BNPI in Bifacial Solar Panels: Complete 2026 Guide
Bifacial solar panels generate electricity from both their front and rear surfaces. Because the rear side also contributes to electrical output, their electrical performance cannot always be described using only the conventional front-side STC condition.
This is where Bifacial Nameplate Irradiance (BNPI) becomes important.
BNPI provides a defined irradiance condition for reporting electrical characteristics of bifacial photovoltaic modules. Under the IEC definition, BNPI corresponds to 1,000 W/m² irradiance on the module front and 135 W/m² on the module rear.
Table of Contents
What Is Bifacial Nameplate Irradiance (BNPI)?
BNPI stands for Bifacial Nameplate Irradiance. It is a defined irradiance condition used for reporting nameplate characteristics of bifacial photovoltaic modules.
The important difference from conventional front-side STC reporting is that BNPI includes irradiance on both surfaces of the bifacial module.
- Front irradiance: 1,000 W/m²
- Rear irradiance: 135 W/m²
This definition is referenced in the IEC framework for bifacial PV measurement and module qualification. IEC TS 60904-1-2 specifies measurement procedures for the I-V characteristics of bifacial PV devices.
Why Is BNPI Important for Bifacial Solar Panels?
A bifacial module can receive light from both sides. The amount of rear-side irradiance depends on factors such as ground reflectivity, module height, row spacing, tilt, surrounding structures and installation geometry.
Using a defined BNPI condition provides a common reference for reporting bifacial module characteristics.
1. Better comparison of bifacial modules
When manufacturers report bifacial electrical characteristics under a defined condition, engineers and buyers have a more consistent basis for comparing module data.
2. Improved understanding of module ratings
A bifacial module datasheet can contain electrical characteristics at conventional STC as well as additional bifacial information associated with BNPI.
3. Useful for PV modelling
Bifacial performance modelling requires information about front-side performance, rear-side irradiance and bifaciality. BNPI provides a defined reference point for the module's electrical characteristics.
4. Important for module testing
For manufacturers and testing laboratories, BNPI is relevant when measuring or calculating bifacial electrical parameters such as maximum power, short-circuit current and open-circuit voltage.
What Is the BNPI Test Condition?
The current IEC definition of bifacial nameplate irradiance specifies the following irradiance combination:
| Parameter | BNPI Condition |
|---|---|
| Front-side irradiance | 1,000 W/m² |
| Rear-side irradiance | 135 W/m² |
| Device type | Bifacial PV device/module |
| Purpose | Nameplate characteristic reporting |
BNPI should not be described as an arbitrary 200 W/m² rear-irradiance condition or as a manufacturer-selected percentage such as "+10% bifacial gain." The IEC definition specifies 135 W/m² rear irradiance together with 1,000 W/m² front irradiance.
BNPI vs STC: What Is the Difference?
| Parameter | STC | BNPI |
|---|---|---|
| Front irradiance | 1,000 W/m² | 1,000 W/m² |
| Rear irradiance | Not included in conventional monofacial STC reporting | 135 W/m² |
| Application | Standard PV electrical rating | Bifacial module nameplate characteristics |
| Rear-side contribution | Not represented by the conventional front-side condition | Included through the defined rear irradiance |
For conventional STC, PV electrical characteristics are referenced to 1,000 W/m² irradiance, 25°C cell temperature and the applicable reference spectrum.
For bifacial modules, additional reporting at BNPI provides information under a defined two-sided irradiance condition.
BNPI vs Bifaciality Factor
BNPI and bifaciality are related, but they are not the same thing.
Bifaciality
Bifaciality describes the relationship between the electrical response of the rear side and the front side of a bifacial PV device.
Different bifaciality coefficients can be reported for:
- Short-circuit current, φIsc
- Open-circuit voltage, φVoc
- Maximum power, φPmax
BNPI
BNPI is the defined irradiance condition at which bifacial nameplate characteristics are reported.
| Term | Meaning |
|---|---|
| BNPI | Defined front and rear irradiance condition for bifacial nameplate characteristics |
| Bifaciality coefficient | Relationship between rear-side and front-side electrical performance |
| STC | Standard reference condition used for PV electrical characterization |
BNPI Calculation Example
A useful way to understand BNPI is to see how the rear-side irradiance is represented relative to the 1,000 W/m² front-side irradiance.
