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Solar System Performance Indicators: a comparative analysis

Updated: May 18

Solar energy has emerged as a key player in the transition towards renewable energy sources, with photovoltaic (PV) systems being widely adopted for electricity generation. Assessing the performance of these solar PV systems is crucial for ensuring optimal energy production and efficiency. In this essay, we will delve into the specifics of solar performance indicators, focusing on the advantages and disadvantages of utilizing Energy Performance Index-SAM (EPI-SAM) and Performance Ratio (PR), which require weather sensors, versus Energy Performance Ratio-Reg (EPI-Reg or EPR) indicator. Additionally, recent advancements in solar performance assessment techniques, such as integration of Sensorless technology pioneered by Soltell Systems, which enables accurate calculation of EPR indicator will be explored.

Digitalisation of distributed solar photovoltaics.
Fig 1. Digitalisation of distributed solar photovoltaics.

Energy Performance Index-SAM (EPI-SAM) and Performance Ratio (PR) are widely used performance indicators that require the installation of weather sensors to monitor various environmental factors such as solar radiation, temperature, and wind speed. EPI-SAM calculates the energy yield of a PV system relative to its expected energy yield under standard test conditions. PR, on the other hand, measures the actual energy output of the system relative to its theoretical maximum output.


Accuracy: EPI-SAM and PR provide accurate assessments of solar PV system performance by taking into account real-time environmental conditions.

Comprehensive Analysis: These indicators offer a comprehensive analysis of system performance by considering multiple factors that influence energy production.

Performance Monitoring: EPI-SAM and PR allow for continuous monitoring of system performance, enabling early detection of issues and optimization opportunities.


Cost: The installation and maintenance of weather sensors required for EPI-SAM and PR can be costly, particularly for large-scale solar installations.

Complexity: Implementing EPI-SAM and PR requires expertise in data analysis and interpretation, making them less accessible to inexperienced users.

Reliability: The accuracy of EPI-SAM and PR depends heavily on the reliability of weather sensors and the quality of data collected, which may vary depending on location and environmental conditions.

Energy Performance Ratio-Reg (EPR) is an alternative performance indicator that utilizes regression analysis to estimate the energy performance of a PV system based on historical energy data. Unlike EPI-SAM and PR, the EPR indicator does not require the installation of weather sensors, making it a more cost-effective and accessible option for financial and operational purposes. The main drawbacks of EPR is however the need for a substantial data history and insufficient accuracy.


Cost-Effectiveness: EPR eliminates the need for expensive weather sensors, reducing installation and maintenance costs.

Simplicity: EPR offers a simplified approach to performance monitoring that is easier to implement and understand, making it suitable for a wide range of users.

Accessibility: EPR provides a more accessible option for performance monitoring, allowing smaller-scale solar installations and less experienced users to assess system performance effectively.


Accuracy Limitations: EPR relies on historical performance data and regression analysis, which may not capture real-time environmental variations as accurately as EPI-SAM and PR.

Limited Environmental Insights: EPR does not take into account real-time environmental conditions, limiting its ability to provide insights into the impact of factors such as weather changes on system performance.

Predictive Challenges: Regression analysis used in EPR may struggle to accurately predict future energy output under changing environmental conditions, leading to potential inaccuracies in performance estimates. In conclusion, the selection of solar performance indicators hinges on several factors, including precision needs, cost considerations, and user proficiency. While EPI-SAM and PR provide thorough performance analysis, they necessitate the use of weather sensors. Conversely, EPR offers a cost-effective and accessible alternative but requires an additional technological edge to achieve adequate accuracy and predictive capabilities.

Recent advancements in solar performance measurement technology have facilitated the calculation of EPR using Sensorless technology developed by Soltell Systems. This innovative approach not only eliminates the reliance on weather sensors but also ensures highly accurate performance assessment. As a result, it contributes significantly to the ongoing progress and widespread adoption of distributed solar energy across residential, commercial, and industrial sectors, where the cost of weather sensors has been a limiting factor.

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