How is the PR (Performance Ratio) calculated ?

[André Mermoud]

The performance ratio is described in the help "Project design > Results > Normalised performance index".

As defined namely by the European Communities (JRC/Ispra), in the norm IEC EN 61724, it is computed by

PR = E_Grid / (GlobInc * Pnom)

where:

- E_Grid = the energy delivered to the grid [kWh],

- GlobInc = Irradiation in the plane of array [kWh/m2]

- Pnom = Array nominal power at STC (nameplate value) [kWp]

The product (GlobInc * Pnom) is numerically equivalent to the Energy which would be produced if the system was always running with its nominal efficiency as defined by the nameplate nominal power [kWh].

NB: The PR includes all the array losses mentioned on the Loss diagram (Shadings, IAM, Soiling, PV conversion, mismatch, wiring resistance, etc) and the system losses (inverter efficiency and AC losses in grid-connected, or storage/battery/unused losses in stand-alone, etc).

As it is referenced to the Incident irradiance, it is not dependent (or marginally) on the meteo data, location, plane orientation,

As it is referenced to the Nominal power, it is not dependent on the module efficiency.

Unlike the "Specific energy production" indicator, expressed in [kWh/kWp/year], the PR is related to the system quality, and allows the comparison between installations in different locations and orientations.

It is often used as Performance Warranty basis.

NB: if you take the values from the arrow-loss diagram, the GlobInc value should be taken just after the transposition (i.e. irradiance losses are included in the PR).

The results may be slightly different, as here the Nominal STC Power is referred to the efficiency at MPP calculated by the model (not the nameplate value used for the "official" definition), which may be different.

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)

Weather-corrected PR

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

The objective is to get rid of the seasonal variations of the PR, mainly due to the varying array temperature. Other contributions varying along the year, like irradiance level, seasonal shadings, 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:

TArrayAver = Σ hours (GlobInc * TArray) / Σ hours (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 [W/m²]

- GRef = 1000 W/m²

- muPmpp = Pmpp temperature coefficient of the PV module [%/°C]

- TArray = Array (cell) temperature of this hour [°C]

- TArrayAver = Array temperature average over the whole year, weighted by GlobInc [°C]

In PVsyst the weather-corrected result variable is named PRTemp. You can get it on the report by using "Settings > Report preferences" in the Report editing menu.

PR for bifacial systems

The definition of the performance ratio should be something like a standard, defined by an official instance, and accepted by everybody.

Now I have not yet seen any reference which would define a performance ratio for bi-facial systems. Therefore PVsyst cannot propose any specific value in the present time.

The value provided presently with the PVsyst results uses the definition of the Monofacial systems, so that the bi-facial gain comes as an increase of this ratio.

NB: The main objective of the PR is to find an indicator for comparing real and simulated data, therefore which may easily be evaluated using simple (and "primary") measured data.

However neither the rear side irradiance, nor the part of the bifacial gain is available in usual measurements.

[Michele Oliosi]

Following the update of IEC 61724-1:2021, since version 7.2.13 you will find a bifacial performance ratio in the results summary window.

This bifacial performance ratio is computed as:

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

(see https://www.pvsyst.com/help/performance_ratio.htm)

Note that this may not correspond to notes 1 and 2 of §3.20 in the norm IEC 61724-1:2021, since it does not include the bifaciality factor. It is instead based on a literal interpretation of the IEC 61724-1:2021 §3.20 main paragraph.

Edited November 11, 2022 by Michele Oliosi
BackShd is positive -> should be added to GlobBak