Quick Facts
What Performance Ratio measures
Performance Ratio (PR) is the international standard quality metric for grid-connected solar PV plants. It expresses how much of the theoretically possible output a plant actually delivers, after every loss between sunlight and AC grid energy. PR is irradiance-normalised, so a plant in Chennai and a plant in Jodhpur can be compared on the same scale even though they receive different annual sun.
A PR of 0.82 means that for every 100 units of solar energy the modules could have produced under perfectly STC-like conditions across the year, the plant delivered 82 units to the grid. The remaining 18 units were lost to temperature derating, soiling, shading, cable resistance, inverter conversion, and mismatch.
The IEC 61724 standard defines how to measure and report PR. Most large project contracts in India use IEC 61724 as the reference document.
How PR is calculated
The formal definition:
PR = E_AC / (P_STC x H_POA / G_STC)Where:
- E_AC is the AC energy output of the plant over the measurement period (kWh).
- P_STC is the installed DC capacity (kWp).
- H_POA is the plane-of-array irradiation in the measurement period (kWh per sq m).
- G_STC is the reference irradiance, 1 kW per sq m.
In words: divide the actual energy delivered by the energy the plant would have delivered if the modules were operating at their full STC efficiency for every kWh per sq m of sunlight received.
A worked example for a 100 kWp Indian rooftop plant:
- Annual AC energy: 1,52,000 kWh
- Annual POA irradiation: 1,950 kWh per sq m
- Theoretical maximum: 100 kWp x 1,950 / 1 = 1,95,000 kWh
- PR: 1,52,000 / 1,95,000 = 0.779 or 77.9%
77.9% is a respectable PR for a rooftop plant in a hot, dusty Indian location.
What is included in PR losses
A typical PR loss waterfall for an Indian solar plant:
| Loss source | Typical magnitude |
|---|---|
| Module temperature (cell hot) | 6% to 12% |
| Soiling and dust | 3% to 7% |
| Inverter conversion losses | 1.5% to 3% |
| DC cable losses | 1% to 2% |
| AC cable losses | 0.5% to 1.5% |
| Mismatch and tolerance | 1% to 2% |
| Shading (partial) | 1% to 5% |
| Inverter clipping (DC oversized) | 0% to 3% |
| MPPT inefficiency | 0.5% to 1.5% |
| Transformer losses (if any) | 0.5% to 1% |
The dominant loss in India is heat. Module temperatures above 50 deg C are routine, and the temperature coefficient of mono PERC at around minus 0.34% per deg C drives a major chunk of the PR gap.
How PR is measured continuously
A plant’s SCADA system reads three streams in real time:
Inverter AC output, summed across all inverters, gives total plant energy.
On-site pyranometers, mounted in the plane of the modules, give POA irradiance.
Cell or back-of-module temperature sensors give the operating temperature for diagnostics.
The PR for every minute, hour, day, and month is computed and stored. Lenders and asset managers receive monthly PR reports. EPC contracts often specify a guaranteed annual PR over the first one or two years, with bonus or penalty clauses.
Improving PR
Cleaning. Soiling is the most controllable PR drag. A clean module costs nothing in capex but adds 3% to 7% over an unwashed module. Quarterly cleaning during dust season is standard practice.
Cooling. Modules mounted with air gap of 100 mm or more beneath the array run cooler than tight-mounted panels. Tracker-mounted modules also run cooler because wind passes over both faces.
DC oversizing matched to inverter rating. Heavy clipping costs PR. Mild clipping (1.2x DC to AC) is usually break-even or slightly positive.
Hot-spot replacement. Cracked, browning, or PID-affected modules drag down their string. Annual EL imaging catches these.
Cable sizing. Tight cable sizing saves capex but loses PR. Reviewing voltage drop budgets at design time prevents this.
PR over the plant’s life
A typical first-year PR for an Indian rooftop plant is 80% to 84%. Without intervention, this drops by around 0.5% to 0.7% per year because of module degradation, soiling buildup between cleanings, and inverter aging. A 25-year plant may end its life at PR of 65% to 70% if poorly maintained, or 72% to 75% if well maintained.
The economic impact is large. A 1% PR drop is a 1% revenue drop. For a 1 MW commercial plant earning Rs 1 crore a year, every 1% of PR is Rs 1 lakh of annual revenue.
Common mistakes with PR
Calculating PR with GHI instead of POA irradiance. POA is the correct denominator for tilted modules.
Mixing PR for nominally identical plants in different climates. PR is already irradiance-normalised, so a Chennai plant with 80% PR and a Bikaner plant with 80% PR are operating at the same quality, even though the Bikaner plant produces 25% more energy.
Treating PR as the only health metric. PR can stay high while individual strings or inverters degrade. Pair PR with string-level current monitoring.
Comparing a tracker plant to a fixed-tilt plant by PR alone. Trackers typically improve CUF much more than PR.
Setting an unrealistically high contractual PR. EPCs penalised for missing PR will design to minimise risk, sometimes at the cost of capex efficiency.
Best practices
Measure POA, not GHI. Install on-plane pyranometers from day one.
Calibrate pyranometers annually. A pyranometer drifting by 3% will appear as a 3% PR error.
Use IEC 61724-1 or IEC 61724-3 as the reference for measurement and reporting methodology.
Track temperature-corrected PR alongside raw PR. Temperature-corrected PR isolates plant quality from weather variation and is the right number to evaluate O&M decisions.
Review PR every month, not just annually. A 3% drop is easy to fix in the same month, hard to recover a year later.
Related glossary terms
- Capacity Utilisation Factor
- What is kWp
- Solar Irradiance
- Temperature Coefficient
- Solar Degradation
- Soiling Loss
- Shading Loss
- Inverter Clipping
Key takeaways
Performance Ratio is the international quality metric for solar PV plants, defined by IEC 61724. It compares actual AC energy output to theoretical STC output for the irradiance received. Indian rooftop and ground-mount plants typically achieve 78% to 85% PR in year one, with heat, soiling, and inverter conversion as the dominant losses. PR declines slowly over the plant’s life but is responsive to cleaning, hot-spot replacement, and inverter maintenance.