Therefore, the rear-side irradiance in the BNPI condition is 13.5% of the front-side irradiance.
Example using bifaciality
For a simplified calculation, if a bifacial module has a front-side STC power of 600 W and a maximum-power bifaciality coefficient of 80%, an illustrative BNPI estimate can be expressed as:
PBNPI ≈ 600 × (1 + 0.80 × 0.135)
PBNPI ≈ 664.8 W
This is a simplified calculation example, not a substitute for a laboratory measurement. Actual reporting and calculation must follow the applicable measurement procedure, module characteristics and uncertainty requirements.
The relationship between BNPI, bifaciality and power can also be used in PV performance modelling. PVGIS, for example, documents the use of BNPI and gives a corresponding simplified relationship between STC power and bifaciality for its bifacial-module treatment.
How Is a Bifacial PV Module Tested?
Bifacial PV testing requires consideration of irradiance reaching both surfaces of the device.
IEC TS 60904-1-2 covers measurement of I-V characteristics of bifacial PV devices under natural or simulated sunlight.
Key electrical parameters
- Isc: Short-circuit current
- Voc: Open-circuit voltage
- Imp: Current at maximum power
- Vmp: Voltage at maximum power
- Pmax: Maximum power
- Bifaciality coefficients: Rear-to-front performance relationships
Measurement approaches
Bifacial measurement can be performed using approaches that provide equivalent results when the relevant requirements are satisfied. The applicable procedures include methods for simultaneous two-sided irradiation and approaches based on equivalent or sequential irradiation.
The choice of method depends on the laboratory equipment and the applicable measurement procedure.
BNPI and Current IEC Standards
The technical basis for bifacial PV electrical characterization has evolved over several editions of IEC documents.
| Standard | Relevance |
|---|---|
| IEC TS 60904-1-2:2024 | Measurement of I-V characteristics of bifacial PV devices |
| IEC TS 60904-1-2:2024 + AMD1:2026 | Current consolidated version including the 2026 amendment |
| IEC 61215-1:2021 | Defines BNPI terminology and requirements related to bifacial module nameplate characteristics |
| IEC TS 63202-3 | Measurement of I-V characteristics of bifacial PV cells |
The current IEC consolidated version of IEC TS 60904-1-2 was published with Amendment 1 in March 2026. IEC states that the document covers I-V measurement of bifacial PV devices and includes updated requirements related to irradiance uniformity, background conditions and bifaciality calculations.
BNPI Does Not Mean Every Bifacial Module Will Produce the Same Field Gain
This is one of the most important points for PV engineers and buyers.
BNPI is a defined reference condition. It does not mean that a module will always receive exactly 135 W/m² on its rear side in the field.
Actual rear irradiance can change significantly depending on:
- Ground albedo
- Module height above ground
- Row-to-row spacing
- Module tilt
- Tracker geometry
- Ground coverage ratio
- Diffuse radiation
- Nearby structures
- Shading
- Site latitude and climate
For this reason, the BNPI rating should not be treated as a direct prediction of annual energy yield.
Why BNPI Matters to Solar Module Manufacturers
For PV manufacturers, bifacial testing and reporting have become increasingly important as bifacial module technology is widely deployed.
1. Electrical characterization
Manufacturers can characterize bifacial module performance using defined measurement conditions and report the relevant electrical parameters.
2. Nameplate information
IEC 61215-1 includes specific requirements for reporting bifacial electrical characteristics at the required irradiance conditions.
3. Quality control
BNPI-related measurements can be incorporated into appropriate module characterization and verification activities, depending on the manufacturer's testing setup and applicable product requirements.
4. Customer documentation
Clear reporting helps EPC companies, designers, testing laboratories and customers understand how bifacial electrical characteristics have been determined.
