Design, installation and performance
System performance ratio
Also known as: performance ratio, PR, yield factor.
Definition
The performance ratio is the relationship between the energy a system actually produces and what it would produce under ideal conditions. It bundles the normal losses from temperature, wiring and the inverter; it usually sits close to 75-80%.
No system converts into usable electricity all the energy it could theoretically capture. Between the panel and the meter, losses accumulate: cell temperature above the 25 °C reference, wiring resistance, inverter efficiency, dust on the glass, occasional shading, mismatch between modules and downtime for maintenance. The performance ratio sums all of that up in a percentage.
It is the factor that turns an optimistic estimate into a credible one. A quote that calculates output as kWp × PSH × 365 without applying a performance ratio is promising a figure the system will not reach. The useful question to ask the installer is what performance ratio they used and which losses they included.
It also serves as a diagnostic tool during operation: comparing the measured performance ratio against the expected one reveals problems — accumulated dirt, a downed string, a degraded panel — before they translate into a higher bill.
The essentials of Performance ratio
- Usual reference value: between 75% and 80%.
- It includes losses from temperature, wiring, inverter, soiling and shading.
- Annual output ≈ kWp × PSH × 365 × performance ratio.
- A measured ratio well below the expected one indicates a problem to investigate.
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Frequently asked questions about Performance ratio
What performance ratio is normal?
On well-designed systems it usually sits around 75-80%. Much higher values in a commercial estimate should raise suspicion, and much lower measured values during operation point to dirt, unforeseen shading or a fault in some component.
How can I improve my system's performance?
With what is in your hands: keeping the panels clean, stopping vegetation growing where it casts shade, checking the monitoring regularly and responding quickly to any alert. Losses from temperature, wiring and the inverter are decided in the design, not during operation.
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