Standards Development

Scope

This part of IEC 61724 outlines terminology and metrics for PV system performance, focusing on various definitions of Performance Ratio. This document primarily addresses performance assessment for flat ‑plate, grid‑connected PV systems, including both fixed‑tilt and tracking configurations.

Purpose

This document defines calculated quantities and Performance Ratio metrics that are used to evaluate PV systems and in PV system performance guarantees.

Performance ratios quantify a system’s energy output relative to expected energy output and are widely used in contractual performance guarantees and for operations and maintenance.

Various performance ratio metrics are defined.

The different performance ratio metrics have different advantages and tradeoffs:

• Temperature-corrected performance ratios reduce seasonal variability.

• For bifacial systems, if the contributions of rear-side irradiance are to be included in the system’s expected energy output, use the bifacial forms of performance ratio, while if rear -side irradiance contributions are considered a bonus that does not count towards expected energy output, use the non-bifacial forms.

• For systems with significant clipping, filtering clipping time periods out of the data allows detection of DC degradation, while using a clipping-corrected performance ratio allows assessment of inverter function and evaluation at high irradiance.

Selection of a specific performance ratio to use for system monitoring depends on the desired approach. In some cases it may be beneficial to use more than one performance ratio metric.