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Page • Driesse et al. 2008 presents an alternative inverter performance model to accurately express the effects of both power level and input voltage on PV inverter efficiency.  Its accuracy was evaluated using CEC measurements on inverters of different sizes and designs. The model is expressed by the following formula: $$\small p_{loss} =...

Page • The effect of a one-degree error in the azimuth angle for a latitude tilt system is similar throughout the year.  The first figure below shows the POA irradiance error pattern on the summer solstice.  At other times of the year the pattern is similar except for the changes in the length of...

Page • The three figures below show the effect of a one-degree error in the tilt angle on the plane of the array irradiance for key times of the year. At the equinoxes, the error is negligible. At the summer solstice the error is negative (defined as (POA36.05° – POA35.05°) and lasts longer...

Page • Effective irradiance is total plane of array (POA) irradiance adjusted for angle of incidence losses, soiling, and spectral mismatch. In a general sense it can be thought of as the irradiance that is “available” to the PV array for power conversion. In the context of the Sandia PV Array Performance Model (SAPM),...

Page • Extraterrestrial radiation ($E_a$) is the intensity (power) of the sun at the top of the Earth’s atmosphere.  It is usually expressed in irradiance units (Watts per square meter) on a plane normal to the sun.  It varies throughout the year because of the Earth’s elliptical orbit, which results in the...

Page • David Faiman presented a module temperature model (Faiman 2008) based on simple heat transfer concepts.  The model form is: $$T_{m}=T_{a}+\frac{E_{POA}}{U_{0}+U_{1}\times WS}$$ where $$T_{m}$$ is module temperature (°C)$$T_a$$ is ambient air temperature (°C)$$E_{POA}$$ is the irradiance incident on the plane of the module or array ($$W/m^{2}$$)$$U_0$$ is the constant heat transfer component ($$W/m^{2}K$$)$$U_1$$ is the convective...

Page • Preliminary measurements were made at Sandia to demonstrate the process of measuring bifacial gains and to illustrate some of the challenges of designing bifacial experiments that are representative. In this short term test, we mounted two bifacial and two monofacial modules in two-up, landscape orientation at 35° tilt. It is...

Page • A fixed tilt array orientation is described with an array tilt angle ($$\theta_T$$) and an array azimuth angle ($$\theta_{A,array}$$). For example, the orientation of a south-facing, fixed-latitude-tilt array in Albuquerque, NM (latitude = 35.05 degrees N) would have a tilt angle of 35.05 degrees from horizontal and an array azimuth of...

Page • The Fuentes cell temperature model is a first-principles energy balance model that, unlike most PV thermal models, includes the effect of thermal transience.  It was originally developed for the PVFORM simulation software and was later adopted by PVWatts (most notably versions 1 and 5).  In addition to the usual plane...

Page • Global Horizontal Irradiance (GHI) is the amount of terrestrial irradiance falling on a surface horizontal to the surface of the earth. GHI can be measured with a variety of instruments. The most common instrument used to measure GHI is called a pyranometer which has a hemispherical (180°) view angle. The...

Page • Sandia National Laboratories in Albuquerque, NM has measured global horizontal spectral irradiance nearly continuously from September 2019 to March 2022. During this time other broadband irradiance measurements (global horizontal, direct normal, diffuse horizontal and global normal) and and weather variables were also recorded. For this dataset PV Performance Labs (PVPL) has pulled...

Page • The Photovoltaic Systems Evaluation Laboratory (PSEL) at Sandia National Laboratories (SNL) in Albuquerque, NM has measured global normal spectral irradiance nearly continuously from August 2013 to April 2018. During this time other broadband irradiance measurements (global horizontal, direct normal, diffuse horizontal and global normal) and weather variables were also recorded....

Page • <Download form at bottom of page> The power industry is seeing large amounts of distributed generation being added onto the electric power distribution system. This presents a new set of issues, especially for renewable generation with variable intermittent power output. It is important to precisely model the impact of solar...

Page • The Hay and Davies diffuse model  divides the sky diffuse irradiance into isotropic and circumsolar components.  Horizon brightening is not included.  An anisotropy index, $$A_i$$ , is defined as: $$A_i=\frac{DNI}{E_{a}}$$, where $$DNI$$ is the direct normal irradiance and $$E_a$$ is the extraterrestrial radiation. The Hay and Davies model formulation for sky diffuse radiation is: $$E_d-DHI\times\left[A_i R_b +\left (1-A_i\right)\frac{1+\cos \left(\theta_T\right)}{2}\right]$$,...

Page • Described by Coello and Boyle (2019), the Humboldt State University (HSU) soiling model predicts time series soiling ratio SR based on accumulated particulate mass density $$\omega$$ (g/m2): $$\DeclareMathOperator\erf{erf}SR = 1 – 0.3437 * \erf(0.17 * \omega^{0.8473})$$ where $$\erf$$ refers to the Gauss error function.  Accumulated mass density $$\omega$$ is the...

Page • 2025 IEA-PVPS T13-31:2025: Floating Photovoltaic Power Plants: A Review of Energy Yield, Reliability, and Maintenance (424 downloads ) IEA-PVPS T13-29:2025: Dual Land Use for Agriculture and Solar Power Production: Overview and Performance of Agrivoltaic Systems (449 downloads ) IEA-PVPS T13-30:2025: Degradation and Failure Modes in New Photovoltaic Cell and Module...

Page • Data from Annex 1 of the IEA PVPS Task 13 report entitled: Climatic Rating of Photovoltaic Modules: Different Technologies for Various Operating Conditions (32286 downloads ) The data set consists of two parts: 1. Module performance data Pmax, Isc, and Voc matrixTemperature coefficient α, β and δSpectral response curveAngular loss...

Page • Bill Marion from NREL developed a correction method that improves the ability of PVWatts to represent the nonlinear behavior at low irradiance, which is evident in x-Si module performance (Marion, 2008). Figure from Marion (2010) presentation: “Overview of the PV Module Model in PVWatts” Marion found that the error could...

Page • Unless the PV array is mounted on a two-axis tracker, the incident angle for the direct component of the solar radiation will not be normal except for a few rare instances, depending on the orientation.  When the angle of incidence is greater than zero, there are optical losses due to...

Page • Before a PV Lifetime system is installed all the modules are characterized indoors on a flash simulator at standard test conditions (STC).  Every year afterwards a sample of modules are removed from the arrays and re-characterized.  All this data will eventually be made available. Guidelines for determining the relationship between...

Page • Inverter saturation, commonly referred to as “clipping”, occurs when the DC power from the PV array exceeds the maximum input level for the inverter.  In response to this condition, the inverter typically adjusts DC voltage to reduce the DC power.  This is done by increasing voltage above the MPP voltage,...

Page • Weather and irradiance data are used as input to PV performance models.  These data are directly measured, derived from measured data, or simulated using a stochastic model. Irradiance is to power as insolation is to energy.  Or in other words: Irradiance is an instantaneous measurement of solar power over some area.  The units of irradiance are watts per...

Page • The isotropic sky diffuse model is the simplest of the POA sky diffuse models and forms the foundation upon which more accurate models build.  The isotropic sky diffuse model assumes that the diffuse radiation from the sky dome is uniform across the sky.  The POA sky diffuse irradiance ($$E_{d,iso}$$) is...

Page • The Kimber soiling model is a simple model of predicting a time series of soiling ratio values based on an assumed linear soiling accumulation rate and the timing of rainfall and other cleaning events. The model’s soiling rate represents the rate of performance loss accumulation rather than some physical rate...