DECK Performance Index Table

Performance Index Table


This table shows the performance index by month for all systems that have both generation data and weather data (irradiance and cell temp). System size must be set accurately in order for the data to be meaningful.  


Note that the Performance Index may be considered as an alternative calculation of actual vs expected, but this metric accounts for available sunlight because the “expected” value in this case is calculated using irradiance and cell temp data from your on-site weather station. You will not see values in this table for systems that do not have a weather station.


Cells of the table are color coded to indicate which results are a good performance index versus a bad performance index, so it’s possible to spot at a glance which systems likely need attention. The color scheme spans from primary green (for systems with high performance index values) to primary red (for systems with low performance index values).


A word about Performance Index calculations:  Performance Indices are tools commonly used to compare the projected solar system output based on the available solar resource to the actual system generation. This calculation relies on actual solar irradiance measurements, and so cannot be computed unless a weather station with a working irradiance sensor has been installed on site.

In addition to the weather station and generation meter, our Performance Index also utilizes (2) user input values that can be changed on the settings page: the DC Nameplate system size, and the “Temperature Coefficient of Power.”  The calculation can be visualized this way:


Our Performance Index is a way to summarize all of the system inefficiencies in a single value.  A perfect system with absolutely no losses would have a performance index of 1.0.  After factoring in typical losses due to inverter inefficiencies, wiring resistance and other imperfections, a new system performing very well should yield a performance index near 0.85.

How DECK calculates your Performance Index:

The DECK Performance Index does diverge from the ISO defined Performance Ratio in two ways.

    • DECK Performance Index factors in the effect of cell temperature on PV module performance.
    • DECK Performance Index is based on irradiance measurements taken at the Plane of Array, and not on the Global Horizontal Irradiance.

Why does my performance index look inaccurate?

You may see inaccurate Performance Index values, such as a ratio of 1.0 or above. The leading cause of inaccurate Performance Index calculations is an error in the customer-defined system size value for your site.  Please check the settings page to verify that the DC Nameplate size is correct.


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