André Mermoud

I think I have defined these functions. You have a button "Set modules" and "Set all modules". And "Match this table" / "Match all tables". When you resize a table it asks for resizing all identical tables. And when attributing the strings "Distribute one Table" and "Distribute all". Perhaps I have forgotten a parameter ?

When defining an amorphous modules, the parameters (specific to the PVsyst model) are not easy to determine. The best way is to let PVsyst choose all parameters at their default value (Rshunt, Rserie, d2muTau). The temperature coefficient muPmpp should be specified according to the datasheets (negative value). Please pass and check the default checkboxes several times until stabilization, as these parameters are highly interdependent (especially d2MuTau <=> reserie). This is a delicate choice. If you have real difficulties you can send me the datasheets and I will analyse the situation.

You are right. This is explained in detail in the FAQ Why sometimes the oberload losses increase significantly without reason ?

And in the version 6, these limit angles are defined within the project (button "Albedo - Settings"). In the version 6, you have a tool for analysing the spread of orientations in Helios3D constructions, and the program will choose the orientation average as calculation basis.

Sorry, in the present time this configuration cannot be calculated by PVsyst. Although it is authorized by the SolarEdge architecture, PVsyst cannot compute configurations with modules of a same string in different orientations. I will perhaps develop this in a future version.

Not in the present time. Using this in the simulation would only be possible outdoor (otherwise we have to estimate the room temperature). It would imply 2 additional parameters : - The internal temperature increase as function of the instantaneous power - The PMax power derate as function of the internal inverter temperature. Did you ever see these parameters in the inverter datasheets ?

In the version 6, the models have in principle not been significantly changed. However several modifications of default values may explain significant differences in the final results: Defaults values Transposition model In the previous versions (up to Version 5), PVsyst proposed the Hay transposition model as default as it was judged more "robust" than the Perez model. In a recent study, Pierre Ineichen found that the Perez model is giving slightly better results (in terms of RMSD of hourly values) in any case (see "Global irradiance on tilted and oriented planes: model validations", P. Ineichen, 2011). Therefore the Perez model is proposed as default in the version 6. The Perez model gives yearly values significantly higher than the Hay model, of the order of 0% to +2% depending on the climate and the plane tilt. PV module: Rserie parameter Up to Version 5, the default Rserie value was chosen in order to obtain a gamma value (Diode ideality factor) of 1.30 for mono- and 1.35 for poly-crystalline modules, according to our first measurements on some modules. This leads to underestimated low-light performances. But: - By comparison with the Sandia model (obtained by outdoor measurements with dozens of modules), we observed that Gamma should rather be of the order of 1.1 to 1.15. - The low-light data, measured indoor (flash-test) by different independent laboratories, are compatible with still lower Gamma values of the order of 0.9 to 1. We still do not understand quite well this discrepancy between indoor and outdoor measurements. However in the version 6, we fixed the default Gamma value to 1.1, which significantly decreases the irradiance losses of previous simulation (by 2-3% depending on modules and climate). This will affect all modules for which the Rseries was not specified in the database by the manufacturers. When manufacturers propose enhanced Rseries resistances, we require that they provide independent low-light measurements for assessing the proposed values. See What explains the difference of yield between different modules? The default gamma value is specified in the Hidden parameters, topic "PV modules". You can change it even in the version 5 if desired. Module quality and Mismatch losses In the previous versions the default "Module quality loss" was chosen as the medium value between the lower tolerance and 0. In the version 6, the database also mentions the higher tolerance limit for modules. The default "Module quality loss" is now defined as the quarter between the lower and the higher tolerance. This doesn't change anything for symmetrically defined tolerances, but will provide a negative loss factor (gain) for positive sorted modules (for example -0.75 for a -0/+3% module). The mismatch loss parameter was previously proposed by default as 2%, corresponding to PV module samples with an Isc dispersion of the order of 5%. Nowadays the PV modules are specified with narrower tolerance limits, and the delivered samples for a given project are often with 2-3% dispersion. Therefore we diminished this mismatch default loss to 1%. Simulation differences Losses with derate factors (Module quality, Mismatch, Soiling) In the version 5, some loss parameters (derate factors) were specified by respect to the STC power, when the result was evaluated as a percentage of the "previous" energy. This gave a discrepancy in the final results, which were higher (by about 10%] than their parameter. In the version 6 the derate loss factors are specified by respect to the "actual" energy and the results are identical to the parameters. Array Energy calculation In the simulations of the version 5, the electrical behaviour calculation was done globally for the whole sub-array. Therefore if you had, for example, 9 strings on 2 MPPT inputs, the calculation was equivalent to 2 inputs of 4 1/2 strings. In the version 6 the calculation is performed for each inverter separately (one with 4 and one with 5 strings). This may induce difference in case of overpower conditions.

