IEC 61724 — Photovoltaic System Performance — is the international standard that defines how solar PV plant performance is measured, calculated, and reported. It establishes the measurement accuracy requirements for irradiance sensors, temperature sensors, and energy meters; defines the standard performance metrics (Performance Ratio, Yield, Capacity Factor); and specifies the data acquisition requirements for monitoring systems.
For solar engineers, project developers, and EPC companies, IEC 61724 matters in three contexts: (1) specifying the monitoring system for a new plant; (2) calculating and reporting Performance Ratio (PR) in the Detailed Project Report (DPR) and IEA for lender review; and (3) post-commissioning O&M performance tracking against the bankable yield report’s P50 target. Getting IEC 61724 right — particularly the sensor class and data resolution requirements — is the difference between a monitoring system that produces defensible performance data and one that generates disputes between the EPC contractor and the project owner during the warranty period.
Direct answer. IEC 61724 defines three classes of measurement accuracy for PV monitoring: Class A (highest accuracy; required for bankable IEA verification and long-term performance reporting), Class B (intermediate accuracy; suitable for operational monitoring), and Class C (lowest accuracy; suitable for basic production tracking only). For utility-scale projects financed by IREDA, IFC, or tax equity investors, Class A or Class B monitoring with 1-minute or 5-minute data resolution is the standard requirement. Performance Ratio (PR) calculated per IEC 61724 is the primary metric for comparing simulated yield against measured production.
IEC 61724 Structure — The Three Parts
IEC 61724 is a multi-part standard:
| Part | Title | Relevance |
|---|---|---|
| IEC 61724-1 | Monitoring — Measurement, data exchange, and analysis | Primary specification for monitoring system design |
| IEC 61724-2 | Capacity evaluation method | Verifying nameplate capacity through measured production |
| IEC 61724-3 | Energy evaluation method | Evaluating long-term energy production against benchmarks |
For most project finance and O&M applications, IEC 61724-1 is the relevant document — it defines what to measure, how accurately, and how to calculate the key performance metrics.
Measurement Class Definitions
IEC 61724-1 defines three measurement accuracy classes (A, B, C) for the primary measured parameters.
Class A — Research Grade / Bankable
Highest accuracy. Irradiance: thermopile pyranometer (ISO 9060 Class A or secondary standard). Temperature: calibrated platinum resistance thermometer (PT100). Energy: revenue-grade meter (IEC 62053 Class 0.2S or 0.5). Required for IEA performance verification, long-term PPA compliance monitoring, and lender reporting.
Class B — Field Grade / Operational
Intermediate accuracy. Irradiance: pyranometer or calibrated reference cell. Temperature: calibrated thermocouple or PT100. Energy: revenue-grade or high-accuracy sub-meter. Suitable for routine O&M monitoring and performance guarantee tracking. Acceptable for most operational reporting contexts.
Class C — Basic Grade / Production Tracking
Lowest accuracy. Irradiance: uncalibrated reference cell or modeled irradiance. Temperature: basic thermocouple or ambient temperature sensor. Energy: standard utility meter or inverter-based production estimate. Not suitable for bankable reporting; adequate only for basic production tracking and fault detection.
Sensor Specifications by Class
Irradiance Sensors:
| Sensor Type | Class | Typical Uncertainty | Use Case |
|---|---|---|---|
| ISO 9060 Class A pyranometer (Kipp & Zonen CMP11, Hukseflux SR20) | A | ±1.0–1.5% daily irradiance | IEA verification, lender reporting, research |
| ISO 9060 Class B pyranometer (Kipp & Zonen CMP6/10, Hukseflux SR05) | B | ±2.0–3.0% daily | O&M monitoring, performance guarantees |
| Calibrated silicon cell reference (SolarEdge, Huawei reference cell) | B–C | ±3.0–5.0% | Operational monitoring; spectral corrections not included |
| Modeled irradiance from nearby weather station | C | ±5.0–10.0% | Basic production tracking only |
Energy Meters:
| Meter Class | IEC Standard | Accuracy | Use Case |
|---|---|---|---|
| Class 0.2S | IEC 62053-22 | ±0.2% | Revenue metering, PPA billing, DFI lender reporting |
| Class 0.5S | IEC 62053-22 | ±0.5% | Revenue metering; adequate for most IEA verification |
| Class 1.0 | IEC 62053-21 | ±1.0% | General energy billing |
| Inverter production counter | Manufacturer spec | ±2.0–5.0% | Internal monitoring only; not for contractual reporting |
Module/Ambient Temperature Sensors:
| Sensor Type | Class | Notes |
|---|---|---|
| PT100 / PT1000 (calibrated, 4-wire) | A | Best-practice for module temperature measurement |
| Thermocouple Type K (calibrated) | A–B | Acceptable; slightly lower accuracy than PT100 |
| Thermocouple Type K (uncalibrated) | B–C | Suitable for operational monitoring only |
| Ambient temperature from weather station | C | For PR calculation only if module temperature is modeled |
Key Performance Metrics Defined by IEC 61724
IEC 61724-1 defines the standard performance metrics used in solar reporting globally.
