In one line
The ratio of a plant's actual energy output to what it would have produced at nameplate efficiency under the measured irradiance — the standard scorecard for system health.
What it means
Performance ratio strips the weather out of a production number so that plants and periods can be compared fairly. A PR of 80% means the system delivered 80% of the energy that its installed capacity would ideally have produced under the irradiance actually received; the missing 20% is temperature, soiling, wiring, inverter, mismatch, availability and clipping losses. Well-designed modern plants achieve 78–85% annually, with temperature-corrected PR used to remove seasonal noise when tracking degradation year over year. PR is defined in IEC 61724, and it is the metric most performance guarantees are written against — which is why the details of its calculation, especially whether it is temperature-corrected and which irradiance sensor feeds it, are worth arguing over at contract stage.
Worth knowing
- PR normalises for weather; specific yield does not.
- Modern plants: 78–85% annual PR.
- Temperature-corrected PR is the fair basis for year-on-year comparison.
- Guarantee disputes usually turn on sensor quality and correction method.
How it's calculated
PR = E_actual ÷ (POA irradiation × P_installed ÷ G_STC)- E_actual — measured energy delivered over the period (kWh)
- POA irradiation — measured plane-of-array irradiation (kWh/m²)
- P_installed — installed DC capacity (kWp)
- G_STC — 1,000 W/m² reference irradiance
Field notes
- 1Agree the PR formula, sensor specification and correction method in the contract — not after the first dispute.
- 2Seasonal PR swings are normal: summer heat depresses it, cool sunny months raise it. Compare like months.
- 3A slow PR decline with stable availability usually means soiling or degradation; a sudden step means equipment.
Common mistakes
- Comparing PR across plants in different climates as if it were a quality ranking — hot sites are structurally lower.
- Calculating PR from satellite irradiance when the guarantee specifies on-site pyranometers.
Frequently asked
What is a good performance ratio for a solar plant?
78–85% annually for a well-designed modern system. Above 85% is excellent; sustained readings below about 75% warrant investigation of soiling, availability, inverter derating or degradation.
What is the difference between performance ratio and specific yield?
Specific yield (kWh/kWp) measures how much energy a plant produced and therefore depends on the local resource. Performance ratio normalises against the irradiance actually received, so it measures how well the plant converted what it got.
Sources
Related terms
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