How PV performance ratio and yield are calculated
Performance ratio compares real production against the ideal output a system would make at standard test conditions. This tool builds PR by multiplying the loss factors instead of adding them: PR = inverter efficiency x (1 - temperature loss) x (1 - soiling) x (1 - shading) x (1 - wiring) x (1 - mismatch) x (1 - degradation) x (1 - other). Multiplying is closer to physical reality than subtracting a single summed percentage.
Well built fixed tilt systems in the US land between 0.78 and 0.85. Temperature is usually the largest single item, since crystalline modules lose roughly 0.35% of output per degree Celsius above 25°C and a rooftop array can run 30°C hotter than ambient on a July afternoon. For a cross check, NREL PVWatts uses a default of 14.08% total system losses made up of soiling, shading, snow, mismatch, wiring, connections, light induced degradation, nameplate rating and availability, with inverter efficiency handled separately at 96%.
Annual production is system size x peak sun hours x 365 x PR. Pull peak sun hours for the plane of array from the NREL National Solar Radiation Database or PVWatts rather than using a national average, because tilt and azimuth shift the number substantially. Any figure used for financing or a power purchase agreement should be confirmed against full simulation software and reviewed by a qualified PV engineer.
Frequently Asked Questions
Shading and soiling are the usual suspects. Compare production string by string: one low string points to shade or a bad connection, while a uniformly low array points to soiling or module degradation. Wash and remeasure before changing anything else.
Crystalline silicon typically shows 1-2% light induced degradation in the first year and then 0.4-0.6% per year. For a 20 or 25 year pro forma, enter the average degradation over the analysis period rather than the first year figure.