You’re looking at two datasheets for bifacial solar modules. The designs seem almost identical, yet their power ratings tell different stories. One claims a higher bifacial gain, but can you trust that number? That figure is more than a simple measurement; it’s the foundation for every energy yield prediction, financial model, and material choice you’ll make.
Measuring the power of a two-sided solar panel is far more complex than for a standard one-sided module. The „unseen“ rear side is a powerhouse of potential, but its contribution can be wildly inconsistent if not measured under rigorously controlled conditions. This variability is a ghost in the machine for solar project development, creating uncertainty where you need precision.
At PVTestLab, we believe that reliable innovation starts with reliable data. That’s why we’ve refined a protocol based on the highest industry standards to deliver exactly what every developer and manufacturer needs: a repeatable, trustworthy power rating. Let’s pull back the curtain on how it’s done.
Why Bifacial Measurement Is So Tricky
With a standard monofacial module, the process is simple: light hits the front, and you measure its power output. A bifacial module, however, has a second, more complicated power source. It captures light that reflects off the ground or rooftop onto its rear side. This extra energy is called bifacial gain.
The challenge? The amount of reflected light isn’t a constant. It depends on:
- Albedo: The reflectivity of the surface below (e.g., white gravel reflects more light than dark soil).
- Mounting Height: How high the module is installed.
- Tilt Angle: The angle of the installation.
- Irradiance: The intensity and angle of the sun.
In the real world, these factors are constantly changing. To create a fair comparison between different modules, testing them in a lab under a standardized, repeatable set of conditions is essential. This is where a high-precision solar simulator becomes indispensable.
The Gold Standard: Introducing the Dual-Light AAA Solar Simulator
Not all simulators are created equal, especially when it comes to bifacial testing. The international standard, IEC TS 60904-1-2:2019, outlines the requirements for accurate measurement. While it allows for different methods, the most direct and precise approach uses a dual-light system.
A dual-light AAA solar simulator illuminates both the front and rear of the module—either simultaneously or in rapid succession—within a single, controlled test. The „Class AAA“ rating ensures the highest grade of precision across three key areas:
- Spectral Match: The spectrum of the simulator’s light closely matches natural sunlight.
- Spatial Non-Uniformity: The light is spread evenly across the entire module surface.
- Temporal Instability: The light intensity remains stable throughout the test flash.
This technology moves beyond estimations and theoretical adjustments to capture the module’s true performance under specific, defined bifacial conditions.
The PVTestLab Protocol: A Step-by-Step Breakdown
Achieving a precise power rating isn’t about a single flash of light. It’s a meticulous process designed to isolate and then accurately combine the performance of each side of the module. Whether clients are working on solar module prototyping or validating new materials, this protocol is the bedrock of our analysis.
Here’s how we do it:
Step 1: Isolate the Front Side
First, we need a baseline. We place the bifacial module in the simulator and completely cover its rear side with a black, non-reflective material. We then perform a flash test at standard test conditions (STC): 1000 W/m² irradiance at a cell temperature of 25°C.
This gives us the standard I-V curve—the module’s performance fingerprint—for the front side alone. This is the same test a monofacial module would undergo.
Step 2: Isolate the Rear Side
Next, we flip the process. The front side is now covered, and we expose the rear side to a controlled flash of light. The irradiance for the rear side is a critical variable. We can set it to different levels—such as 100 W/m², 200 W/m², or 300 W/m²—to simulate different real-world albedo conditions. This gives us the I-V curve for just the rear side.
This step is especially crucial when conducting lamination process trials for new transparent backsheets or encapsulants, as it directly measures how well these new materials facilitate rear-side power generation.
Step 3: Calculate the Combined Bifacial Performance
With two clean data sets, one for the front and one for the rear, we can calculate the module’s combined performance. Using the principle of superposition and the one-diode model, our software combines the two I-V curves.
This isn’t just simple addition. The process accurately models how the currents from the front and rear cell strings combine within the module’s circuitry. The result is a single, comprehensive I-V curve that represents the module’s total output under the specified front and rear irradiance conditions.
The peak of this new curve gives us the key metric we’ve been looking for: the equivalent maximum power (Pmax,eq). This is the reliable, repeatable power rating that can be printed on a datasheet and used for accurate performance modeling.
Why This Precision Matters
This protocol does more than just generate a number; it creates confidence.
- For Module Developers: It provides an accurate benchmark to validate new designs and technologies.
- For Material Suppliers: It offers concrete data on how a new transparent backsheet or encapsulant performs in a real module.
- For Project Financiers: It delivers a bankable power rating that reduces uncertainty in energy yield forecasts.
By eliminating the guesswork, you can make smarter decisions, accelerate your R&D cycle, and bring more effective products to market faster. For anyone working through these challenges, our expert process engineers can help design a test plan tailored to your specific goals.
Frequently Asked Questions (FAQ)
What is a bifaciality factor?
The bifaciality factor is the ratio of the rear side’s efficiency to the front side’s efficiency, measured under the same standard illumination. While our protocol measures this, we focus on the combined Pmax,eq, as it’s a more direct and practical measure of the module’s total power output in a realistic scenario.
Why can’t you just test the module outside?
Outdoor conditions are constantly changing due to clouds, sun angle, and atmospheric conditions. A lab-based AAA solar simulator provides perfectly stable and repeatable conditions, making it essential for comparing different modules or validating design changes. Outdoor testing is valuable for long-term performance data, but not for a standardized power rating.
How does this measurement relate to energy yield modeling?
The Pmax,eq values we measure at different rear-side irradiances (e.g., 100, 200, 300 W/m²) become critical inputs for energy yield simulation software like PVSyst. These values allow modelers to more accurately predict how a module will perform across a full year in a specific location with a known albedo.
Does the spectrum of light on the rear side matter?
Yes, absolutely. Reflected light from the ground often has a different spectral composition than direct sunlight. A Class AAA simulator’s ability to match the solar spectrum is crucial for both front and rear-side measurements to ensure the data reflects real-world physics.
From Data to Decision
Understanding a bifacial module’s true power potential begins with a measurement you can trust. A precise, repeatable protocol using a dual-light AAA simulator transforms ambiguity into certainty. It provides the foundational data needed for every subsequent step in the value chain—from R&D and manufacturing to project deployment and financing.
When you’re ready to move from concept to reality, having this level of data-driven insight is no longer a luxury; it’s a necessity.