Field Testing Guide

Testing Bifacial PV Modules in the Field: Accounting for Rear Irradiance and Albedo

A practical guide to IV curve tracing and Isc measurement for bifacial modules. What to measure, what equipment you need, and how to account for rear-side irradiance in both workflows.

Introduction

Bifacial PV modules are now the dominant module technology shipped globally, and for good reason. By capturing reflected light off the ground and surrounding surfaces on their rear side, they can deliver meaningful energy gains over equivalent monofacial modules under the right conditions. But that same characteristic is what makes field performance testing more complex.

The core problem: standard field testing procedures were built for monofacial modules. Whether you're tracing a full IV curve or doing a quick Isc check with a clamp meter, the conventional workflow measures front-side plane-of-array (POA) irradiance and uses that alone as the basis for correction and comparison. Apply that unchanged to a bifacial module and you're ignoring every watt generated by the rear face, which means understated power readings, skewed performance ratios, and capacity test results you can't defend.

This guide covers both common field measurement approaches: IV curve tracing for full module characterisation, and short-circuit current (Isc) measurement for rapid string-level screening. Both sections explain how to correctly account for rear irradiance in the workflow. If you are new to the concept of effective irradiance, start with the Geff explanation guide before working through the steps below.

Two Measurement Approaches, One Rear Irradiance Problem

This guide covers both IV curve tracing and Isc measurement. Both require the same rear irradiance sensor setup, and both use the same Geff calculation principle. The difference is which bifaciality coefficient you apply and what output parameter you are correcting. IV curve tracing gives you full module characterisation correctable to STC; Isc measurement gives you a fast string-level proxy for effective irradiance and module health. For a plain-language explanation of how the Geff calculation works, see the Geff calculation guide.

The two methods work well together in practice. Run Isc sweeps across all strings to identify outliers quickly, then deploy the IV tracer on flagged strings to understand the nature of the underperformance. For a full side-by-side breakdown of instruments, correction formulas, and when to use each, see the dedicated comparison page.

Learn More: IV Curve Tracing vs. Isc Measurement for Bifacial PV →

The Standards That Govern This Work

IEC TS 60904-1-2: The Bifacial Measurement Standard

IEC TS 60904-1-2 Published in 2019, this is the foundational standard for measuring IV characteristics of bifacial PV devices. It defines three bifaciality coefficients: φIsc (short-circuit current), φVoc (open-circuit voltage), and φPmax (maximum power). For field work, φIsc and φPmax are the values you'll use most. One applies to Isc-based correction and the other to IV curve STC correction.

IEC 61724-1: System Performance Monitoring

IEC 61724-1 The standard for ongoing performance monitoring of fielded PV systems. It requires rear-side irradiance contributions for bifacial systems, defines two options for measuring rear irradiance in the field, and specifies the Bifacial Irradiance Factor (BIF) and bifacial Performance Ratio formulation.

IEC 61724-2: Capacity Evaluation

IEC 61724-2 Covers formal system capacity testing. Research from NREL's BEST facility found that the IEC 61724-2 approach using a calibrated bifacial reference module, which derives effective irradiance directly from the reference module's Isc, produced the lowest measurement uncertainty of the methods tested.

IEC 60891: IV Curve Correction Procedures

IEC 60891:2022 Method 4 is the current recommended approach for bifacial module IV correction. Critically, the irradiance input must be Geff, not front-side irradiance alone.

IEC 60904-3: Reference Irradiance Conditions

IEC 60904-3 Defines the reference spectral irradiance (AM1.5G) and the standard outdoor deployment condition for bifacial rating: a module tilted at 37 degrees, over a surface with albedo 0.21 (light bare soil), at a height of at least 1 metre. Under these conditions, expected rear-side irradiance falls in the 145-165 W/m² range.

Equipment You'll Need

The rear irradiance instrumentation is identical for both IV tracing and Isc measurement. What differs is the primary measurement device.

Primary Measurement Instruments

Rear Irradiance & Albedo Measurement Equipment

IEC 61724-1 defines three sensor options. The same setup serves both IV tracing and Isc measurement workflows.

