolerance during testing.
- Modules should be electricically connected following the manufacturer's specified wiring configuration, with bypass diodes in their normal operating state.
Testing Items
The low irradiance performance test captures a complete set of electrical parameters at the reduced irradiance level and compares them with the corresponding STC values. The typical measured and derived quantities include:
- Maximum power output (Pmax) at 200 W/m² and 25 °C.
- Short-circuit current (Isc), open-circuit voltage (Voc), and current and voltage at the maximum power point (Imp, Vmp) at the low irradiance level.
- Fill factor (FF) under low irradiance conditions.
- Low irradiance conversion efficiency and the ratio of low-light efficiency to STC efficiency, often expressed as a relative performance factor.
- The low irradiance performance factor, defined as the measured power at 200 W/m² divided by the value expected from a purely linear extrapolation from STC power.
- Optionally, the full irradiance-dependent efficiency curve from 100 W/m² to 1100 W/m² in accordance with IEC 61853-1.
Testing Methods
Applicable Standards
- IEC 61215-2 – MQT 09, Performance at low irradiance, defining the measurement at 200 W/m² for design qualification and type approval.
- IEC 61853-1 – Irradiance and temperature performance measurements and power rating at irradiance levels from 100 W/m² to 1100 W/m².
- IEC 60904-1 – Measurement of I-V characteristics of photovoltaic devices.
- IEC 60904-2 and IEC 60904-6 – Requirements for primary and secondary reference PV devices.
- IEC 60904-7 – Computation of the spectral mismatch correction for measurements of photovoltaic devices.
- IEC 60904-9 – Classification of solar simulators for electrical performance testing.
- IEC 60891 – Procedures for temperature and irradiance corrections to measured I-V characteristics.
Test Conditions
The standard low irradiance test point is defined as 200 W/m² irradiance with a cell temperature of 25 °C ± 2 °C and a spectral distribution matching the AM 1.5G reference spectrum. When the irradiance reduction is achieved with neutral density filters, wire mesh attenuators, or reduced simulator power, the spectral transmission of the attenuation method must be characterized and a spectral mismatch correction must be applied in accordance with IEC 60904-7, because filter-induced spectral shifts can otherwise bias the result.
Equipment Requirements
- A solar simulator meeting the classification requirements of IEC 60904-9, with verified spatial non-uniformity, temporal stability, and spectral match at the reduced irradiance level.
- A calibrated reference device traceable to a recognized national metrology institute, with a spectral response closely matched to the sample under test.
- A precision electronic load or I-V curve tracer with sufficient resolution and accuracy at the lower current levels produced under weak light.
- A temperature-controlled environment or mounting arrangement, together with rear-surface temperature monitoring, to hold the sample at the specified temperature.
Test Procedure
The module is positioned in the test plane of the simulator, the temperature sensor is affixed, and the sample is allowed to reach thermal equilibrium at 25 °C. The reference device is used to set and continuously verify the 200 W/m² irradiance level. A complete I-V characteristic is then recorded, and the measured data are corrected for any residual temperature deviation and spectral mismatch. The corrected parameters are compared with the module's STC results to calculate the low irradiance performance factor, which quantifies whether the module performs above, at, or below linear scaling when the irradiance drops to one fifth of the standard level.
Data Evaluation and Reporting
IEC 61215-2 defines the measurement method and requires the low irradiance result to be documented as part of the qualification test record. Acceptance in commercial practice is typically assessed against the manufacturer's declared low-light performance data, project technical specifications, or tender requirements. A performance factor close to or above 1.0 indicates that the module retains its expected relative efficiency under weak light, while a value significantly below 1.0 signals elevated shunt losses or recombination that will reduce energy harvest in cloudy or low-sun climates.
Applications
- Energy yield prediction and bankability studies, where accurate low-light characterization improves the fidelity of simulation models for climates with frequent overcast conditions.
- Product certification and type approval in accordance with the IEC 61215 series for modules entering global markets.
- Input data generation for the IEC 61853 energy rating framework, which combines irradiance and temperature dependence with climate data to produce a comparable energy rating metric.
- Research and development support for cell process optimization, particularly efforts to raise shunt resistance and reduce recombination losses.
- Technology benchmarking for tenders and procurement, enabling project developers to compare module families on weak-light behavior in addition to STC efficiency.
- Verification of manufacturer marketing claims regarding superior low-light or cloudy-day performance.
- Application-specific selection, such as building-integrated photovoltaics, high-latitude installations, and agrivoltaic systems, where extended periods of diffuse and low-intensity illumination are expected.
FAQ
What irradiance level is used for the low irradiance performance test?
The reference test point specified in IEC 61215-2 (MQT 09) is 200 W/m² at a cell temperature of 25 °C with the AM 1.5G spectrum. For the extended energy rating characterization of IEC 61853-1, measurements are performed at multiple irradiance levels between 100 W/m² and 1100 W/m² to map the full irradiance dependence of the module.
Is the low irradiance test a pass or fail test?
The primary function of MQT 09 within the IEC 61215 sequence is measurement and reporting rather than a fixed pass/fail threshold. In commercial practice, the measured result is evaluated against the manufacturer's declared data, project specifications, or tender requirements. Sustained accredited third-party laboratories report the measured low-light power and the derived performance factor so that buyers can make informed comparisons.
Why do some modules perform better than others under weak light?
Low-light behavior is controlled mainly by shunt resistance and recombination. At 200 W/m² the photogenerated current is only about one fifth of the STC value, so leakage currents through shunt paths consume a proportionally larger fraction of the output. Cells with high shunt resistance and low recombination retain more of their open-circuit voltage and fill factor, yielding a higher relative efficiency under weak illumination.
How is the 200 W/m² level produced in the laboratory?
The reduced irradiance is achieved either by operating the solar simulator at reduced intensity or by attenuating the full-intensity beam with calibrated neutral density filters or mesh screens. In all cases, the spectral transmission of the attenuation method is characterized, and a spectral mismatch correction in accordance with IEC 60904-7 is applied to ensure traceable and comparable results.
What is the difference between IEC 61215-2 MQT 09 and IEC 61853-1?
MQT 09 provides a single-point measurement at 200 W/m² used within the design qualification sequence, whereas IEC 61853-1 defines a matrix of measurements at multiple irradiance and temperature combinations to produce a comprehensive performance matrix used for power rating and, together with IEC 61853-2, -3, and -4, for climate-specific energy rating.
How significant is low-light performance for annual energy production?
In many climates, a large share of the annual irradiation arrives at levels well below 1000 W/m², and modules frequently operate under diffuse light. Even a few percentage points of difference in relative low-light efficiency can translate into measurable differences in annual kilowatt-hour yield, which is why low irradiance test data is an important input for financial models and levelized cost of energy calculations.