IEC 61724-1 Monitoring
Defines monitoring system classes A/B/C, the parameters to record (irradiance, temperature, AC/DC power, energy), sensor accuracy and placement, sampling and data quality — the foundation for any performance claim.
IEC 61724
Photovoltaic system performance monitoring
IEC 61724 defines how to monitor and evaluate the performance of a photovoltaic system: the Performance Ratio, monitoring classes A/B/C, sensor accuracy, and the methods to verify capacity and energy against guarantees. It turns 'is the plant working?' into a measurable, contestable number.
Document structure
IEC 61724-2 Capacity evaluation method
A short-test method to verify a plant's power capacity against a target, correcting measured output to reference conditions.
IEC 61724-3 Energy evaluation method
A method to verify energy delivered over a longer period against a model — the basis of energy performance guarantees in EPC and O&M contracts.
Key concepts
- Performance Ratio
(PR) - The headline metric: actual energy produced divided by the energy theoretically possible from the measured irradiance and the installed capacity. PR strips out the weather, isolating how well the system itself performs. Good plants sit around 0.80-0.85.
- Reference, array and final yield
- Reference yield (from irradiance), array yield (DC output) and final yield (AC energy delivered), all normalised per kWp. Their ratios localise losses — array-to-final gaps point to inverter or AC-side losses; reference-to-array gaps to module, shading or DC losses.
- Monitoring classes A / B / C
- Three tiers of monitoring rigour. Class A is the highest accuracy (large or high-value plants, performance guarantees); Class C is basic (small systems). The class sets the required sensor accuracy, redundancy and recording.
- Irradiance sensor: pyranometer vs reference cell
- Plane-of-array irradiance is the denominator of the PR, so its sensor matters. Thermopile pyranometers are accurate but slow; reference cells match the modules' spectral and angular response. Class A demands the tighter accuracy.
- Temperature correction
- Module power falls as temperature rises. A temperature-corrected PR separates real under-performance from a simply hot day, making PR comparable across seasons and sites.
- Availability
- The fraction of time the plant (or each inverter) was able to produce. Distinguishing energy lost to faults/downtime from energy lost to low irradiance is essential to a fair performance assessment.
- Capacity vs energy evaluation
- Part 2 answers 'does it reach rated power?' in a short test; Part 3 answers 'does it deliver the contracted energy?' over weeks or months. Contracts usually rely on one or both to settle performance guarantees.
Notes & guidance
From ‘is it working?’ to a number
Anyone can see a solar plant is producing. The hard question is whether it is producing as much as it should, given the sunshine it actually received. IEC 61724 answers that with the Performance Ratio and a defined monitoring method — turning a vague impression into a measurable, contestable figure that contracts and lenders can rely on.
Performance Ratio — the headline metric
The Performance Ratio (PR) is the actual energy produced divided by the energy theoretically possible from the measured irradiance and the installed capacity. Because it normalises against the sunlight the plant actually saw, PR isolates the system’s own behaviour from the weather. Well-run plants sit around 0.80-0.85; a falling PR is the first sign of soiling, degradation, inverter faults or shading.
The yields behind it — reference, array and final yield — localise where energy is lost: between DC and AC (inverter/AC side) versus before the array (modules, shading, DC).
Monitoring classes A / B / C
A PR is only as trustworthy as the sensors behind it. IEC 61724-1 defines three classes of monitoring rigour: Class A (highest accuracy — large plants, performance guarantees), B, and C (basic, small systems). The class fixes the required accuracy of irradiance and temperature sensors, redundancy and data recording. Getting the irradiance sensor right — pyranometer vs reference cell — matters because it is the denominator of every PR.
Verifying capacity and energy
Parts 2 and 3 turn monitoring into contract proof. Part 2 (capacity) verifies the plant reaches rated power in a short test, correcting to reference conditions. Part 3 (energy) verifies the energy delivered over weeks or months against a model. These are the methods that settle the performance guarantees written into EPC and O&M contracts — the field counterpart to the factory qualification of IEC 61215 and the commissioning of IEC 62446.
Applicable industries
- Utility-scale and commercial solar (O&M, asset management)
- EPC contractors and performance-guarantee settlement
- Independent monitoring and technical advisors
- Lenders verifying yield assumptions