This project is a collaboration between the National Physical Laboratory (NPL) and Statkraft and aims to establish a novel performance metric for the accurate evaluation of oversized and bifacial photovoltaic (PV) systems. This will lead to increasing the value of future solar projects, reducing financial risks and improving fault identification, thus energy production, revenue and CO2 offsets from renewable generation.

The performance ratio (PR) is the most established metric for assessing solar photovoltaic (PV) system performance in the Solar Industry. It is the ratio between the actual to the expected generation based on a system's rated power. It is widely applied as a contractual guarantee for a power plant's performance and therefore may determine the value of a project over its projected lifetime.

A guaranteed PR value is agreed between parties based on software simulation of the PV system and an initial evaluation. Nevertheless, PR measurements can be strongly affected by differences between actual weather conditions during the measurement period (typically 1 year) and "typical meteorological year" data used in modelling. The most serious weather-induced errors occur on "oversized" PV systems.

Oversizing occurs when a PV array has a rated DC power larger than its inverter's rated AC output power; a common practice in PV industry, which leads to inverter "clipping", outputting a flat generation curve during high insolation periods. Clipping can be exaggerated by actual weather conditions, impacting PR, especially in high insolation years. This effect is further enhanced for bifacial PV systems, which harvest additional power from the rear side of PV modules. This effectively makes the systems even more oversized and the existing performance metrics no longer reliable.

Despite the proposed corrections for temperature variations, current IEC standards for performance metrics fail to assess the performance of oversized PV plants. In this project, we aim to identify an improved metric and assess its merit, in order to accurately assess the PV systems that we develop.

NPL and Statkraft will define this metric for monofacial and bifacial systems and will benchmark with current standard metrics against seasonal and annual weather variability, for different PV system configurations and faults. The metric will be validated against real data, multiple locations and systems' configurations.

The proposed metric will be:

* Insensitive to the actual weather conditions during the evaluation period.
* Sensitive to underperformance of systems compared to their expected performance given the actual weather conditions.
* Simple and unambiguous to use in contracts.