Irradiance

SunSolve calculates irradiance using two different methods, chosen according to what the result is used for:

• Ray-traced, broadband-corrected irradiance — used for dedicated sensor readings and full-surface absorbers. This captures the full 3D detail of the scene: finite array size, real shading, reflections and non-uniform albedo. Sensors and custom objects can be placed anywhere in the scene, independent of the modules, and may or may not track the sun.
• View-factor irradiance — used by the thermal model, which needs a fast estimate of the total incident power on each module surface at every timestep, and also reported as the in-plane irradiation totals in the waterfall loss chart. It is always calculated for the module itself, using its actual size, location and tilt, but assumes an idealised, infinite array around it with isotropic diffuse light and a uniform albedo.

Because the two methods rely on different assumptions, they will not always agree. For a large, regular array with uniform albedo and no unusual structure shading, they typically align closely. They are more likely to diverge for small or irregular arrays, spatially varying albedo, or areas near obstructions, since only the ray-traced method models finite geometry, real shading and reflections directly. They can also diverge simply because a sensor or custom object is not co-located with a module, or has a different tilt or tracking behaviour.

The two methods also differ in how they interact with the ray tracing itself. View-factor calculations run as a separate step and have no effect on the ray-traced result. Sensors and full-surface absorbers, by contrast, participate directly in the ray tracing: they physically intercept incident light, which can shade other objects or modules positioned behind them.

Aside from its role in the thermal model, and any shading created in the scene, none of the irradiance quantities described on this page feed directly into the electrical output of the modules. Module current is determined directly from the ray-traced photon flux absorbed in the collecting regions of each cell; see Determination of cell light-generated current.

Where irradiance is calculated and reported

LocationMethodDetails

Dedicated sensors, Ray-traced + broadband correction, Average irradiance absorbed by the sensor’s front or rear surface,, assuming perfect absorption at all angles and wavelengths.

Full-surface absorbers, Ray-traced + broadband correction, Absorbed energy density on a custom object’s surface,, using the material’s actual reflectance.

Thermal model, View-factor, Total POA irradiance used to solve module operating temperature.

In-plane irradiation, View-factor, Summary-table and waterfall-chart irradiation totals.

Ray tracing and the broadband correction

During ray tracing, SunSolve tracks the wavelength-dependent absorption on the active facets of sensors and custom objects, subject to the same assumptions described in Ray tracing methodology.

A dedicated sensor assumes perfect absorption: every ray that reaches it is absorbed, irrespective of incidence angle (IAM = 1) or wavelength (QE = 1). A custom object configured as a full-surface absorber, by contrast, uses the reflectance of its assigned material (for example, galvanised steel or grass): the reflected fraction returns to the scene, and only the absorbed fraction is reported.

SunSolve’s ray tracer models light only in the 300-1200 nm band. Converting the absorbed photocurrent directly into an irradiance value would therefore miss the energy carried by wavelengths outside this band.

To account for this, SunSolve applies the broadband correction: it scales the photocurrent absorbed within the modelled band up to an equivalent full-spectrum irradiance in W/m².

The equivalent broadband irradiance on a surface is:

$$G=\frac{J{L}_{\text{surface}}}{J{L}_{\text{GHI}}}\times GH{I}_{\text{all}}$$ TermDescription

$J{L}_{\text{surface}}$, The photon current absorbed by the surface,, integrated over the modelled 300-1200 nm band.

$J{L}_{\text{GHI}}$, The equivalent photon current of the global horizontal irradiance (GHI) over the same 300-1200 nm band.

$GH{I}_{\text{all}}$, The total broadband GHI from the weather file (W/m²),, including wavelengths beyond 1200 nm.

The ratio $J{L}_{\text{surface}}\mathrm{/}J{L}_{\text{GHI}}$ is the fraction of the horizontal in-band irradiance that reaches the surface. Multiplying by the broadband $GH{I}_{\text{all}}$ scales that fraction to the full-spectrum irradiance.

View factor irradiance

The view factor implementation follows the standard approach as described by [Marion2017] and [Mermoud2014]. This has been demonstrated to be consistent with 3D ray tracing under this idealized scenario:

• infinitely long rows,
• infinitely many rows,
• isotropic insolation for diffuse light,
• isotropic reflection from the ground with a constant albedo,
• a flat ground,
• no reflection from modules,
• no reflection or shading from structures like posts and torque tubes, and
• no transmission through modules or gaps between modules.

The view factor model assumes albedo is wavelength‑independent.

The view factor model is solved separately for each module orientation and position in the y-direction. Each solve includes the following components:

1. Sky to module front for direct light

2. Sky to module rear for direct light

3. Sky to module front for isotropic light

4. Sky to module rear for isotropic light

5. Sky to ground to module rear for direct light

6. Sky to ground to module rear for isotropic light

The ground is solved by integrating across a minimum of 10 equally spaced points. Where a wavelength dependent albedo has been defined it is converted into a single averaged value (with no weighting applied to wavelength).

Legacy: POA equivalent irradiance output

Before the Sensors and full-surface absorber options were added, the POA equivalent irradiance time-series output was the only way to approximate a broadband irradiance sensor from ray tracing results. It applies the same broadband correction described above, but at the resolution of individual cells or modules rather than a placed object. It remains available for backwards compatibility, but a sensor or full-surface absorber is now the recommended way to obtain this kind of result.