Use our Formula Composer to track your solar PV performance - Wattics
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Use our Formula Composer to track your solar PV performance

Few PV system owners track theoretical output often owing to the complexities surrounding off-grid calculations and miss out on information about their systems performance. Through the Formula Composer tool, Wattics provides a simplified solution that gives you the confidence to analyse, monitor and track your PV system performance. In this article, you will learn how to build your own PV model to help you gain insight into the performance of your solar PV system.

Key Learning Outcomes:

  • Learn how to create your own PV model 
  • Learn how to analyse your PV performance  
  • Learn how to set up alerts for continuous monitoring

Key tools used: Formula Composer, Notifications & Alerts, Portfolio Analyst

Table of contents

  1. Regression analysis with the Wattics Formula Composer 
  2. Building your PV Regression Model using the Wattics Formula Composer
  3. Creating a monitored point for tracking PV performance 
  4. Understanding your PV system performance
  5. Setting up alerts to identify an ill-performing PV system

1 – Regression analysis with the Wattics Formula Composer

The Formula Composer allows you to create mathematical calculations to study different data sets to determine whether any relationships exist between them – without having to perform complex analysis often done in Excel. This tool is especially useful for regression analysis where formulas are useful in establishing metrics to determine if two simultaneous events are related in real time, or as we will see in the following example, using external data such irradiance and comparing it to PV system generation/output to determine the performance of your PV system.

The aim of creating this example PV model, which will be explored in more detail further on in this guide, is to provide improved visibility of your solar generation performance. You can plot your generation with respect to local sunlight received to produce a model that calculates theoretical generation volume, highlights abnormalities, and identifies a PV system that is underperforming.

(* Local sunlight, or irradiance, is the amount of light energy falling on the PV cell typically stated in Watts/square metre. (W/m2). Irradiance data can be found in a number of locations, these are summarised here in a complete list of resources: irradiance data.)

In this example, the new data we have uploaded is irradiance. Instructions on uploading new data to the Wattics platform are provided here:

We’ll start by navigating to the Formula Composer tool which you will find in the Dashboard tab of your Wattics dashboard, which is the calculator icon in your dashboard:

2 – Building your PV Regression Model using the Wattics Formula Composer

For this user guide, we will use the example of the Grand Irish Solar Company in the Wattics dashboard as an example to help you successfully create your own PV generation model. 

On accessing the Formula Composer, you will see two blank tabs labelled Y-axis and X-axis (see screenshot below). Here we are going to click on the X-axis which enables you to run the Formula Composer as a regression analysis tool that is the foundation of your PV generation model. To populate the tabs, type ‘@’ which will provide a drop-down menu of all your monitored points. Continue to type the name of your desired data point to speed up selection. In this example, we are looking at the @Grand Irish Solar Company.

**For other formulaic calculations, you can populate the Y-axis tab to evaluate some specific formula with respect to the default time setting of the X-axis

In this example, we are interested in how strong the correlation is between PV generation and irradiance data for The Grand Irish Solar Company. Based on this we will know how accurately we can use irradiance to predict PV output.

Regression Formula:

We populate the Y-axis by selecting The Grand Irish Solar Company > Generation > PV Plant (represents the existing PV output) from the drop-down menu and repeat for the X-axis by selecting The Grand Irish Solar Company > Metrics > Irradiance (represents the irradiance data uploaded)

Filling these tabs will automatically create a scatter chart containing this data with a line overlaid interpolating the data points. Each point plots kWh PV generation against W/m2 irradiance. The regression formula and R2 values are displayed above the chart. The R2 value represents the proportion of variance for a dependent variable – here PV generation. 

The closer the R2 value is to 1 the higher the likelihood of being able to accurately predict generation output based on irradiance data.

The R2 value here is 0.79 based on a linear relationship which does not indicate strong correlation between PV generation and irradiance. To see if we can improve this, we can adjust the regression order to apply different curves that may fit the data better by clicking on the drop-down menu.

By changing the regression order to 3, or a cubic expression, we increase the R2 value to 0.84, which makes the correlation slightly stronger and so more accurate at predicting PV generation based on irradiance.

3 – Creating a monitored point for tracking PV performance

Once you have determined that there is a strong correlation between generation and irradiance, you can continue to monitor the performance of solar generated against localised irradiance by creating a data point, which you can use for further analysis and reporting.

Please note that this example is based on aggregated data based on the granularity selections in the Control Panel (specifically for this example) and so could not be applied to provide daily estimations of solar generation. For daily estimations, it is recommended that a lower granularity be applied to your analysis.

On clicking ‘create new point’ you will be prompted to fill out additional information to complete the creation of your new data point:

  1. Enter a name for the new data point e.g. Simulated PV
  2. Define Date point type eg. Electricity
  3. Select the site under which you would like your data point to appear in your menu tree 
  4. Specify whether the formula should be applied to values or cost
  5. Insert your units
  6. Select the start date from which you would like to have the data populating 
  7. Deselect the inclusion of this new point from Site and Organisation level calculations to omit this data from analysis performed at Site and Organisation level
  8. Click your agreement to add the new data point to your Wattics subscription and hit Save

The platform will then create this new point under your menu tree. Please allow a few minutes for your data to populate as there may be reprocessing required, if historical data is taken into account.

4 – Understanding your PV system performance

Once you have created your new data point you can review your underlying regression formula by going into the Portfolio Analyst tool, locating the new data point, in this example – PV Sim 2 – in your left menu tree.





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