Performance Ratio (PR)
The most widely used metric for comparing actual performance to potential performance:
PR = E_AC / (H_i × P_0 / G_STC)
Where:
- E_AC = AC energy output measured at the AC meter (kWh)
- H_i = plane-of-array irradiation (kWh/m²) measured by the monitoring system
- P_0 = installed DC peak power at STC (kWp)
- G_STC = 1,000 W/m² (STC reference irradiance)
Interpretation:
PR = 0.80 means the system produced 80% of the energy it would have produced if it operated continuously at STC efficiency. The gap (20%) represents all combined losses: thermal, soiling, wiring, inverter, availability, degradation.
Temperature-Corrected PR:
IEC 61724 also defines temperature-corrected PR (PR_TC) which removes the temperature effect from the PR calculation, allowing fair comparison of systems in different climates:
PR_TC = PR × (1 + γ × (T_actual − T_STC))
Where γ is the module temperature coefficient and T_actual is the average module temperature during the measurement period.
Temperature-corrected PR is particularly useful for comparing the performance of systems in hot climates (India, MENA) with the simulated PR from PVsyst (which already models temperature losses) — if the measured PR_TC matches the simulated PR closely, the system is performing as designed.
Reference Yield (Y_r) and Final Yield (Y_f)
IEC 61724 defines a set of yield metrics that decompose overall performance:
| Metric | Formula | Interpretation |
|---|---|---|
| Reference Yield (Y_r) | H_i / G_STC (kWh/kWp) | Equivalent hours at STC irradiance; the “solar resource” at the site |
| Final Yield (Y_f) | E_AC / P_0 (kWh/kWp) | AC energy per unit of installed peak power |
| System Losses (L_s) | Y_r − Y_f (kWh/kWp) | Total system losses expressed as equivalent peak-sun hours |
| Performance Ratio | PR = Y_f / Y_r | Fraction of reference yield delivered as AC |
Example for a 10 MW Indian project, January:
- Measured H_i: 190 kWh/m² (plane-of-array irradiation)
- Measured E_AC: 1,460 MWh
- Y_r = 190 kWh/m² / 1 kW/m² = 190 kWh/kWp (reference hours)
- Y_f = 1,460,000 kWh / 10,000 kWp = 146 kWh/kWp
- PR = 146 / 190 = 0.768 (76.8%)
Data Acquisition Requirements
IEC 61724-1 specifies the data resolution requirements for monitoring systems:
| Application | Minimum Data Resolution | Storage Requirement |
|---|---|---|
| IEA verification / lender reporting | 1-minute intervals | 10+ years |
| Performance guarantee tracking | 5-minute intervals | Contract period + 2 years |
| O&M operational monitoring | 5–15 minute intervals | 5 years |
| Basic production tracking | 15–30 minute intervals | 2 years |
| Post-event analysis (fault investigation) | 1-minute or sub-minute | 6 months minimum |
SCADA and DAS (Data Acquisition System) Requirements:
For utility-scale plants, IEC 61724-compliant monitoring requires:
- Data logger at plant level — collecting data from all sensors and meters at the specified resolution
- Time synchronization — NTP (Network Time Protocol) synchronization for accurate timestamp correlation
- Data validation — automated checks for sensor out-of-range values, communication outages, and data gaps
- Data storage and backup — local storage with remote backup to prevent data loss
- Remote access — secure data access for lender reporting and O&M teams
DAS commissioning tip. The most common IEC 61724 compliance failure in newly commissioned Indian utility-scale plants: the DAS (Data Acquisition System) collects 15-minute interval data instead of 5-minute or 1-minute — because the installer used default inverter SCADA settings without verifying IEC 61724 resolution requirements. Requesting IEC 61724 Class B minimum compliance from the SCADA vendor at the procurement stage — and verifying the interval setting during DAS commissioning — prevents a data quality dispute when the independent engineer or lender's technical advisor reviews the first quarterly performance report.
