What is Performance Ratio (PR)?
The ratio of a solar plant's actual production to its theoretical ideal production; the key indicator of efficiency.
Performance Ratio (PR) is the ratio of a solar power plant's actual production to the ideal energy it could theoretically produce, expressed as a percentage. It compares what would be produced if all the solar energy falling on the plant became electricity against what was actually produced. PR rolls all real-world losses — shading, temperature, soiling, cable and inverter losses — into a single coefficient.
For a well-operated plant, the typical annual PR range is 75–85%. Above 80% is good, above 85% excellent. PR varies with season and temperature: it drops as panel temperature rises in summer and rises on cool, sunny days. So it is the temperature-corrected annual PR, not a single day, that is meaningful to monitor.
PR directly affects return on investment. A plant running at 75% PR instead of 80% loses thousands of kWh of production per year. Monitoring PR continuously means catching a shading- or soiling-related loss the same day, not months later.
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