Performance ratio

Defined as the ratio of final yield to reference yield. It expresses the combined impact of temperature effects, optical and electrical losses, and balance‑of‑system inefficiencies on the system’s delivered energy relative to the ideal reference yield. Note that it neglects rear side irradiance. Collection of that light increases the PR by increasing the E_out.

When working with bifacial systems it may be preferable to analyse bifacial performance ratio.

Output name

Performance ratio

Symbol

PR

Units

-

Formula

$$\frac{E_{out}\mathrm{/}{P}_0}{H_{POA,f}\mathrm{/}{G}_{i,\text{ref}}}$$

SunSolve PR calculation details

SunSolve calculates the front-side and rear-side in-plane irradiation (H_POA,f and H_POA,r) for the PR and PR_bifi equations using a view factor model (i.e. not ray tracing). That view factor model includes losses due to row-to-row shading, but no other optical losses. It does not include transmission though rows due to module or bay spacing. The value used is the average value across the entire module surface. Where more than one module is present it is the average value across all modules in the system.

Thus SunSolve’s performance ratio calculation differs from PVsyst in how it treats row-to-row shading:

SunSolve approach:

• $H_{POA,f}$ includes the effects of row-to-row shading.
• $H_{POA,r}$ includes the effects of row-to-row shading.

PVsyst approach:

• $H_{POA,f}$ uses GlobInc which does not include row-to-row shading.
• $H_{POA,r}$ uses GlobBack + BackShd, which includes row-to-row shading via the VF model.
• This mixed approach is documented in their Performance Ratio (PR) help page

Some points of note:

• Under ideal conditions (e.g. no structural shading, no IAM) the results of view-factor solving match those of ray-tracing.
• The IEC 61724-1 standard defines in-plane irradiance as the “sum of direct, diffuse, and ground-reflected irradiance” and discusses measurement via sensors. It is not explicit regarding the inclusion of row-to-row shading on the value.
• E_out is determined in SunSolve via ray tracing, and thus incorporates the detailed shading losses of the scene.
• For bifacial systems, it is not practical or sensible to calculate unshaded rear-side irradiance. The rear irradiance is fundamentally determined by ground reflection and inter-row geometry, making row-to-row shading an intrinsic part of the rear-side resource. Both SunSolve and PVsyst recognize this reality in their PR_bifi calculations.
• Additionally, when making actual site measurements, POA sensors typically capture some degree of row-to-row shading depending on their location within the array.