What is performance ratio (PR) and how do you read it?
Daily energy alone does not tell you whether a plant is healthy. PR judges output against that day's sunlight.
Imagine a PV plant produced 10,000 kWh yesterday and 7,000 kWh today. Is that a problem? Not if it was overcast; maybe if it was clear and sunny. Raw energy includes the weather, so it is not enough on its own.
Definition
Performance ratio is the ratio of actual energy to the energy expected under the same irradiance and temperature. It is expressed as a percentage and largely removes the effect of weather.
PR = Σ E_actual / Σ E_expected × 100Where does expected energy come from?
Expected power is computed from installed capacity, measured plane-of-array irradiance and module temperature. Standard test conditions are 1000 W/m² and 25 °C. Crystalline silicon modules lose power as they heat up, so PR calculated without temperature correction looks misleadingly low in summer.
P_expected = P_installed × (G / 1000) × (1 + γ × (T_module − 25))How to read it
- PR falling slowly on all inverters at once usually points to a plant-wide effect such as soiling.
- PR dropping on one inverter at certain hours of the day can point to shading or a problem in that string.
- PR suddenly dropping to zero is not an efficiency issue; it means production stopped and needs separate handling.
- At dawn and dusk, when irradiance is low, PR is noisy; leaving those intervals out gives a cleaner figure.
How m4chanical GES calculates it
The dashboard computes energy-weighted PR only over 15-minute intervals with irradiance of 100 W/m² or more. The default warning threshold is 85% and can be adjusted for your plant. Intervals below the threshold are listed as 'underperformance' events.
Note: there is no single good or bad PR value. The right threshold depends on module technology, climate and measurement quality, and should be set per plant.
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