Wiring (ohmic) losses: Remember that the ohmic loss goes with the square of the current, therefore of the Power ! The basic loss parameter is the resistance of the wiring: Pwirloss = Rw * I² [W or kW] However in PVsyst the loss parameter may also be expressed as a loss percentage when running at STC. Therefore: as an example, if we admit a system of 10 kW with a loss of 2% at STC (i.e. under 1000 W/m²): - Under 1000 W/m2, the loss will be R * Istc² = 20 W (2 % of 10'000 W) - Under 500 W/m2, the current will be half, the loss will be R * (Istc / 2)² = R * Istc/4 = 5 W, i.e. 1% of 5000 W In other words, with 2% loss at STC, when running at half the power (under 500 W/m²), the relative loss will be 1% and under 250 W/m2 it will be 0.5%. The loss has to be evaluated at each simulation time step according to the actual power, and the cumulated loss over the year will be of the order of 60% of the specified value in % of STC (depending on climate). Transfo iron loss: The Iron loss is a permanent loss (as soon as the transformer is connected to the grid). It is a 24/24H loss (or eventually about half of this time if you switch OFF the line connexion by night). The iron loss only depends on the grid voltage, therefore it is constant. Only the Ohmic part of the transfo loss is related to the yield, and obeys the rule described above. Overload loss: During the sizing time, the overload estimation results of a very quick and coarse calculation, using the histogram representation of the output of the array along the year. This histogram involves global parameters like an average array temperatures for each power class, and doesn't take into account the inverter's Pnom dependency on the temperature, as well as all array losses. Moreover, it is based on the monthly irradiation values of the project's site, which may not be the same as the Meteo file's values. Therefore, the loss estimation of this sizing tool is not quite accurate, and is often overestimated. The referennce ("exact") value can only be obtained with the detailed hourly simulation. This gives usually lower overload losses, as all the the losses of the array are correctly taken into account. Unavailability: The parameters define an unavailability duration. The unavailabîlity periods (up to 5) may be specified explicitly, or you can ask for a random distribution. Now a failure in winter or in summer, or by clear/covered day, of by night/day, will not have the same consequences on the production of course. Therefore the energy loss is not equal to specified duration. In the present time, it is not possible in PVsyst to specify an unavailability loss with a pre-defined annual value.

You have defined an external transformer. For such a device you have 2 kinds of losses: - Ohmic-like losses: these are proportionnal to the square of the current (or the power as the voltage is constant). - Iron loss: this is proportionnal to the grid voltage, therefore constant. This loss remains of course during thre night. Now in the simulation you have the opportunity of disconnecting the transformer from the grid during not-operating time. Therefore you can chek whether a HV switch would be profitable, i.e. if its price will be compensated by the saved energy price.