BNPI and Solar Module Quality Testing
From a quality-control perspective, bifacial module testing requires attention to both the measurement condition and the test setup.
Important factors include:
- Irradiance level and uniformity
- Front and rear illumination
- Temperature measurement
- Spectral conditions
- Reference-device calibration
- Electrical measurement accuracy
- Test-system repeatability
- Data correction and uncertainty
The 2026 amendment to IEC TS 60904-1-2 includes updates to irradiance non-uniformity requirements, non-irradiated background conditions and the calculation of bifaciality.
This makes it important for PV testing teams to use the current applicable edition rather than relying only on older procedures.
BNPI vs Real-World Bifacial Gain
| Parameter | BNPI | Real Project |
|---|---|---|
| Front irradiance | Defined at 1,000 W/m² | Changes with weather and sun position |
| Rear irradiance | 135 W/m² | Site dependent |
| Ground albedo | Not a universal field value | Depends on surface |
| Module height | Reference condition | Project specific |
| Row spacing | Not a universal field parameter | Project specific |
| Purpose | Nameplate characterization | Energy-yield evaluation |
Why BNPI Is Becoming More Important in Bifacial PV
As bifacial modules become an important part of modern PV installations, standardized characterization becomes increasingly useful.
A common reference condition helps connect:
- Module testing
- Manufacturer datasheets
- Quality assurance
- PV simulation
- Module comparison
- Project engineering
However, engineers should always separate laboratory/nameplate characterization from site-specific energy-yield prediction.
Key Takeaways
- BNPI means Bifacial Nameplate Irradiance.
- The defined BNPI condition is 1,000 W/m² front + 135 W/m² rear.
- BNPI is different from conventional STC.
- BNPI is different from the bifaciality coefficient.
- Bifaciality coefficients can be reported for Isc, Voc and Pmax.
- Actual rear irradiance in a solar project depends on site and system design.
- BNPI should not be treated as a guaranteed field energy gain.
- IEC TS 60904-1-2 is the key IEC document for bifacial I-V characterization.
- The current consolidated version includes the 2026 amendment.
Frequently Asked Questions About BNPI
1. What does BNPI stand for?
BNPI stands for Bifacial Nameplate Irradiance.
2. What is the BNPI irradiance condition?
The BNPI condition is 1,000 W/m² on the front side and 135 W/m² on the rear side of the bifacial PV device.
3. Is BNPI the same as STC?
No. Conventional STC reporting uses the front-side reference condition, while BNPI adds a defined rear-side irradiance condition for bifacial PV characteristics.
4. What is the difference between BNPI and bifaciality?
BNPI defines the irradiance condition used for reporting bifacial nameplate characteristics. Bifaciality describes the relationship between rear-side and front-side electrical performance.
5. Does 135 W/m² rear irradiance mean every project will receive 135 W/m²?
No. 135 W/m² is the defined BNPI reference condition. Actual rear irradiance varies with albedo, module height, geometry, shading, weather and other site conditions.
6. Which IEC standard covers bifacial I-V testing?
IEC TS 60904-1-2 covers measurement of I-V characteristics of bifacial photovoltaic devices. The current consolidated version incorporates Amendment 1:2026.
7. Can BNPI be used directly to calculate annual solar generation?
No. BNPI is a defined module characterization condition. Annual energy yield requires site-specific modelling using irradiance, temperature, albedo, geometry, system losses and other relevant parameters.
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Technical References
- IEC TS 60904-1-2:2024 + AMD1:2026 — Measurement of current-voltage characteristics of bifacial PV devices.
- IEC 61215-1:2021 — Terrestrial photovoltaic modules: design qualification and type approval.
- IEC TS 63202-3 — Measurement of I-V characteristics of bifacial photovoltaic cells.
- PVGIS — Bifacial nameplate irradiance and bifacial PV calculation guidance.
Note: Standards are periodically revised. Always verify the applicable edition and project/customer requirements before using this information for formal compliance, certification or contractual testing.