Supporting Items

Workflow A: Isc Measurement

Rapid Screening • String-Level Health Check • O&M

Use this workflow for rapid string-level health screening, routine O&M checks, or any situation where you need to survey many strings quickly. Isc gives you a direct proxy for effective irradiance and flags strings with soiling, mismatch, shading, or degradation issues before committing to full IV curve tracing.

1. 1
   Document Site Conditions Record module tilt angle, mounting height above grade at the lower module edge, row pitch, and a description and estimated albedo of the ground surface. Photograph the inter-row ground cover. This documentation is the same for both Isc and IV workflows and only needs to be done once per site visit.
2. 2
   Select Test Strings and Representative Location Identify which strings you are testing. For module-level Isc, select representative mid-array, mid-row modules. Avoid array edge strings and end-of-row positions, as they see elevated rear irradiance and are not representative of the array as a whole.
3. 3
   Set Up Front and Rear Irradiance Sensors Mount your front-side reference cell in the plane of array. Mount your rear-facing sensor coplanar with the module rear face at mid-module height. These sensors remain in place for the duration of your Isc sweep. You don't need to move them between strings as long as they remain representative of the array section being tested.
4. 4
   Confirm Stable Irradiance Conditions Wait for steady-state irradiance before beginning measurements. Front-side GHI should be stable within +/-2% over the measurement window. Avoid testing under moving cloud cover. Log irradiance readings continuously so you have GF and GR at the time of each string measurement.
5. 5
   Measure Isc and Record Simultaneous Irradiance Isolate the string at the combiner or disconnect, connect your measurement instrument, and measure Isc. At the same moment, record GF and GR from your sensors. Also record module temperature if available. Note the time of each measurement for cross-referencing with logged irradiance data.
6. 6
   Calculate Effective Irradiance for Isc Correction Just as with IV curve tracing, Isc correction requires an effective irradiance that accounts for both sides of the module:

   Geff = GF + (GR x φIsc)

   Where φIsc is the short-circuit current bifaciality coefficient from the module datasheet (per IEC TS 60904-1-2). This is often very close in value to φPmax but is technically the correct coefficient for Isc calculations.
7. 7
   Correct Isc to STC and Compare to Nameplate Apply temperature and irradiance correction to translate the measured Isc to STC conditions:

   Isc,STC = Isc,measured x (1000 / Geff) x [1 + αIsc x (25 - Tmodule)]

   Where αIsc is the temperature coefficient of short-circuit current (from the module datasheet, in %/°C or A/°C) and Tmodule is the measured module temperature in °C. A deviation of more than +/-3-5% from nameplate Isc, after confirming your irradiance and temperature inputs are accurate, warrants further investigation with a full IV curve trace.
8. 8
   Flag Outliers and Proceed to IV Tracing If Needed Compare Isc,STC values across all strings tested. Strings that fall significantly below the fleet average (typically more than 3-5%) should be flagged for follow-up IV curve tracing to characterise the nature of the underperformance, whether soiling, shading, module degradation, bypass diode failure, or wiring issues. Isc screening tells you where the problem is; IV tracing tells you what it is.

Workflow B: IV Curve Tracing

String-Level Commissioning • Acceptance Testing • Performance Verification

Use this workflow during commissioning to verify that each string is performing to specification before the system is handed over. IV curve tracing at the string level gives you Voc, Isc, Pmax, and fill factor for every string — STC-corrected and comparable to nameplate — providing the baseline documentation needed for warranty protection, EPC sign-off, and future performance comparison.

At commissioning, bifacial strings must be tested with rear irradiance accounted for. Testing during the first energisation without capturing GR will produce STC-corrected results that understate actual string performance, making it impossible to establish a valid baseline or defend future warranty claims based on that data.