When the PV power exceeds the DC power Pnom(dc) corresponding to the inverter Pnom(ac) value, the inverter has to displace the operating point along the P/V curve of the array, in order to just draw the necessary power. This displacement is usually towards higher voltages. The power loss is (Pmpp - Pnom(dc)) for this hour. Overload, usual conditions Now if the voltage corresponding to this Pnom(dc) is over the VmppMax of the inverter, there is no possible operating point with both conditions [Pnom(dc)] and [Vnom(dc) < VmppMax] : the inverter has to stop ! When you come to this situation, the solution is to diminish the number of PV modules in series. Overload with high voltage operating point NB: this behavior arises when the hypothesis of PVsyst are met, i.e. the inverter really stops working above the VmppMax value. This is not necessarily the case in the reality. You should ask the manufacturer for identifying the exact behavior of his inverter. In some cases the real VmppMax is higher than the value specified on the datasheet (we have a case where the specified VmppMax is 800V, and the cut happens at 904V).

In the results panel you have a button for viewing graphs in hourly values. But during the simulation, you can also create a CSV file with a choice among any variable involved in the simulation. This file may then be analysed in a spreadsheet like EXCEL. For this, just before performing the simulation, you have a button "Output File".

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.

First, please carefully check on you report whether all parameters are identical. Compare also the Loss diagrams. If produced by different versions, you can look for eventual modifications on the historical evolution of the software in the help "Overview > Historical evolution of the software", or on our site http://www.pvsyst.com, "Software / Software development". Now if you start from Meteo data in monthly values, the synthetic hourly data file is constructed using a stochastic process: with 2 different executions you will have completely different years (sequences of days and or hours in a day). This may lead usually to discrepancies of the order 0.5 to 1% in the yearly result. These are indeed unavoidable uncertainties due to the stochastic models used. Therefore if you want to perform a quite identical simulations as a colleague, you have to share the exact hourly meteo file used (*.MET).

PVsyst is not able to give definitive answers when comparing the yield of 2 different modules of same technology. A detailed analysis of the parameters has to be performed for such an assessment. Let's limit this analysis to Crystalline modules (mono or poly). The yield (specific energy production, or Performance Ratio) depends on the STC values, but also on 2 additional parameters Rserie and Rshunt (and also the behaviour of the Rshunt value according to irradiance, i.e. Rsh(0) and Rsh(exp)). These parameters are not mentioned in the Standards and in the usual specifications, but they have a great influence on the Low-light performance, and therefore on the annual yield. In absence of further information, these values are set at default values in PVsyst. If Rshunt can be measured (which is very difficult and unreliable for crystalline technology, when using flash-test I/V curve data), it may be specified. But manipulating the defaults for Rshunt irradiance evolution is not advised, as these values seem to reflect a rather "stable" behaviour. The main effect on the low-light behaviour of the model is the Rserie value, which is not directly measurable (see I can't specify my measured Rserie). When not available, PVsyst chooses a default value which corresponds to a pre-defined Gamma value (diode ideality factor) when solving the one-diode model equations. According to my early outdoor measurements on some modules, this value is presently set to Gamma=1.35 for poly and 1.30 for mono, which leads to poor low-light efficiency performances. After comparisons of the one-diode model with the US Sandia model (based on outdoor measurements) on several modules, I discovered recently that this value should be reduced to about Gamma = 1.10 to 1.15. This gives relative low-light efficiencies (by respect to STC) between -0.5% to 0% under 800 and 600 W/m2, and therefore an increased annual yield (about 2% to 3% higher than the previous case, depending on climate). Now when analysing Low-light efficiency data provided by the manufacturers (measured with flash-tests by 3rd-party institutes ), it appears that these efficiencies are still higher, usually around +0.1 to +0.4% for 800 and 600 W/m2 by respect to STC. This discrepancy between models established on outdoor and indoor measurements are not well understood. They may be partially attributed to the fact that the flash-test uses full beam component, when the outdoor have a part of diffuse component, depending on the irradiance and suffering of IAM effect. Therefore in the PVsyst database: - either the Rserie/Rshunts are not specified, and established to Default values (i.e. checkbox "default" checked). This gives under-estimated performances in the present version 5, but will be improved in the future version 6, by requiring Gamma values = 1.10 and 1.15. NB: These modified values may already be defined in the version 5 in the "Hidden parameters". - or these parameters are specified by the manufacturers. In this case we don't accept Gamma values below 1.10, unless the manufacturer can provide an assessment using Low-light efficiencies, measured by an independent institute. Other parameters may give deviations between modules: - The "Module quality loss" parameter takes half the lower tolerance as default value. This will induce a difference of 1.5% between modules with +/-3% tolerance, and positive-sorted (-0/+5%) modules. - Some modules are defined with STC values Imp*Vmp higher than the nameplate specification (for taking positive-sorting into account). The simulation will use the Imp*Vmp for calculation, and the PNom for reference, resulting in overestimated indicators like Specific production or Performance Ratio.

