What are the essential Key Performance Indicators (KPIs) for assessing and improving the operational efficiency of photovoltaic? What are their advantages and disadvantages? These and other questions are addressed in the report “Technical Key Performance Indicators for Photovoltaic Systems: Challenges and Best Practices” prepared by IEA PVPS Task 13. The study explores the role of key technical indicators for solar systems and offers a set of best practices for effective data management in the photovoltaic sector.
The report clearly shows that KPIs are essential for evaluating PV systems in multiple contexts, including operational efficiency, financial feasibility, and sustainability metrics, providing a solid foundation for effective decision-making.
The reliability of calculations underlying these KPIs depends on the ability to obtain high-quality data and standardized definitions, which help reduce biases, improve transparency, and ensure fair performance assessments across different projects. This is why the authors emphasize the urgent need for standardization and harmonization in data management processes. The report provides valuable insights into current gaps in this field and advocates for global harmonization, which could significantly impact the sustainability and profitability of photovoltaic operations.
Key Performance Indicators for Photovoltaic
What do we mean when we talk about photovoltaic KPIs?
The P50 energy yield describes the probability that a photovoltaic system will generate a specific amount of energy over one year. The “50” indicates there is a 50% chance that the actual energy yield will be higher and a 50% chance it will be lower. Variants such as P75, P90, P95, and P99 can also be used.
- Advantage: Widely used and easy to understand.
- Disadvantage: Requires a deep understanding of uncertainties related to meteorological data, which are increasingly affected by climate change.
The Performance Ratio (PR) represents the ratio between the actual energy produced by a system and the energy it could have produced under ideal conditions.
- Advantages: Simple and easily understood by all stakeholders.
- Challenges:
- Sensitive to data losses and production exclusions (e.g., due to curtailment or grid-related events), which are becoming more frequent with rising instances of mandatory production limitations.
- Negatively impacted by high irradiation levels in oversized systems with a high DC/AC ratio due to clipping (power limitation).
The Availability indicator measures the percentage of time a photovoltaic system is capable of generating electricity during a given analysis period.
- Advantages:
- When time-based, it is easy to understand and use for all stakeholders, regardless of technical background.
- When energy-based, it accounts for energy losses, giving greater weight to downtimes with significant losses.
- Challenges for energy-based availability:
- In deregulated markets, energy may have less value during periods of high irradiation.
- Less transparent, as it relies on simulation models to calculate expected energy. For these reasons, time-based availability remains the most widely used indicator in contracts.
The Soiling Rate measures the ratio between the actual measured power of a dirty photovoltaic cell and the power it would generate if clean. While calculation methods are not standardized, IEC 61724-1:2021 provides general guidance.
- Advantages: When calculated reliably, it allows for informed decisions on cleaning schedules and business plan projections.
- Challenges: Obtaining a reliable assessment of soiling-related losses is complex, and each method comes with specific pros and cons.
The Degradation Rate quantifies the irreversible losses (e.g., material degradation) that occur in a photovoltaic system over time.
- Advantages: A key KPI for predicting and monitoring the performance decline of a system over its lifetime.
- Challenges: Reliable calculations require several years of data (at least two, with significant uncertainties persisting even with five years), and statistical methods are heavily influenced by climatic variability.
The Energy Performance Index (EPI) expresses the ratio of measured yield to predicted yield.
- Advantages: For well-functioning systems, the EPI is generally more stable (around 100%) than the Performance Ratio, as it is less influenced by system design and meteorological variability.
- Disadvantages:
- Design tools are not typically integrated into monitoring platforms, making EPI calculations a semi-manual process.
- Requires more sophisticated and less intuitive equations compared to PR.
These KPIs provide critical insights into the performance of photovoltaic systems, offering a foundation for optimizing operations and enhancing sustainability in the renewable energy sector.
