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Photovoltaic Performance

Overview

A common way to judge a photovoltaic (PV) system is energy delivered over a period (daily, monthly, or annual). Energy is the time integral of instantaneous power: each power value is the best estimate of output at one moment, given sun geometry, irradiance, temperature, and modelled losses.

Quantity Meaning
Power Instantaneous electrical output (e.g. W)
Energy Integral of power over time (e.g. Wh, kWh)

How PVGIS estimates PV power (overview)

Power is built from in-plane irradiance, then effective irradiance after optical and spectral adjustments, then conversion using temperature-related efficiency, and finally system losses. Sun position sets the solar incidence angle on the collector; that angle links irradiance geometry to the beam component on the surface.

Step What PVGIS does
1 Choose the time span and time resolution for the series.
2 Compute solar altitude as a function of time and location.
3 Compute solar azimuth as a function of time and location.
4 Build sun masks (e.g. above horizon, very low sun, below horizon, shade if applicable).
5 Derive direct irradiance components on horizontal and inclined planes as required by the model chain.
6 Derive diffuse and ground-reflected components when the sun is above the horizon.
7 Sum direct, sky-diffuse, and ground-reflected terms into global in-plane (GII) irradiance.
8 Ingest ambient temperature, wind speed, and spectral inputs where the model uses them.
9 Apply efficiency (temperature and low-irradiance behaviour, technology-specific).
10 Form power as irradiance × effective performance, then apply system loss factors to obtain delivered output.

Performance chain

Delivered PV performance is estimated along a chain that starts from in-plane irradiance (II), adjusts it for reflectivity and spectral effects into effective irradiance (EI), converts EI to photovoltaic power (PP) using module temperature and efficiency, and applies system losses (SL) to produce the final output.

Box label Meaning
In-Plane Irradiance (II) In-plane irradiance into the chain
Reflectivity = f(Incidence) Reflectivity effect (RE)
Spectral Factor Spectral effect (SE)
Effective Irradiance (EI) = II + RE + SE Effective irradiance after RE and SE
Module Temperature = f(Air Temperature, Wind Speed) Temperature / low-irradiance effect (TE)
Photovoltaic Power (PP) = EI + TE Power after EI and TE
System Loss (SL) Loss step after PP
Final output ∑PP = PP + SL Delivered output
Arrow Formula
RE RE = II × Reflectivity
SE SE = II × Spectral factor
TE TE = EI × Efficiency coefficients
SL SL = P × System loss factor 
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flowchart TB
    II["<img src='../../icons/sun-wireless-outline.svg' width='18' height='18'/><br/>In-plane irradiance (II)"]
    RE["<img src='../../icons/sun-angle-outline.svg' width='18' height='18'/><br/>Reflectivity = f(incidence)"]
    SE["<img src='../../icons/noun-sun-525998_modified.svg' width='18' height='18'/><br/>Spectral factor"]
    EI["<img src='../../icons/noun_global_horizontal_irradiance_new.svg' width='18' height='18'/><br/>EI = II + RE + SE"]
    TE["<img src='../../icons/thermometer.svg' width='18' height='18'/><br/>Module temp = f(Tair, wind)"]
    PP["<img src='../../icons/noun-solar-panel-6862742.svg' width='18' height='18'/><br/>PP = EI + TE"]
    SL["<img src='../../icons/noun-panel-4586042.svg' width='18' height='18'/><br/>System loss (SL)"]
    OUT["<img src='../../icons/noun-solar-energy-853048.svg' width='18' height='18'/><br/>Final output sum PP = PP + SL"]

    II --> RE
    RE -->|II x Reflectivity| EI
    II --> SE
    SE -->|II x Spectral factor| EI
    EI --> TE
    TE -->|EI x Efficiency coeffs| PP
    PP --> SL
    SL -->|P x System loss| OUT

    classDef noBox fill:transparent,stroke:transparent,color:#000000;
    class II,RE,SE,EI,TE,PP,SL,OUT noBox;

Power-rating model

The power-rating model in PVGIS 1 is aimed especially at crystalline silicon modules. Its coefficients are informed by indoor and outdoor campaigns at the European Solar Test Installation (ESTI).

  1. Thomas Huld, Gabi Friesen, Artur Skoczek, Robert P. Kenny, Tony Sample, Michael Field, and Ewan D. Dunlop. A power-rating model for crystalline silicon pv modules. Solar Energy Materials and Solar Cells, 95(12):3359–3369, 2011. URL: https://www.sciencedirect.com/science/article/pii/S0927024811004442, doi:https://doi.org/10.1016/j.solmat.2011.07.026. ↩