We update the database using the requests of the manufacturers, and publish it with each new issue of PVsyst (about every 1-2 months). We can't of course follow all the new products of all manufacturers in the world. Therefore, please ask your provider/manufacturer to take contact with us (support@pvsyst.com), for updating the database. The database of PVsyst holds now (in 2021) about 19'600 PV modules and 5’800 inverters. Nevertheless you can easily create your own components by yourself. The easiest way is to choose a similar existing device in the database, modify its parameters according to the manufacturer's datasheets, and save it under a new name, therefore creating a new file in your database. For that, you have a tutorial on youtube , / Component database NB: We cannot give any warranty on the database. When using a component in the reality, you are advised to always check the component's parameters with recent datasheets . The values are normally specified by the manufacturers, but they may let some errors that we may not detect. On the other hand, the datasheets may be modified by the manufacturers, without notice.

We cannot give any warranty on the values of the database. When using a component in the reality, you are advised to always check the component's parameters with recent datasheets. Indeed, the datasheets may be modified by the manufacturers, without notice. And there may always be some errors during the retranscription of data (either in the Manufacturer's transmitted data, or during our treatment).

Since March 2016 The PHOTON database is no more available. They have changed their web site and their Database page is not more reachable. However PVsyst is not managing this tool in any way. We have no relationship with the PHOTON Team. And we don't know any alternative database sufficiently detailed for being used as input in PVsyst. The PVsyst database holds about 12'500 PV modules and 4'500 inverters. We update the database using the requests of the manufacturers, and publish it with each new issue of PVsyst. We can't of course follow all the new products of all manufacturers. It would be very big task, and we don't want to include data without the acknowledgement of the manufacturer. Nevertheless you can easily create your own components by yourself. The easiest way is to choose a similar existing device in the database, modify its parameters according to the manufacturer's datasheets, and save it under a new name, therefore creating a new file in your database. For Crystalline modules, except Isc, Vco, Impp and Vmpp, nb of cells in series and module sizes, you can let all the other parameters at their default value. NB: PHOTON still sells a database readable through MS_Access. However there is no link between this database and PVsyst: for using it you will have to recopy all the values manually in PVsyst. On the other hand, make sure that this database is still regularly updated. Procedure before march 2016: Importing data from the PHOTON database is in principle straightforward. In "Tools" / "PVModules", you have a button "Import from PHOTON" When clicking this button, you can immediately go to your Web browser and "Paste the address of the Photon site. After that you choose your component, and select/copy the whole page from the first word up to the end. Returning in PVsyst and opening a "New" component (PV module or Inverter), you should have a button "Import from PHOTON". Troubles: If this button is not visible, there may be 3 problems: - If you have an old version of PVsyst, < 5.4: please download and install the latest version, as the PHOTON web page has completely changed. - If you are using Explorer 9, a problem of access appeared some months ago: In the menu of the browser, you should open "Options"/"Display and compatibility parameters", and here give the authorization to the PHOTON site: http://www.photon.info.com. - With Explorer 9 the format of the copied data has changed recently: please update to the latest version of PVsyst (V5.58 or higher). NB: In the PHOTON database, parameters are not always complete. If some required parameters are missing PVsyst will sometimes not be able to import this component.

