Performance Ratio

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Performance Ratio

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See also the previous page Normalized Performance index.

The Performance Ratio is the ratio of the energy effectively produced (used), with respect to the energy which would be produced if the system was continuously working at its nominal STC efficiency. The PR is defined in the norm IEC EN 61724.

In usual Grid-connected systems, the available energy is E_Grid. In stand-alone systems, it is the PV energy effectively delivered to the user, i.e. E_User - E_BackUp. In pumping systems, this is E_PmpOp.

The energy potentially produced at STC conditions is indeed equal to GlobInc * PnomPV, where PnomPV is the STC installed power (manufacturer's nameplate value). This equivalence is explained by the fact that at STC (1000 W/m², 25°C) each kWh/m² of incident irradiation will produce 1 kWh of electricity.

Therefore for a grid-connected system:

     PR  =  E_Grid  /  (GlobInc * PnomPV)



The PR includes the optical losses (Shadings, IAM, soiling), the array losses (PV conversion, aging, module quality, mismatch, wiring, etc)  and the system losses (inverter efficiency in grid-connected, or storage/battery/unused losses in stand-alone, etc).

Unlike the "Specific energy production" indicator, expressed in [kWh/kWp/year], this indicator is not directly dependent on the meteo input or plane orientation. This allows the comparison of the system quality between installations in different locations and orientations.

Namely, the PR is not dependent on the PV module efficiency.  As an example an amorphous module and a crystalline high-efficiency module will lead to comparable PR.  Only the low-light performance and temperature dependency will induce differences.

A tracking system  will have a similar PR than a fixed sheds arrangement. Even sometimes slightly  lower because the array temperature (related to GlobInc) may be higher.

With equivalent yield performances, a tracking system with backtracking will have a significantly higher PR than a non-backtracking one, because the effective irradiance in the collector plane is lower (due to the not optimal orientation). In a conventional tracking system the mutual shadings highly affect the PR.

In big systems (sheds or trackers), the far albedo is only "seen" by the first row, so that its effect is negligible (shading factor = (n-1)/n, where n = nb. of sheds).  Therefore if you increase the albedo in your project, the GlobInc value will increase, the albedo loss will increase for a same production, and the PR will  decrease.

Self-consumption and storage

The PR is an indicator of the availability of solar energy for final uses. Therefore, when a part of the energy is used internally (E_Solar), this should obviously be included in the PR evaluation. With systems including a storage, the storing losses (battery charge/discharge inefficiency, DC-AC and AC-DC conversion devices)  should also be included in the PR.

Therefore in the above formula, the E_Grid should be replaced by E_Grid + E_Solar :

PR  =  (E_Grid + E_Solar)  /  (GlobInc * PnomPV)

Bifacial PR

If the above definition of the Performance Ratio calculation is applied to bifacial systems, then the bifacial contribution from the rear side of the PV modules will become a gain, which will increase the PR. For systems with high tilt, like for example East-West facing vertical PV systems, this can easily lead to PR values larger than 100%. Conceptually this poses no problem, as long as the bifacial contribution is just interpreted as a gain.

The revised IEC 61724 -1 standard (Ed.2 from 2021), introduces the concept of a bifacial Performance Ratio. The basic idea is that the additional irradiance contribution on the rear side of the PV modules is added to the Global incident irradiance.

This is problematic if the PR value is used to compare different system designs, since the rear side irradiance depends on design choices like mounting height, row spacing and PV module tilt and azimuth. Section in the IEC standard specifies that the rear side irradiance contribution is to be measured on PV modules close to the center of the PV system, and without exceptional sources of shadings, as to ensure "representative" conditions. This definition however leaves a wide margin of interpretation. Furthermore, the rear side irradiance distribution is non-uniform by nature, and it is not possible to define a single representative measurement spot for all sun and weather conditions.

Another ambiguity of the IEC standard is in the use of the bifaciality factor as a multiplicative factor for the backside irradiance. Main paragraph §3.20 implies that the sum of front-side and backside irradiances is to be considered, and this is the choice made in PVsyst (see formula below). However, notes 1 and 2 of §3.20 indicate that the bifaciality factor may be included in the definition as a multiplicative factor for the backside irradiance. When measuring the total effective irradiance with a bifacial reference device such as a bifacial reference cell, this second option may be the most natural one.

In PVsyst, in order to calculate the bifacial PR, the rear side irradiance is approximated as GlobBak + BackShd, where GlobBak is the effective irradiance on the rear side of the PV modules, and BackShd are the losses induced by the 'Structure shading factor' in the bifacial model definitions.

The bifacial PR then becomes

PRbifi = PR / (1 + (GlobBak + BackShd)/(GlobInc))

Please note, that this concept of bifacial PR is by nature blind to changes in ground albedo, mounting height and to the effect that row spacing and PV module orientation have on the bifacial contribution.

In the simulations of bifacial systems, the bifacial PR calculation will be displayed separately in the main results summary. By definition, it will be smaller than the standard PR value.


Use of PR  as in-site production assessment:

The PR is an important metric in the PV industry, it is often used as a contractual condition / warranty when commissioning a PV system, or for the verification of the annual yield.

This is not an easy task, as the PR is not constant over the year, and the effective GlobInc value is not always available.

Weather-corrected PR

For short time analysis (commissioning, one-week tests), a NREL paper proposes a  "Weather-corrected Performance ratio".  It has since then been included in the norm IEC 61724-1.

The objective is to get rid of the seasonal variations of the PR, mainly due to the varying array temperature. Other weather contributions like irradiance level, wind velocity, varying soiling, etc are not taken into account in this approach.

The proposition is to define an average array temperature, which is an average  over all operating hours in the year, weighted by the incident irradiance GlobInc.

Then for a specified period, the PR (corr)  is defined by the following equation:

  PR (corr)  =  E_Grid  /  ( PNomPV  * Σ hours   ( GlobInc / GRef  * (1 + muPmpp * (Tarray - TArrayAver) ) )

Where :

-  GlobInc       = incident irradiance in hourly values

-  GRef           = 1000 W/m²

-  muPmpp     = Pmpp temperature coefficient of the PV module

-  TArray         = Array (cell) temperature of this hour

-  TArrayAver = Array temperature average over the whole year, weighted by GlobInc, i.e.:

TArrayAver =   Σ hours (GlobInc * TArray)  / Σ hours (GlobInc)


Mathematically, if the  TArrayAver is calculated with the same data, the  yearly PR(corr) value should be equal to the yearly PR.  

We have implemented this in the PVsyst version 6.74. However the result is not so convincing, because in most PV systems, other contributions are season-dependent.

With shed-systems, applying this PR correction seems to over-correct the PR seasonal behavior. This is due to the mutual shadings, which are more pronounced in winter.  

This correction is almost unusable with tracking systems, as the seasonal variations due to tracking are largely dominating the temperature effect.

Therefore these results will be available on the report only when you ask it in the "Report Preferences", menu "Settings" in the Report editing window.