Performance ratio
Ratio of actual AC energy output to the theoretical output implied by plane-of-array irradiation and nameplate capacity; a location-independent quality metric for PV plants.
The performance ratio (PR), defined in IEC 61724-1, normalises a PV system output by the solar resource it actually received, so plants in different climates can be compared on technical quality.
Yf is final yield (kWh per kWp) and Yr reference yield (plane-of-array irradiation in kWh/m² divided by 1 kW/m², i.e. peak sun hours). A PR of 0.82 means 18% of the theoretical STC energy was lost.
Typical values: well-designed modern utility plants 80–88%; residential rooftops 75–85%; older or poorly maintained systems 60–75%. Main loss contributions in a sunny climate:
- Cell temperature above 25 °C: 4–10% (see temperature coefficient)
- Soiling: 1–5%
- Inverter conversion: 1.5–3%
- DC and AC cabling, mismatch: 1.5–3%
- Shading, reflection, low-irradiance and spectral effects: 2–5%
- Clipping and availability: 0–3%
PR is lower in hot climates and in summer, and slightly higher in cool, sunny conditions. Temperature-corrected PR is used in O&M contracts so that warranted values are not penalised by weather.
Related terms
All terms →- Specific yield
Annual energy produced per unit of installed PV capacity, expressed in kWh/kWp per year; combines solar resource and system performance.
- Temperature coefficient
The percentage change in a PV module power, voltage or current per degree Celsius of cell temperature away from 25 °C.
- Soiling loss
Reduction in PV or CSP output caused by dust, pollen, bird droppings, snow or other deposits blocking light from reaching cells or mirrors.
- Peak sun hours
Daily solar irradiation expressed as the equivalent number of hours at 1,000 W/m²; numerically equal to kWh/m² per day.
- Inverter clipping
Energy lost when the DC power available from a PV array exceeds the inverter AC rating, so the inverter caps output at its maximum.