The efficiency behaviour involved in the results of PVsyst is a "pure" application of the "One-diode" standard model, corrected for exponential Rshunt as function of the irradiance. In the simulation process, the "Irradiance loss" is the deficit of efficiency as function of the irradiance during the simulation, by respect to the efficiency at nominal conditions (STC, 1000 W/m²). You can see the graph "Efficiency as function of Irradiance level" in the "graphs" tool of the PV module definition. The "Low-light efficiency" is mainly dependent on the Rserie parameter, and the exponential behaviour of the Rshunt (as well as di²MuTau for amorphous technology). - The Rserie resistance: the loss goes with the square of the current (R * I²), therefore increasing quadratically with power. Take now a module under 200 W/m2 irradiance: if the Rserie is high (bad), the losses are higher at STC, and the STC will be lower. Therefore inversely, for two modules having a same Pnom at STC, the module having a high Rserie will have a higher relative efficiency at low levels. - The Rshunt exponential behaviour : when the irradiance diminishes, the Rshunt increases exponentially (and therefore de corresponding loss diminishes). Now if you have a good Rshunt at STC (low losses), you have nothing to gain when coming the low irradiances levels. As a contrary, with a bad Rshunt at STC (as in amorphous modules), the Rshunt loss will strongly diminish toward low levels, therefore increasing the efficiency. See detailed explanations. Therefore modules with identical Pnom at STC , but a bad (high) Rserie, or a high Rshunt dynamics will behave better under low irradiances, i.e. under real operating conditions during the year ! Here is a typical diagram of the efficiency as a function of Irradiance. During the simulation, the corresponding "Low-light" loss is the difference between the STC efficiency (1000 W/m2 @25°C) and the effective efficiency under the present irradiance and 25°C.

In the version 6, you have the opportunity of defining Low-light efficiencies, and use them for the determination of the Rseries. You can do these definitions in 2 different dialogs: Set Rs for optimizing[/i]", this will enable the optimization according to Low-light at each opening (reading) of the module.

Rshunt is the inverse of the slope of the I/V curve around Isc. According to our measurements at sun, it has an exponential behaviour (increases when the irradiance diminishes). For the crystalline modules that we have measured, this behaviour may be approached with an exponential paramater RshExp = 5.5, and an intercept Rsh(0 W/m²) = 4 times the Rsh(1000 W/m²). Rserie is one of the 4 unknown parameters when establishing the one-diode model using a reference I/V curve (i.e. imposing that the curve passes through the 3 reference points (0,Isc), (Vmp, Imp), (Voc, 0) at STC). When we avail of a measured I/V curve, Rserie may be determined by minimizing the errors between the measurement and the modelled I/V curve. NB: Rsmax is the maximun possible value allowing the curve to pass through the 3 reference points, i.e. to solve the equations. Now for crystalline modules or CIS, the manufacturer's data don't specify an I/V curve, so that PVsyst has to do some hypothesis for proposing default values: - 1/Rshunt is taken as a given fraction of the (Isc-Imp)/Vmp value, - Rserie is chosen for getting a fixed value of the Diode Ideality factor Gamma in the equations. These values have strong influence on the Low-light efficiency. Therefore when we avail of reliable low-light efficiency data, the Rserie value may be chosen for approaching them at best.

