Revision Date: 10/7/2025
I-V Curve Measurement of Bifacial Modules Using Fluke SMFT-1000 and PVA-1500
Introduction:
Bifacial solar modules have gained popularity in the solar industry due to their ability to capture
sunlight from both sides, potentially increasing energy yield compared to traditional monofacial
modules. Despite these advancements, the capacity rating under Standard Test Conditions
(STC) remains based only on light from the module's front side. Not including backside
irradiance means the additional energy production from the rear side is not fully captured in
the standard module power rating.
Standard Test Conditions and Bifacial Gain
Standard Test Conditions (STC) for solar modules include an irradiance of 1000 W/m², a cell
temperature of 25°C, and an air mass of 1.5. However, these conditions do not account for the
bifacial gain, which is the additional power generated by the rear side of bifacial modules. This
gain depends on factors such as ground reflectance and installation height, which are not
standardized in STC measurements. Therefore, comparing the measured results to the
manufacturer's rating under STC while testing bifacial modules is challenging.
Standard Test Conditions (STC) for solar modules include an irradiance of 1000 W/m², a cell
temperature of 25°C, and an air mass of 1.5. However, these conditions do not account for the
bifacial gain, which is the additional power generated by the rear side of bifacial modules. This
gain depends on factors such as ground reflectance and installation height, which are not
standardized in STC measurements. Therefore, comparing the measured results to the
manufacturer's rating under STC while testing bifacial modules is challenging.
Testing Bifacial Modules with Fluke I-V Curve Tracers
The Fluke SMFT-1000 and PVA-1500 tools capture tilt angle, module temperature, and frontside
irradiance during an I-V curve test. Capturing these data points allows the software to translate
the measured I-V curve to what the curve would have been under standard test conditions of
1000 W/m2 and 25°C cell temperature. This information helps compare the actual performance
of a module or string of modules to what the manufacturer says the results should be or to
other test results.
Since bifacial modules gain performance from backside irradiance, which is not included when
rated under standard test conditions, their field performance will exceed their rating at STC.
This difference can make it challenging to compare the measured results to the manufacturer's
rating since the actual performance of the module will often exceed what's expected. Bifacial
modules are usually compared between measurements instead of comparing the measured
results translated to STC to the manufacturer's rating. When comparing test results of different
modules or strings of modules, it's critical to ensure that they are the same module
manufacturer and model, have the same number of modules in series, and are mounted at the
same tilt angle and orientation.
Comparing Test Results
To identify outliers and ensure consistent performance across different strings of modules,
compare each string's fill factor (SMFT & PVA), performance factor (PVA only), and I-V curve.
The fill factor (FF) is a metric used to identify curves that are operating less efficiently due to
deviations from normal I-V curve shape. The fill factor is calculated as the ratio of the
maximum power (the power at the maximum power point or MPP) to the product of open circuit
voltage (Voc) and short-circuit current (Isc) ((Imp x Vmp) ÷ (Isc x Voc) = Fill Factor). A
higher fill factor indicates higher efficiency. A low fill factor may indicate issues with the string
of modules, which would require further troubleshooting.
The 'Performance Factor' (PF) compares the actual power output to the expected power output
under actual conditions. It is a metric provided in the PVA-1500 PV analyzer and data analysis
tool software. Since the performance factor is a ratio of measured vs. expected power based on
frontside STC specifications, the performance factor with bifacial modules will typically exceed
100%. (Note: PF of 100%-120% is typical, up to 130% with snow-covered ground). Performance factor can still be used to
identify outliers, modules or strings with a performance factor lower than those of comparable
modules or strings under comparable conditions
The I-V curve of different tests can be overlaid in the SMFT-1000 and PVA-1500 software to
identify outliers quickly. The measured I-V curve translated to STC will typically outperform the
manufacturer's expected I-V curve at STC because of the backside gain. Still, they should be
similar to other comparable modules or strings.
You can identify any underperforming modules or strings by comparing the fill factor,
performance factor, and the I-V curve of different strings. This comparison helps pinpoint issues
such as shading, soiling, or module degradation, allowing for targeted maintenance and
optimization. Using the Fluke SMFT-1000 and PVA-1500 for I-V curve tracing of bifacial modules
provides comprehensive insights into their performance. Accurate testing and detailed analysis
help optimize the installation and maximize the energy yield of bifacial solar modules.
Conclusion
Testing the performance of bifacial modules provides valuable insights into their effectiveness.
While the bifacial factor is not included in modules' STC specs, using frontside irradiance
measured with Fluke SMFT-1000 and PVA-1500 allows for a consistent and comparable analysis
of bifacial module performance. By standardizing the approach to testing and comparison, solar
professionals can reliably assess the efficiency and quality of bifacial modules, even without
accounting for backside irradiance. This method identifies underperforming modules and
informs decisions for troubleshooting and maintaining solar arrays.