1. 1
   Document Site and String Configuration Before testing, record the string circuit configuration: number of modules per string, module model and serial number range, string combiner assignment, and string identifier. Photograph the string from both front and rear to document ground surface conditions, mounting height at the lower module edge, row pitch, and inter-row ground cover. This documentation links every IV curve result to a specific physical string for the life of the asset.
2. 2
   Select Test Strings Systematically At commissioning, the goal is full coverage — ideally every string. Where project scope limits this, prioritise strings from the centre of each sub-array, avoiding end-of-row and array-edge positions which see non-representative rear irradiance. Document which strings were tested and which were not, and the rationale for any omissions.
3. 3
   Set Up Front and Rear Irradiance Sensors Mount your front-side reference cell coplanar with the module front face at mid-string height. Mount your rear-facing reference cell or pyranometer coplanar with the module rear face at mid-module height on the same or an immediately adjacent module. Rear irradiance non-uniformity can exceed 30% between the lower module edge and module centre — mid-height placement is essential for a reading representative of the full module area. Keep sensors in place throughout the string test sequence; relocate only when moving to a different sub-array section with meaningfully different ground surface or mounting geometry.
4. 4
   Attach Module Temperature Sensor Affix a PT100/PT1000 RTD or thermocouple to the rear surface of a module in the string under test. Position it away from the rear irradiance sensor mount to avoid shadow influence on the temperature reading. Module temperature at commissioning is critical for accurate STC correction — a 10°C error in temperature measurement introduces approximately 0.4% error in corrected Pmax for standard c-Si modules.
5. 5
   Confirm Stable Measurement Conditions Front-side POA irradiance should be stable within ±2% over the measurement period, and rear irradiance similarly stable. Avoid testing during periods of variable cloud cover. Clear-sky conditions between 09:00 and 15:00 solar time with irradiance above 700 W/m² provide the most reliable commissioning results. Log front-side and rear irradiance continuously throughout the string test sequence to tie each IV curve result to its exact measurement conditions.
6. 6
   Isolate the String and Connect the IV Tracer Isolate the string at the combiner box or string disconnect. Verify polarity before connecting the IV tracer leads. For 1500V systems, confirm that your IV tracer and test leads are rated for the full open-circuit voltage of the string at the lowest expected ambient temperature — not just the STC nameplate Voc. Record the string identifier and test time before initiating the sweep.
7. 7
   Capture the IV Curve and Record Simultaneous Environmental Data Initiate the IV sweep. At the exact time of the sweep, record GF (front-side POA), GR (rear-side POA), and module temperature. If your IV tracer has an integrated reference cell input, confirm it is reading front-side POA irradiance — not GHI from a horizontal sensor. For commissioning documentation, capture at least three sweeps per string and use the median or average of stable, consistent results.
8. 8
   Calculate Effective Irradiance (Geff) Combine front and rear irradiance using the string's module φPmax bifaciality coefficient from the module datasheet (per IEC TS 60904-1-2):

   Geff = GF + (GR x φPmax)

   This is the irradiance value that enters your STC correction. Using GF alone will systematically understate corrected Pmax and produce a commissioning baseline that does not reflect the string's actual operating condition.
9. 9
   Apply STC Correction per IEC 60891 Method 4 IEC 60891:2022 Using Geff and measured module temperature, apply Method 4 correction to translate the field IV curve to STC. Enter Geff — not GF — as the irradiance input. Verify that your IV tracer supports a custom Geff input rather than relying on a fixed reference cell reading. A tracer that accepts only a direct reference cell value cannot perform a correct bifacial STC correction without modification of the input.
10. 10
    Compare to Nameplate and Flag Anomalies Compare the STC-corrected Pmax, Isc, and Voc against the module nameplate multiplied by the number of modules in the string. Note that bifacial module datasheets typically state front-side Pmax only; the rear-side contribution is captured in your Geff and will be reflected in the corrected result. A string result more than 3–5% below the expected nameplate value warrants investigation before sign-off — check for wiring errors, reversed polarity, module damage, or shading before accepting the result as a performance baseline.
11. 11
    Record and Archive Commissioning Results For each string tested, document and archive: string ID, test date and time, GF, GR, Geff, φPmax used, measured field Voc, Isc, and Pmax, STC-corrected Voc, Isc, and Pmax, fill factor, module temperature, ambient temperature, and weather conditions. This record is the reference baseline for all future O&M performance comparisons, warranty claims, and degradation assessments. Store raw IV curve files alongside the summary data.

