What is the performance ratio of a PV system and what affects it?

Performance ratio (PR) compares actual energy output to theoretical output based on irradiance: PR = Actual_kWh / (Rated_kWp x Irradiance_kWh/m^2 x 1000W/m^2 STC). Typical values are 75-85%. Losses affecting PR: temperature (modules lose 0.4-0.5%/C above 25C), soiling (dust, dirt - 2-7%), shading, wiring/mismatch (2-3%), inverter efficiency (2-4%), and downtime. Higher PR indicates better system quality and operation. Monitoring PR over time detects degradation (typically 0.5%/year). Bifacial modules can show PR >100% due to rear-side gain.