Capacity-Based Performance Ratio (PR) Calculator
The Capacity-Based Performance Ratio (PR) measures how effectively a solar PV plant converts available plane-of-array solar irradiance into electrical energy.
It is normalized using installed DC capacity and reference irradiance as defined in IEC 61724.
Inputs Used for Capacity-Based Performance Ratio
- Energy Generated (kWh): Total electrical energy produced by the plant during the selected period.
- Plant DC Capacity (kWp): The total nameplate DC capacity of the solar plant based on installed PV modules.
- POA Insolation (kWh/m²): The measured plane-of-array insolation (sunlight received) in kWh/m² during the same time period.
- Irradiance @ STC: Fixed reference value of 1 kW/m² as defined in IEC 61724. This value is not user-adjustable.
This calculator evaluates plant performance independently of system size by comparing actual energy yield against the theoretical energy available from measured solar insolation.
How to Interpret Capacity-Based Performance Ratio
Capacity-Based Performance Ratio should be interpreted as a long-term efficiency indicator under measured irradiance conditions, not as a real-time health signal.
- PR above 80%: Generally indicates strong operational performance when calculated using high-quality, time-aligned energy and irradiance data.
- PR between 70% and 80%: Common for operational plants where thermal losses, soiling, inverter losses, and other system inefficiencies reduce available energy.
- PR below 70%: Indicates significant losses or unreliable input data. Common causes include plant outages, soiling losses, irradiance sensor errors, or incorrect DC capacity values.
When Capacity-Based Performance Ratio Gives Misleading Results
Capacity-Based PR becomes unreliable when irradiance or DC capacity data does not accurately represent the PV array.
- Incorrect POA sensor tilt or orientation compared to the PV array.
- Dirty or uncalibrated irradiance sensors reducing measured insolation.
- Incorrect DC capacity values due to module replacements or degradation assumptions.
- Module temperature effects are not corrected and can lower PR during hot periods.
Worked Example
The example below shows how Capacity-Based Performance Ratio is calculated using real operational data from a solar power plant.
Input Values
- Energy Generated: 450 kWh
- Installed DC Capacity: 100 kW
- Measured Insolation: 5.5 kWh/m²
- Reference Irradiance (STC): 1.0 kW/m²
Calculation Result
Calculated Performance Ratio:
81.8%
Who Should Use Capacity-Based Performance Ratio
- Solar plant owners tracking long-term performance trends.
- O&M engineers diagnosing aggregate losses at plant level.
- Lenders and auditors evaluating operational performance during due diligence.
Technical FAQs on Capacity-Based Performance Ratio
Why is DC capacity used instead of AC capacity in this calculation?
IEC 61724 defines PR using installed DC capacity because generation occurs at the module level. Using AC capacity would mix inverter limits and clipping effects with generation performance.
How sensitive is capacity-based PR to irradiance sensor errors?
Capacity-based PR is highly sensitive to irradiance measurement accuracy. Even small POA sensor misalignment, soiling, or calibration drift can significantly inflate or depress PR values. Sensor validation is mandatory before using PR for diagnostics.
Is capacity-based PR suitable for comparing two different solar plants?
Direct comparison between different plants is risky. Differences in module technology, temperature profile, DC oversizing, and sensor placement can distort results. Capacity-based PR is most reliable for trend analysis within the same plant.
Can inverter clipping artificially reduce Capacity-Based PR?
Yes. In DC-oversized plants, inverter clipping during high irradiance periods limits AC energy output while POA insolation continues to accumulate. This reduces calculated PR even though the plant is operating as designed.