Common Commissioning Mistakes to Avoid

Using Front-Side Irradiance Alone for STC Correction

The most consequential error in bifacial string commissioning. Correcting to STC using only front-side POA irradiance systematically understates the corrected Pmax of every string, establishing a commissioning baseline that appears to show underperformance when the system is actually operating correctly. Every future O&M measurement will then compare against a baseline that was already low, masking real degradation. Always calculate Geff using φPmax for IV curves and φIsc for Isc measurements.

Testing Without Stabilising Conditions First

IV curves captured under rapidly changing irradiance or with the array warming up after initial energisation will produce inconsistent results that cannot be reliably corrected to STC. Allow the array to stabilise thermally and wait for steady irradiance conditions — ideally above 700 W/m² with less than ±2% variation — before beginning commissioning tests.

Placing the Rear Sensor Near the Lower Module Edge

NREL's bifacial field research consistently shows rear irradiance non-uniformity of 30% or more between the lower module edge and the module centre. A rear sensor positioned at the lower edge will underread the average rear irradiance seen by the module, understating Geff and producing corrected results that overstate apparent performance. Position rear sensors at mid-module height.

Testing Edge Strings and Calling Them Representative

End-of-row and array-edge strings see higher rear irradiance than mid-array strings due to unobstructed ground reflection on at least one side. Using them to establish a commissioning baseline will set a standard that mid-array strings cannot meet under the same conditions. Test mid-array, mid-row strings for baseline documentation, and note edge strings separately if tested.

Not Recording the Ground Surface Condition

Ground albedo directly affects rear irradiance and therefore Geff. Commissioning tests conducted over bare disturbed soil will produce different Geff values than the same strings measured six months later over established grass cover. Document the actual ground surface condition with photographs on the day of commissioning. This context is essential when comparing future O&M measurements against the commissioning baseline.

Skipping the Bifaciality Coefficient

The φPmax value from the module datasheet (measured per IEC TS 60904-1-2) is the correct weighting factor for rear irradiance in the Geff calculation. Do not substitute 1.0 or omit the coefficient entirely — this is mathematically incorrect and will produce inflated Geff values, leading to overcorrected STC results that appear artificially high at commissioning and create a false baseline for future comparison. If the datasheet does not list φPmax separately from φIsc, contact the manufacturer for the IEC TS 60904-1-2 test report.

Failing to Archive Raw IV Curve Files

Summary tables of Voc, Isc, and Pmax values are useful but insufficient for commissioning documentation. Raw IV curve files contain the full current-voltage characteristic and can be re-analysed later with different correction coefficients, used to identify degradation modes that are not visible in summary parameters, or submitted as evidence in warranty disputes. Archive raw files alongside summary data for every string tested.

Quick Reference: Standards and Their Role in Field Testing

Final Thoughts

String-level commissioning of bifacial arrays requires two things that monofacial commissioning does not: a rear irradiance sensor and the discipline to apply the correct Geff calculation before correcting results to STC. Neither is complicated, but both are easy to skip under site pressure — and skipping either produces a commissioning baseline that cannot be defended against a future performance dispute.

The most important output of a commissioning test campaign is not the individual string results — it is the baseline dataset. A correctly documented IV curve for every string, with Geff-corrected STC values and archived raw files, is what allows any future O&M technician to determine whether a string has degraded, whether a replacement module matches the original, or whether a warranty claim is supported by evidence. Investing the additional time to capture rear irradiance at commissioning pays off every time that dataset is referenced over the life of the asset.

Use Workflow A (Isc screening) to move quickly across the full array and flag strings with anomalous current output. Use Workflow B (IV curve tracing) to establish the documented STC baseline on every string, or at minimum on flagged strings and a representative sample. Both workflows require the same site setup and rear irradiance instrumentation — the investment in getting that right at commissioning is not duplicated effort.

Questions about specific test equipment for bifacial commissioning? Browse our equipment options or get in touch. We're happy to talk through what makes sense for your project type and scale.