Short definition
The German term Anlagennutzungsgrad generally describes the utilization or technical performance of a system. However, the term is not used consistently. In PV monitoring, it sometimes refers to the Performance Ratio (PR), which is the interpretation used in this article.
System utilization ratio at a glance
- The PR compares the specific AC energy yield of a PV system with its irradiation-based reference yield.
- Accounting for the available solar energy allows different periods to be compared more meaningfully.
- Deviations may indicate yield losses, technical faults or incorrect measurements.
- The term Anlagennutzungsgrad may also refer to the capacity factor, which measures something different.
How is the Performance Ratio calculated?
The PR compares the measured AC energy yield of the PV system with a reference yield derived from the irradiation in the plane of the modules:
The variables are defined as follows:
- : measured AC energy yield at the defined measurement point
- : installed DC nominal power of the PV system under Standard Test Conditions
- : plane-of-array irradiation over the period under consideration
- : reference irradiance of
The quotient of the AC energy yield and the installed DC nominal power is referred to as the final system yield. Dividing the irradiation by the reference irradiance gives the irradiation-based reference yield. Both values have the dimension of time, making their ratio dimensionless and commonly expressed as a percentage.
The measurement point for the AC energy yield must be clearly defined. Depending on whether the yield is measured at the inverter output, downstream of a transformer or at the grid connection point, different conversion, cable and transformer losses are included in the PR.
A PR of 100% represents an idealized system without losses relative to the underlying reference conditions. Over longer periods, actual values are normally lower. Relevant causes include:
- Inverter, cable and transformer losses within the defined system boundary
- Elevated module temperatures
- Shading and soiling
- Mismatch between modules
- Curtailment and system downtime
- Auxiliary energy consumption by technical components within the defined system boundary
Values above 100% can occur over short intervals. Low module temperatures may temporarily allow modules to generate more than their nominal power determined under Standard Test Conditions. Implausible values can also result from unsuitable sensor data, inconsistent measurement intervals or inadequate time synchronization.
Using the Performance Ratio in PV monitoring
The PR supports the assessment of system performance without evaluating the energy yield solely based on the installed PV capacity. Accounting for irradiation considers differences in the available solar energy. Other influences, particularly module temperature, remain included in the conventional PR.
Typical applications include:
- Comparison over time: The PR can be calculated for days, months or years to identify changes in system performance.
- Comparison of multiple systems: PV systems of different sizes and at different locations can be compared using their PR. This requires comparable calculation methods, measurement points, system boundaries and data quality.
- Identification of deviations: A decreasing or unusually low value may indicate soiling, shading, inverter failures, system downtime or measurement problems.
- Operational analysis: Operators can assess how repairs, cleaning or technical modifications affect system performance.
The PR is not an independent fault diagnosis. An unusual value initially indicates a deviation whose cause must then be investigated using additional measurements, operating states and event data.
Difference between Performance Ratio and capacity factor
The German term Anlagennutzungsgrad may refer not only to the PR but also to the capacity factor, which describes the utilization of the installed nominal power over time:
The PR and capacity factor answer different questions:
| Metric | Reference values | Meaning |
|---|---|---|
| Performance Ratio | Energy yield and irradiation | How high is the actual specific yield in relation to the irradiation-based reference yield? |
| Capacity factor | Energy yield and theoretical maximum yield | What proportion of the energy theoretically available at continuous nominal power was generated? |
The capacity factor depends strongly on the location, weather, season and system orientation. The PR accounts for the available solar energy and is therefore better suited to assessing technical system performance. Reports should explicitly state which of the two metrics is meant by Anlagennutzungsgrad.
Measurements, limitations and technical requirements
A reliable PR calculation requires at least the AC energy yield at the defined measurement point, the installed DC nominal power and representative plane-of-array irradiation data. Measurement points, evaluation periods and measurement intervals must be clearly defined and synchronized.
The position, orientation, cleanliness, calibration and accuracy of the irradiance sensor have a significant effect on the result. The sensor should represent the irradiation conditions of the PV modules as closely as possible. For subsystems with different inclinations or orientations, a single irradiance measurement may be insufficient. Multiple sensors or separate PR calculations that are subsequently weighted by installed capacity may be appropriate.
It must also be defined how curtailment, system downtime, data gaps and implausible measurements are handled. Temperature effects remain included in the conventional PR. Additional module and ambient temperature measurements or temperature-corrected metrics may therefore be useful for more detailed root-cause analysis.
How can EcoPhi support the evaluation?
EcoPhi can collect, combine and visualize PV yields, system capacity and irradiation data from suitable meters and device interfaces. Based on these measurements, the PR can be calculated on a project-specific basis for defined periods and displayed in dashboards.
The evaluation of deviations or the use of defined PR thresholds for alarms can also be configured on a project-specific basis. The specific implementation depends on the available data sources, measurement intervals, device interfaces and project requirements. Comparing multiple systems reliably also requires standardized calculation and data-cleaning rules.
System utilization ratio summarized
The German term Anlagennutzungsgrad is not defined unambiguously. In PV monitoring, it may refer to the Performance Ratio, which relates the AC energy yield to the available plane-of-array irradiation. The calculation method, measurement point, system boundary and data processing must be defined clearly to obtain meaningful results.
Frequently asked questions about the system utilization ratio
What is a good system utilization ratio for a PV system?
There is no universally applicable target value. The PR depends on factors such as the system design, measurement point, module temperature, shading, curtailment and measurement quality. Comparisons with technically and metrologically comparable systems or previous periods are usually more meaningful.
What should be checked if the Performance Ratio decreases?
First, irradiation and yield data should be checked for data gaps, synchronization errors and implausible values. Operating states, inverter availability, soiling, shading, curtailment and module temperatures can then be investigated.
Is the system utilization ratio the same as the capacity factor?
Not necessarily. The German term Anlagennutzungsgrad may refer to either the PR or the capacity factor. Reports should therefore always specify the exact metric, including its formula and reference values.
What irradiation data are required for the PR?
The plane-of-array irradiation is required for the same period as the measured energy yield. Multiple irradiance sensors or separate calculations may be necessary for subsystems with different orientations.