IEC 61724 in the DPR and Bankable IEA
For Indian utility-scale projects, the Detailed Project Report (DPR) submitted to IREDA, PFC, or state DISCOM must include a monitoring system specification. IEC 61724 provides the framework for this specification.
DPR Monitoring System Section (IEC 61724-aligned):
- Monitoring system classification — State which IEC 61724 class (A or B) the monitoring system meets
- Irradiance sensor specification — Type (pyranometer vs reference cell), ISO 9060 class, calibration interval
- Energy meter specification — Type, IEC 62053 accuracy class, revenue-grade vs sub-meter
- Module temperature sensor — Type, calibration
- Data resolution — Interval (1 min, 5 min, 15 min), storage period
- SCADA integration — Connection to plant-level SCADA and remote monitoring
Annual Performance Report (for lender/investor reporting):
IEC 61724 provides the framework for the annual performance report structure:
- Monthly PR table (measured PR vs PVsyst P50 target)
- Monthly yield table (Y_f measured vs Y_f simulated)
- Loss analysis (comparing measured vs modeled loss categories)
- Availability data (scheduled and unscheduled downtime)
- Soiling loss validation (measured vs assumed in PVsyst)
IEC 61724 vs Indian MNRE Monitoring Requirements
| Requirement | IEC 61724 | MNRE/CERC Guidelines |
|---|---|---|
| Irradiance sensor | ISO 9060 Class A or B pyranometer | Weather station with irradiance sensor |
| Energy meter | IEC 62053 Class 0.2S–0.5 | Revenue-grade meter per CEA metering code |
| Data resolution | 1-minute or 5-minute (Class A/B) | 15-minute intervals typically specified |
| PR calculation | IEC 61724-1 formula | MNRE uses similar PR formula |
| Monitoring period | Continuous for project life | Per PPA terms |
| Reporting | Lender-facing quarterly reports | DISCOM and MNRE compliance reports |
MNRE’s monitoring requirements are generally less stringent than IEC 61724 Class A — but DFI-financed projects must meet IEC 61724 standards regardless of MNRE minimum requirements. IEC 61724 Class B monitoring effectively satisfies both MNRE requirements and DFI lender standards for most utility-scale projects.
Common Performance Monitoring Errors and IEC 61724 Fixes
| Error | Impact | IEC 61724 Fix |
|---|---|---|
| Pyranometer soiling uncorrected | Measured irradiance lower than actual → PR appears artificially high | Clean pyranometer on same schedule as modules; use separate clean reference sensor |
| Energy meter class too low (Class 1.0 instead of Class 0.5) | ±1.0% energy measurement error → PR uncertainty too large for lender reporting | Specify IEC 62053 Class 0.5 or better for main revenue meter |
| Data intervals too coarse (15 min instead of 5 min) | Cannot detect transient faults, soiling events, or shading patterns accurately | Re-configure DAS to 5-minute intervals; retroactively acceptable for operational monitoring, but re-configuration needed for IEA verification |
| Module temperature sensor not mounted per IEC 61724 | Temperature measurement not representative of actual operating temperature | Mount sensor on module rear per IEC 61724-1 Section 5.2 (center of a representative module, insulated from the frame) |
| Missing data gaps not documented | PR calculation over periods with data gaps is incorrect | Implement automated data gap detection and interpolation per IEC 61724 guidance |
How Heaven Designs Specifies Monitoring Systems for Bankable Projects
- Solar Ground Mount Design — Utility-scale design packages include IEC 61724-aligned monitoring system specification in the DPR and IFC design documents.
- Solar Rooftop Detailed Engineering Design — Commercial project design includes monitoring system specification aligned with IEC 61724 for C&I performance guarantee tracking.
