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Performance Ratio — The Universal Solar KPI
Performance Ratio (PR) is the most widely used metric for assessing the overall health and efficiency of a solar PV system. It compares actual energy output to the theoretical maximum output if the system operated at STC efficiency throughout the period. A PR close to 1.0 indicates an excellent system; a declining PR over time indicates degradation or fault development.
PR = Eactual (kWh) / [Prated (kWp) × Hirradiance (kWh/m²)]
Example: 6.6 kWp system produces 1,200 kWh in a month. Irradiance measured at 180 kWh/m².
PR = 1,200 / (6.6 × 180) = 1,200 / 1,188 = 1.01 — excellent
Typical PR Values in Australia
System Type / Condition | Typical PR |
|---|---|
New, well-designed residential system | 0.80 – 0.85 |
Commercial flat roof, tropical | 0.75 – 0.82 |
Off-grid with battery (additional conversion losses) | 0.65 – 0.75 |
System with significant shading or soiling | 0.60 – 0.75 |
System with undiagnosed inverter fault | <0.60 — investigate immediately |
Calculating PR Without an Irradiance Sensor
If the system lacks a local irradiance sensor, use BoM gridded solar data for the site's location and time period as a proxy for measured irradiance. Tools like PVwatts and Solargis provide modelled PR calculations without on-site monitoring hardware. While less accurate than measured PR, modelled PR still identifies significant performance deficits.
Engr. Jason Morales — Founder, SolarEnergyPH
