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 61724-1 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 energy yield assessment 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.
Quick answer. IEC 61724-1 is the IEC standard for PV system performance monitoring. Its 2021 edition defines two accuracy classes: Class A for utility-scale and lender-grade reporting, and Class B for operational monitoring; the basic Class C of the 2017 edition was dropped. Class A records data at intervals of 1 minute or less, and Performance Ratio is the headline metric.
TL;DR
- IEC 61724-1 defines monitoring accuracy classes, A (lender-grade) and B (operational), plus a basic Class C in the 2017 edition, with matching irradiance sensor, energy meter, and temperature sensor specifications for each.
- Performance Ratio (PR = Y_f / Y_r) is the core metric the standard defines, along with Reference Yield, Final Yield, and temperature-corrected PR for cross-climate comparison.
- DFI-financed projects usually specify Class A (1-minute records) or Class B monitoring; Indian MNRE/CERC requirements are generally less stringent and typically use 15-minute intervals.
- A common failure on newly commissioned Indian plants is a DAS left at a 15-minute default when the contract specifies Class A (1-minute) data.
- The DPR monitoring system section should state the IEC 61724 class, sensor specifications, data resolution, and SCADA integration to avoid disputes during the performance guarantee period.
What Is the IEC 61724 Standard? 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 measurement accuracy classes for the primary measured parameters. The 2017 edition had Classes A, B and C; the current 2021 edition keeps Classes A and B. The class descriptions below are a practical summary, so check the exact sensor requirements in the standard.
Class A: High Accuracy / Lender-Grade
Highest accuracy. Irradiance: high-grade thermopile pyranometer (ISO 9060:2018 Class A, formerly "secondary standard") or a high-accuracy reference cell. Temperature: calibrated platinum resistance thermometer (PT100). Energy: revenue-grade meter (IEC 62053 Class 0.2S or 0.5). Used for energy yield verification, long-term PPA compliance monitoring, and lender reporting.
Class B: Medium Accuracy / 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 (2017 Edition Only)
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 (e.g. Kipp & Zonen CMP10/CMP11, Hukseflux SR20) | A | ±1.0-2.0% daily irradiance | Yield verification, lender reporting, research |
| ISO 9060 Class B pyranometer (e.g. Kipp & Zonen CMP6, Hukseflux SR15) | 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 yield 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 (see our performance ratio benchmarks for typical values):
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 power temperature coefficient (negative, e.g. −0.0035/°C) and T_actual is the irradiance-weighted average module temperature during the period. In hot weather the correction raises PR, because it adds back the temperature loss. IEC 61724-1 applies the correction interval by interval; the formula above is the period-average approximation.
Temperature-corrected PR is useful for tracking a plant from season to season and for comparing plants in hot climates (India, MENA) with plants elsewhere. To check a plant against its PVsyst model, compare the uncorrected measured PR with the simulated PR, because PVsyst already includes temperature losses.
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 sets the maximum recording interval by class: 1 minute or less for Class A and up to 15 minutes for Class B. Beyond that, the practical resolution and storage depend on the application:
| Application | Practical Data Resolution | Typical Storage |
|---|---|---|
| Yield verification / lender reporting | 1-minute intervals | Project life |
| 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. A common IEC 61724 compliance failure in newly commissioned Indian utility-scale plants: the DAS (Data Acquisition System) records 15-minute data when the contract specifies Class A (1-minute), because the installer left the default SCADA settings. Specifying the IEC 61724 class and recording interval to the SCADA vendor at procurement, 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 Yield Reports
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 (Class A), ≤15 minutes (Class 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 the MNRE DCR compliance minimums. IEC 61724 Class B monitoring satisfies MNRE requirements, while many DFI lenders specify Class A for 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 where Class A is specified) | Cannot detect transient faults, soiling events or shading patterns accurately | Re-configure the DAS to 1-minute records; past 15-minute data stays usable for operational monitoring only |
| Module temperature sensor not mounted per IEC 61724 | Temperature measurement not representative of actual operating temperature | Mount the sensor on the module rear as IEC 61724-1 describes (on a representative module, away from the frame and junction box) |
| Missing data gaps not documented | PR calculation over periods with data gaps is incorrect | Implement automated data gap detection and filling per IEC 61724 guidance, and document it |
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.
For utility-scale developers who need the monitoring specification carried through to actual construction and commissioning, Heaven Green Energy’s ground mount solar park EPC team handles DAS installation and SCADA integration alongside the civil and electrical build.
Related posts: IEC 62548 PV Array Design Standard | IEC 62446 Acceptance Testing | PVsyst Loss Diagram Interpretation | 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: lenders expect a monitoring system specification in the DPR; DFI-financed projects usually specify 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 or high-accuracy reference cells, calibrated temperature sensors, revenue-grade meters) with records at 1-minute intervals or shorter. Class B uses medium-accuracy instruments and allows records up to 15 minutes.
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, an ISO 9060 Class B thermopile pyranometer (for example Kipp & Zonen CMP6) with regular recalibration is a common choice. For DFI-financed projects or Class A compliance, specify an ISO 9060 Class A pyranometer (for example CMP10/CMP11 or Hukseflux SR20) and follow the manufacturer’s recalibration interval (often every two years).
Silicon reference cells (like those built into SolarEdge or Huawei SCADA systems) are acceptable for operational monitoring. 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?
IEC 61724-1 allows records up to 15 minutes for Class B and requires 1 minute or less for Class A. For performance guarantee tracking, many contracts specify 5 minutes or shorter. 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.