- MW-Scale PMC — Owner’s engineer oversight of monitoring system commissioning and IEC 61724 compliance verification.
- Solar SCADA KPI Dashboard Guide — Operational SCADA KPIs that build on IEC 61724 performance metrics.
Related posts: IEC 62548 PV Array Design Standard | PVsyst Loss Diagram Interpretation | Solar Plant Performance Ratio Benchmarks | PVsyst Soiling Loss Modeling India and Africa
The IEC official website is the authoritative source for IEC 61724 and related solar performance standards. The NREL PV performance monitoring research provides technical validation of IEC 61724 measurement methodologies. The IEA PVPS Task 13 performance reports apply IEC 61724 metrics to global utility-scale performance benchmarking. The MNRE solar guidelines reference performance monitoring requirements that align with IEC 61724 principles for DPR documentation.
Glossary: PVsyst, Performance Ratio, P50/P90.
FAQ
What is IEC 61724 and why does it matter for Indian utility-scale solar?
IEC 61724 is the international standard for PV system performance monitoring — it defines how to measure irradiance, energy, and temperature; how to calculate Performance Ratio (PR) and Yield metrics; and what data resolution and storage requirements apply to the monitoring system. For Indian utility-scale projects, IEC 61724 matters because: IREDA and PFC require DPR monitoring system specifications; DFI-financed projects (ADB, IFC, AfDB) require IEC 61724 Class A or B monitoring; and the O&M performance guarantee tracking methodology references IEC 61724 PR calculations. Getting the monitoring system class and data resolution wrong at the design stage creates disputes during the performance guarantee period.
What is the difference between IEC 61724 Class A and Class B monitoring?
Class A monitoring uses the highest-accuracy instruments: ISO 9060 Class A thermopile pyranometers, calibrated PT100 temperature sensors, and IEC 62053 Class 0.2S revenue meters with 1-minute data resolution. Class B uses intermediate accuracy: ISO 9060 Class B pyranometers (or calibrated reference cells), calibrated temperature sensors, and Class 0.5 meters with 5-minute resolution. For utility-scale projects with DFI financing or strict performance guarantee terms, Class A is preferred. For operational monitoring of plants where lender reporting uses modeled rather than measured irradiance for PR calculation, Class B is typically sufficient.
How is Performance Ratio calculated per IEC 61724?
Per IEC 61724-1, PR = Y_f / Y_r, where Y_f is the Final Yield (AC energy delivered per kWp installed: kWh/kWp) and Y_r is the Reference Yield (plane-of-array irradiation divided by STC reference irradiance: kWh/kWp). Equivalently, PR = E_AC / (H_POA × P_STC / 1000). For a bankable performance report, both the energy meter and the irradiance measurement must meet IEC 61724 accuracy requirements. Using inverter production counters (typically ±3–5% accuracy) instead of calibrated revenue meters produces PR values with insufficient accuracy for lender reporting.
What irradiance sensor should I specify for a 10 MW Indian solar project?
For a 10 MW Indian project requiring IEC 61724 Class B compliance (suitable for most IREDA-financed and operationally monitored projects), specify a Kipp & Zonen CMP6 or CMP10 (ISO 9060 Class B thermopile pyranometer) with annual calibration. For DFI-financed projects or those requiring Class A compliance, specify a CMP11 or Hukseflux SR20 (ISO 9060 Class A secondary standard) with bi-annual calibration. Silicon reference cells (like those built into SolarEdge or Huawei SCADA systems) are acceptable for operational monitoring only — they are not ISO 9060 classified and have spectral sensitivity differences that introduce systematic errors in plane-of-array irradiance measurement.
What data interval does IEC 61724 require for a performance guarantee?
For performance guarantee tracking and lender reporting per IEC 61724, the minimum recommended data interval is 5 minutes (Class B) or 1 minute (Class A). Data at 15-minute intervals can calculate daily and monthly PR with acceptable accuracy but misses transient events (cloud-induced irradiance variability, short-duration faults) that affect individual day production. For O&M teams investigating underperformance, 1-minute data enables precise correlation of production drops with irradiance, soiling events, or inverter faults. Specify 5-minute intervals as the minimum in the SCADA procurement specification, with 1-minute as the preferred target for larger utility-scale plants.
