Performance Ratio (PR)

Performance Ratio (PR) is a measure of how efficiently a solar system converts available sunlight into electricity, accounting for all real-world losses. It compares actual energy output to the theoretical output based on the amount of solar radiation received and the system’s nameplate capacity.

A well-designed and maintained residential solar system typically achieves a performance ratio of 75–85%. A PR of 80% means the system is converting 80% of the available solar energy into usable electricity, with the other 20% lost to temperature effects, inverter conversion, wiring, shading, and other factors. PR is useful because it normalizes for weather and location — you can compare the performance of systems in different climates fairly. A system in cloudy Seattle with an 82% PR is performing better relative to its conditions than a system in sunny Phoenix with a 75% PR, even though the Phoenix system produces more total kWh. Monitoring your system’s PR over time helps identify degradation or maintenance issues. A sudden drop in PR could indicate shading from new tree growth, a malfunctioning inverter, or dirty panels that need cleaning.