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IEC TS 61724-3: Photovoltaic System Performance — Energy Evaluation Method
Tip: IEC TS 61724-3 defines a standardized methodology for evaluating the energy performance of photovoltaic (PV) systems. It is the essential reference for commissioning tests, performance guarantee verification, and ongoing monitoring of grid-connected PV power plants.
1. Scope and Purpose of PV Energy Evaluation
IEC TS 61724-3 is the third part of the IEC 61724 series, which addresses photovoltaic system performance monitoring and evaluation. While IEC 61724-1 establishes the general monitoring requirements and IEC 61724-2 covers the capacity test (power measurement), this part focuses specifically on energy evaluation — comparing the actual energy yield of a PV system against the expected yield predicted by simulation models under actual weather conditions. The standard provides a systematic framework for quantifying the difference between predicted and measured energy production and for attributing this difference to specific causes.
The scope of IEC TS 61724-3 applies to grid-connected PV systems of all sizes, from small rooftop installations to utility-scale solar farms. The standard defines the energy evaluation period, which should be a minimum of 12 continuous months to capture seasonal variations in solar resource and temperature effects. However, for commissioning purposes, the standard also defines procedures for shorter evaluation periods (typically 1 to 3 months) with appropriate uncertainty adjustments. The evaluation is based on comparing the measured energy at the point of common coupling (the utility meter) with the simulated energy calculated using on-site weather data.
Warning: A critical distinction in IEC TS 61724-3 is between the “capacity test” (IEC 61724-2) and the “energy evaluation” (IEC 61724-3). A PV system can pass a capacity test under clear-sky conditions yet significantly underperform in annual energy terms due to partial shading, soiling, inverter clipping at non-ideal times, or sub-optimal tilt/orientation. Both tests are necessary for comprehensive performance assessment.
The standard introduces several key performance metrics. The performance ratio (PR) is the ratio of net energy output to the theoretical energy that would be produced if the system operated at its rated efficiency under the actual irradiance conditions. A PR of 0.80 means the system delivers 80% of the energy that would be expected from a perfectly operating system with no losses. The final yield (Yf) is the net energy output divided by the rated array power (kWh/kWp), while the reference yield (Yr) is the total in-plane irradiance divided by the reference irradiance (1000 W/m2). The PR is the ratio Yf/Yr, normalising out the effects of local solar resource variability.
2. Energy Evaluation Methodology and Procedures
IEC TS 61724-3 prescribes a step-by-step methodology for energy evaluation. The process begins with the definition of the evaluation boundary — typically the point of common coupling where utility metering occurs. All energy flows within this boundary must be accounted for, including parasitic loads from inverters, trackers, and monitoring equipment. The standard then requires the development of a simulation model calibrated to the as-built system configuration, using on-site measured weather data (irradiance, ambient temperature, wind speed) as inputs.
Key Performance Metrics per IEC TS 61724-3:
Performance Ratio: PR = Yf / Yr
Final Yield: Yf = Enet / Prated (kWh/kWp)
Reference Yield: Yr = Hpoa / Gref (hours)
Energy Loss: Ltot = Yr − Yf
Where:
Enet = net energy exported (kWh)
Prated = rated array power (kWp)
Hpoa = total in-plane irradiance (kWh/m2)
Gref = reference irradiance = 1 kW/m2
The standard defines specific procedures for data quality assurance, which is arguably the most challenging aspect of PV energy evaluation. Irradiance sensors must be calibrated annually and maintained to within ±3% accuracy. Temperature sensors should be mounted on the back of representative modules. The data acquisition system must record at intervals no longer than 10 minutes, with 1-minute or finer resolution recommended for systems with rapid cloud transients. Missing data periods exceeding 5% of the evaluation period require special treatment, and the standard provides statistical imputation methods for gaps up to 10%.
| Evaluation Type | Minimum Duration | Typical Uncertainty | Primary Application |
|---|---|---|---|
| Short-term commissioning | 1–3 months | ±8–12% | Performance guarantee verification, defect identification |
| Annual evaluation | 12 months | ±3–5% | Contractual compliance, degradation tracking |
| Long-term monitoring | >3 years | ±2–4% | Degradation rate determination, O&M optimisation |
The uncertainty analysis required by IEC TS 61724-3 is one of its most important contributions. The standard mandates that all energy evaluation results be accompanied by a quantitative uncertainty statement, following the ISO Guide to the Expression of Uncertainty in Measurement (GUM). Sources of uncertainty include pyranometer calibration (±2–3%), temperature measurement (±0.5–1 °C), electrical measurement (±0.5–2%), and simulation model assumptions (±2–5%). The combined uncertainty for an annual evaluation typically ranges from ±3% to ±5%. This uncertainty must be explicitly considered when determining whether a system meets its performance guarantee — a measured PR of 0.78 with ±4% uncertainty may be statistically indistinguishable from a guaranteed PR of 0.80.
Good Practice: When designing a PV system with a performance guarantee, specify the evaluation methodology, uncertainty treatment, and dispute resolution process in the contract. Reference IEC TS 61724-3 explicitly and agree on the simulation tool, weather data source, and data quality criteria before construction begins. This prevents costly disagreements during the commissioning phase.
