PlantmarginIs your solar plant performing? Performance ratio and soiling explained
A solar system that produces 10% less than it should rarely raises an alarm. The lights stay on and the bill is still lower. Over a year, that 10% can be worth more than the yearly maintenance contract.
Three measures that matter
Specific yield is kWh produced per kWp installed. It compares systems of different sizes, but it rises and falls with the weather.
Performance ratio (PR) removes the weather. It compares the energy produced with the energy the array would produce with no losses under the sunshine actually measured:
PR = energy produced ÷ (kWp × irradiation in kWh/m² ÷ 1 kW/m²)
A healthy commercial system in Rwanda runs at a PR of roughly 0.75 to 0.82 across the year, measured as described in IEC 61724-1.
Inverter comparison puts each inverter against the best one on the same site. They see the same sun, so a persistent gap points to a local fault.
What lowers performance
| Cause | What you see | Typical fix |
|---|---|---|
| Dust and soiling | Slow decline, recovers after rain | Cleaning, set by measured soiling |
| Blown string fuse | One inverter drops by a fixed share | Replace the fuse, find the cause |
| Inverter trip | One inverter at zero | Reset, check the event log |
| Shading growth | Morning or afternoon losses | Trim trees, move equipment |
| Hot modules | Midday dip on hot days | Ventilation behind modules |
A worked example
Example: an inverter block of 170 kWp received 6.0 kWh/m² of sunshine in a day and produced 765 kWh.
PR = 765 ÷ (170 × 6.0) = 0.75
Its neighbour, the same size, produced 840 kWh, a PR of 0.82. The 75 kWh difference, every sunny day, is worth investigating: at FRw 22 per kWh that is about FRw 1,650 a day from one block.
Setting the cleaning interval
Clean when the energy lost to dust is worth more than a cleaning visit. Measured soiling makes this a calculation, not a calendar.
Plantmargin reads each inverter, an irradiance sensor and the module temperature, works out the PR and flags the block that is falling behind, with the money it is costing.