The "Low-light efficiency" is mainly dependent on the Rserie and Rshunt (exponential behaviour) parameters (as well as di²MuTau for amorphous technology). Rserie impact The loss goes with the square of the current (Rs * I²), therefore increasing quadratically with power. Take now a module designed for an irradiance under 100 W/m2: if the Rserie is high (bad), the losses are higher at high irradiances (orange curve). Now the characteristics of the module you buy is specified at STC. Therefore if you want to design a module of the same Power at STC as your "good" module, this new module with high Rseries should be globally of better quality at low irradiances: Therefore a module of same STC performance, but with bad Rseries will give better low-light performances ! In other words: the module is globally of much better quality, but the high Rserie value penalizes the STC performances. Rshunt impact The shunt resistance at STC has a very low impact on the Low-light efficiency. However the exponential behavior as function of the irradiance will decrease the Rshunt loss, and therefore enhance the efficiencies under 400 W/m². The picture shows the case of a crystalline module, with a dynamics Rsh(0) / Rsh(STC) of 4. However the amorphous modules have a much lower Rshunt at STC, and a much higher dynamic Rsh(0) / Rsh(STC) of around 12. The figure shows that the efficiency difference would be significant between different Rshunt if they were constant. But the recovery of the exponential behavior leads to very similar curves. Therefore the recovery of low-light efficiency is better with low Rshunts and high exponential dynamics.

The Rserie specified in the parameters of the module is not the measured Rserie dV/dI, but the intrinsic Rserie of the one-diode model. It doesn't include the exponential contribution that you see on the graph below. In the PV module's parameters dialog, the measured value dV/dI, named "Apparent Rserie" is mentioned just below this value in the program. When the Rserie is specified by manufacturers (as "measured Rserie"), it corresponds to this "apparent" value. I/V curve with Rshunt and Rserie

The model implemented in PVsyst includes a spectral correction for amorphous modules, but not for crystalline nor other technologies. The correction for amorphous modules is based on a parametrization as a function of air mass and Kt proposed by Betts and al "SPECTRAL IRRADIANCE CORRECTION FOR PV SYSTEM YIELD CALCULATIONS", 19th European Photovoltaic Solar Energy Conference, 7-11 June 2004, Paris, France. According to my own experiments, when putting amorphous modules (tandem or tripple juction) at sun, recording one I/V curve every 10 minutes, and applying the one-diode model to each measurement, the errors Pmpp (measure-model) have RMSD of the order of 2.3% of Pnom over a full year (i.e. under any irradiance and temperature conditions). Without applying the spectral correction, this RMSD increases by about 0.6%, which is therefore the order of magnitude of the spectral effect over a full year for this amorphous module. (see my article "Performance assessment of a simulation model for PV modules of any available technology", Mermoud, A. & Lejeune, T., 2010, 25-th European Photovoltaic Solar Energy Conference and Exhibition (EU PVSEC), Valencia, Spain, 6-10 september 2010) For crystalline modules, the sensitivity to the spectral variations according to weather are rather small (see the conclusions of the article of Betts). My measurements show errors Pmpp (measure-model) with a RMSD of the order of 1.2% of Pnom over a full year, in any irradiance and temperatures conditions. And still less for CIS (RMSD = 1% over 6 years!). Therefore if a spectral corrections has to be applied, it would be within this 1% to 1.2 % dispersion. Now the one-diode model is based on the STC performances, which are specified for an AM1.5 spectrum. The eventual variations of spectrum along the year would be around this STC Pmpp value. But who knows exactly the spectral contents of the irradiance for any weather (and especially the covered conditions), except through Air mass and Kt correlations ? NB: The Sandia model does indeed define a spectral correction for crystalline modules, as a correlation by respect to the Air mass (therefore valid for clear sky conditions). Applied to simulations in middle-Europe, this induces a spectral gain of the order of 0.4 to 0.5% over the year. However this correlation is probably over-estimated during the simulation, because it is not related to the clearness index Kt, so that it is applied identically to cloudy conditions.