3. Engineering Design Insights for PV Energy Performance
IEC TS 61724-3 reveals several engineering insights that directly influence PV system design and operation. The most significant is the importance of soiling losses — the accumulation of dust, pollen, bird droppings, and other contaminants on module surfaces. In many regions, soiling causes annual energy losses of 3–5%, but in arid or agricultural areas, losses can exceed 15% without adequate cleaning. The standard requires that soiling be either directly measured (using soiling stations with cleaned and naturally soiled reference cells) or estimated with appropriate uncertainty. For design engineers, this means incorporating soiling assumptions into energy yield models and designing cleaning access into the plant layout.
A second critical insight relates to inverter clipping and DC/AC ratio optimisation. Modern PV systems often install more DC capacity than inverter AC capacity (DC/AC ratios of 1.1 to 1.4), trading off clipping losses at high irradiance against increased energy capture during low-light conditions. IEC TS 61724-3 requires that clipping losses be explicitly accounted for and validated against measurements. The standard provides guidance on distinguishing between clipping losses (which occur at high irradiance and are expected by design) and other losses such as inverter derating or curtailment (which indicate operational issues).
| Loss Category | Typical Range | IEC 61724-3 Treatment | Engineering Mitigation |
|---|---|---|---|
| Soiling | 2–15% annual | Measured or estimated with uncertainty | Automated cleaning, anti-soiling coatings, tilt optimisation |
| Shading (near/far) | 1–10% annual | Simulated from 3D model | String-level MPPT, module-level optimisers, layout redesign |
| Temperature | 3–8% annual | Calculated from measured temperature | Increased mounting height, bifacial modules, albedo enhancement |
| Inverter efficiency | 1–3% annual | From inverter model or measured | Oversizing inverters, multi-MPPT design, high-efficiency topology |
| Cable losses | 0.5–2% annual | Calculated from design | Oversized cables, higher voltage, distributed inverters |
| Degradation | 0.3–1.0%/year | Linear or non-linear model | Low-degradation module technology, PID-resistant cells |
Critical: IEC TS 61724-3 highlights that soiling loss uncertainty is frequently the largest uncontrolled contributor to the overall evaluation uncertainty. A PV plant with no soiling measurement capability may have soiling-related uncertainty of ±5% or more, completely overwhelming the evaluation’s ability to detect genuine performance issues. Installing soiling monitoring stations is a relatively low-cost investment that dramatically improves evaluation confidence.
For engineers involved in PV plant commissioning, the standard provides a structured approach to root cause analysis when measured performance deviates from expectations. The process begins by confirming the quality of the weather and electrical data, then systematically eliminating potential causes: irradiance sensor calibration drift, inverter-specific performance issues, string-level mismatch, soiling, and simulation model inaccuracies. The standard recommends a tiered approach, starting with plant-level analysis and progressively drilling down to inverter, string, and module levels as needed. This methodology has proven effective in diagnosing issues ranging from simple sensor misalignment to complex partial-shading and string mismatch problems.
Finally, IEC TS 61724-3 emphasises the role of continuous monitoring beyond the initial evaluation period. The annual performance ratio trend is one of the most valuable indicators of PV plant health. A declining PR trend may indicate progressive soiling, increasing module degradation, or accumulating balance-of-system failures. By establishing the evaluation methodology during commissioning, the standard enables consistent long-term performance tracking that directly supports operations and maintenance decision-making.
Frequently Asked Questions
Q1: What is the difference between IEC 61724-2 (capacity test) and IEC 61724-3 (energy evaluation)?
IEC 61724-2 measures instantaneous power output under specific clear-sky conditions to verify that the system can achieve its rated capacity. IEC 61724-3 measures cumulative energy production over weeks to months under actual weather conditions. A system can pass a capacity test but fail an energy evaluation due to issues like soiling, partial shading, or inverter clipping patterns that only manifest over time.
Q2: What weather data is required for an IEC 61724-3 energy evaluation?
The standard requires at minimum: plane-of-array (POA) irradiance measured by a pyranometer or reference cell, ambient temperature, and wind speed. Additional recommended measurements include module temperature (back-surface), spectral irradiance, and albedo (for bifacial systems). The weather station should be located within the PV plant boundary and maintained according to the manufacturer’s specifications with annual calibration.
Q3: How is the performance ratio (PR) normalised for temperature effects?
IEC TS 61724-3 defines both the “simple” PR (which includes temperature effects as a loss) and the “temperature-corrected” PR (which normalises for temperature). The temperature-corrected PR is more useful for comparing system performance across different climates or seasons. It adjusts the measured energy to what would have been produced at a standard module temperature (typically 25 °C), calculated using the module’s power temperature coefficient provided by the manufacturer.
Q4: Can IEC TS 61724-3 be applied to bifacial PV systems?
Yes, but with additional considerations. IEC TS 61724-3 provides guidance specific to bifacial systems, including the need for rear-side irradiance measurement (using albedometers or dedicated rear-side reference cells), the use of bifacial module models that account for rear-side contribution, and the higher uncertainty associated with bifacial energy yield modelling. The standard acknowledges that bifacial system evaluation uncertainty is typically 2–3% higher than for monofacial systems due to the complexity of rear-side irradiance estimation.
