PV Capacity Testing for Sites with Multiple Array Orientations

PV Capacity Testing for Sites with Multiple Array Orientations

For sites featuring multiple array orientations (or multiple Planes of Array, POA), the ASTM E2848 test procedure must be extended by a method to calculate a single, representative irradiance for the entire site. This article explains how to calculate the weigheted average POA irradiance.

The focus of this article is on the case where each array orientation has its own POA sensor. For complex sites with limited sensor equipments (e.g. only one GHI sensor for the entire site), refer to the article Cost-Effective Capacity Testing: Transposing GHI for Sites with Multiple Arrays and Bifacial Modules.

Combining Multipe POAs into a Weighted Average

The individual POA irradiance measures are typically combined via weighted averaging using one of the following 2 methods.

1. Module Area Weighting

The standard recommendation is to combine the individual POA irradiances using a module area-weighted average.

$$E_{\text{POA}}=\frac{{\sum }_i{E}_{\text{POA},i}\times N_i}{N}$$

where:

• $E_{\text{POA}}$ is the weighted average POA irradiance
• $E_{\text{POA},i}$ is the measured POA irradiance of the $i$ -th array
• $N_i$ is the number of panels installed in the $i$ -th array
• $N$ is the total number of panels at the site

2. Weighting with STC rating

If modules with different STC ratings are installed at a site, the weighted averaging calculation should additionally account for the module STC rating of each panel:

$$E_{\text{POA}}=\frac{{\sum }_i{E}_{\text{POA},i}\times N_i\times ST{C}_i}{{\sum }_i{N}_i\times ST{C}_i}$$

where:

• $ST{C}_i$ is the STC rating of the modules installed at the $i$ -th array

Note that the Heliotest app directly implements the Weighting with STC rating method since Module Area Weighting where all $ST{C}_i$ are the same, is just a subset of it.

Capacity test execution

Finally, the resulting single Global POA Irradiance $E_{\text{POA}}$ is then used in the multilinear regression analysis required by the ASTM E2848 capacity test method.

How Heliotest handles this

When you configure a site with multiple array orientations in Heliotest, the weighted average POA irradiance is calculated automatically during the capacity test. You define each sub-array's orientation, module count, and STC rating, and the platform applies the STC-weighted averaging method described above to both the measured and modeled datasets. No manual spreadsheet formulas are needed — the weighting is applied consistently across every test run.