The Pmpp value calculated by the one-diode model in PVsyst may be different from the Nameplate value. Indeed you have 3 values which may all be different: - Pnom (nameplate), - specified Vmpp*Impp at STC, - Vmpp*Impp as computed by the model at STC. - The Nameplate Pmpp is sometimes not equal to the Impp*Vmpp as specified by the manufacturer at STC. Since recently, some manufacturers include the positive sorting in the STC values. That is, for a given nameplate module, they define STC Vmpp*Impp values superior by 1-3%. This of course distorts the interpretation of the PR in the simulation results (produces an artifical increase of the PR). In the database of PVsyst, we don't accept a discrepancy (PNom - Imp*Vmp(STC)) higher than 0.2%, and we correct the Imp value in order to get the exact Pnom (some old modules may have not been corrected). In the versions >= 6.39, you have an information when a discrepancy occurs. And the modules imported from PHOTON are corrected in this way. - The Impp*Vmpp calculated by the one-diode model at STC may be higher than the Imp*Vmp specified value. Indeed, the model is established for passing the I/V (or P/V) curve through the 3 specified points (0,Isc), (Vmp, Imp), (Voc,0). Now, nothing ensures that the specidied (Vmp,Imp) point is the true maximum of this P/V curve. If the true maximum is different, it will of course be higher. This discrepancy may be sometimes up to 1% ! This last effect could be considered as a weakness of the one-diode model: the model doesn't reproduce the exact I/V curve shape of the manufacturer. But more likely this indicates that the manufacturer's specifications are not correct! For establishing their datasheets, manufacturere usually avail of the mesurements of one set or identical modules (of very close powers). We don't know how they extend the (Vmp, Imp) specifications to the other power classes. According to the matching of most modules, we consider that the one-diode model is reliable in this respect. NB: in the PV module definition dialog, when pointing the mouse on the PNom value, all 3 values will be displayed. Effect on the PR and the Specific production The PR (Performance ratio) is related to the Pmpp(STC) value. The "official" definition (as shown on the results page) uses the Nameplate value PNom. But if you evaluate the PR from the simulation results (e.g. the loss diagram), it will involve the Pmpp of the module at STC, as computed by the one-diode model. The PR will be increased in the same proportion as the ratio Pmpp (Model)/Pmpp(Nameplate).

The temperature Coefficient of the open circuit voltage muVoc is not an input parameter, it is a result of the one-diode model. It is closely related to the choice of the Rserie value. You could try to choose the RSerie value giving your specified coefficient, but it is not always possible (the Rserie values may be out of its allowed range "0 to Rsmax"), and it is not a good idea, as other criteria are much more significant for the choice of Rserie (namely the low-light efficiency). NB: Rsmax is the maximun possible value allowing the curve to pass through the 3 reference points (i.e. solving the equations). It is not possible to adjust at the same time a specidfied muPmpp value (temperature coefficient on Pmpp) and muVoc. This is a property of the one-diode model. And the muPmpp value is very important, as it is involved in the operation during the simulation. While the muVoc is not implied in the simulation process. It essentially acts on the sizing limit for the determination of the maximum possible voltage (which should not overcome the VmaxAbs of the inverter, nor the Vmax of the modules, for the lower design temperature conditions). You have the choice of using two different values during the system sizing: - by default, the muVco as result of the model (which depends on the temperature) - if you define the muVoc as specified by the manufacturer in the module's parameters, you can ask for using this value during the sizing process ("Project's settings" button in the project's dialog) NB: you will find the "manufacturer" specification for muVoc value in the PV module definition dialog, page "Additional data > Secondary parameter" NB: In some special conditions (namely with modules specified with a very low gamma value, i.e. Rseries very close to its maximum), for calculation reasons the the lower value of the diode saturation current is limited to 0.1 pA. This limit is sometimes attained with very low temperatures (< 0°C), which leads to positive muVco values. Don't worry: the model still works correctly, only the temperature behavior below this temperature is slightly affected ! We are here at the boundaries of the one-diode model. If you want to decrease this low-temperature limit behaviour, you should increase a little bit the Voc (STC) value, and readjust the Rserie for getting the equivalent Low-light behaviour. This will result is a more robust model.