PEREZ

This function computes the diffuse irradiance on a tilted surface using the Perez model.

The model estimates sky diffuse irradiance (excluding ground-reflected irradiance) from surface orientation, sun angles, relative airmass, and irradiance components. Users can optionally select different sets of empirical coefficients via the model argument.

Excel Usage

=PEREZ(surface_tilt, surface_azimuth, dhi, dni, dni_extra, solar_zenith, solar_azimuth, airmass, perez_model)

• surface_tilt (float, required): Panel tilt from the horizontal (degrees).
• surface_azimuth (float, required): Panel azimuth (degrees).
• dhi (float, required): Diffuse horizontal irradiance, must be >=0 (W/m^2).
• dni (float, required): Direct normal irradiance, must be >=0 (W/m^2).
• dni_extra (float, required): Extraterrestrial normal irradiance (W/m^2).
• solar_zenith (float, required): Solar apparent zenith angle (degrees).
• solar_azimuth (float, required): Solar azimuth angle (degrees).
• airmass (float, required): Relative (not pressure-corrected) airmass values. Must be >=0 (unitless).
• perez_model (str, optional, default: “allsitescomposite1990”): Name of the Perez coefficient set to use.

Returns (float): The sky diffuse component (W/m^2), or an error string.

Example 1: Calculate sky diffuse for basic inputs

Inputs:

surface_tilt	surface_azimuth	dhi	dni	dni_extra	solar_zenith	solar_azimuth	airmass	perez_model
30	180	100	800	1367	40	180	1.5	allsitescomposite1990

Excel formula:

=PEREZ(30, 180, 100, 800, 1367, 40, 180, 1.5, "allsitescomposite1990")

Expected output:

118.793

Example 2: Perez sky diffuse using the 1988 composite coefficients

Inputs:

surface_tilt	surface_azimuth	dhi	dni	dni_extra	solar_zenith	solar_azimuth	airmass	perez_model
25	180	110	750	1330	42	175	1.35	allsitescomposite1988

Excel formula:

=PEREZ(25, 180, 110, 750, 1330, 42, 175, 1.35, "allsitescomposite1988")

Expected output:

135.151

Example 3: Perez sky diffuse using the Sandia 1988 coefficients

Inputs:

surface_tilt	surface_azimuth	dhi	dni	dni_extra	solar_zenith	solar_azimuth	airmass	perez_model
40	135	90	680	1325	55	140	1.8	sandiacomposite1988

Excel formula:

=PEREZ(40, 135, 90, 680, 1325, 55, 140, 1.8, "sandiacomposite1988")

Expected output:

115.115

Example 4: Perez sky diffuse for a higher-airmass morning case

Inputs:

surface_tilt	surface_azimuth	dhi	dni	dni_extra	solar_zenith	solar_azimuth	airmass	perez_model
50	90	130	520	1390	70	100	2.7	usacomposite1988

Excel formula:

=PEREZ(50, 90, 130, 520, 1390, 70, 100, 2.7, "usacomposite1988")

Expected output:

198.361

Python Code

Show Code

from pvlib.irradiance import perez as result_func

def perez(surface_tilt, surface_azimuth, dhi, dni, dni_extra, solar_zenith, solar_azimuth, airmass, perez_model='allsitescomposite1990'):
    """
    Determine diffuse irradiance from the sky on a tilted surface using one of the Perez models.

    See: https://pvlib-python.readthedocs.io/en/stable/reference/generated/pvlib.irradiance.perez.html

    This example function is provided as-is without any representation of accuracy.

    Args:
        surface_tilt (float): Panel tilt from the horizontal (degrees).
        surface_azimuth (float): Panel azimuth (degrees).
        dhi (float): Diffuse horizontal irradiance, must be >=0 (W/m^2).
        dni (float): Direct normal irradiance, must be >=0 (W/m^2).
        dni_extra (float): Extraterrestrial normal irradiance (W/m^2).
        solar_zenith (float): Solar apparent zenith angle (degrees).
        solar_azimuth (float): Solar azimuth angle (degrees).
        airmass (float): Relative (not pressure-corrected) airmass values. Must be >=0 (unitless).
        perez_model (str, optional): Name of the Perez coefficient set to use. Valid options: 1990 Composite, 1988 Composite, Sandia 1988, USA 1988, France 1988, Phoenix 1988, El Monte 1988, Osage 1988, Albuquerque 1988, Cape Canaveral 1988, Albany 1988. Default is 'allsitescomposite1990'.

    Returns:
        float: The sky diffuse component (W/m^2), or an error string.
    """
    try:
        tilt = float(surface_tilt)
        azim = float(surface_azimuth)
        dh = float(dhi)
        dn = float(dni)
        dn_e = float(dni_extra)
        zen = float(solar_zenith)
        s_azim = float(solar_azimuth)
        am = float(airmass)

        mod = str(perez_model) if perez_model is not None else "allsitescomposite1990"

        if am < 0:
            return "Error: airmass must be >= 0"
        if dh < 0 or dn < 0:
            return "Error: Irradiance values (dhi and dni) must be >= 0"

        res = result_func(
            surface_tilt=tilt,
            surface_azimuth=azim,
            dhi=dh,
            dni=dn,
            dni_extra=dn_e,
            solar_zenith=zen,
            solar_azimuth=s_azim,
            airmass=am,
            model=mod,
            return_components=False
        )

        return float(res)
    except Exception as e:
        return f"Error: {str(e)}"

Online Calculator

surface_tilt *

Panel tilt from the horizontal (degrees).

surface_azimuth *

Panel azimuth (degrees).

dhi *

Diffuse horizontal irradiance, must be >=0 (W/m^2).

dni *

Direct normal irradiance, must be >=0 (W/m^2).

dni_extra *

Extraterrestrial normal irradiance (W/m^2).

solar_zenith *

Solar apparent zenith angle (degrees).

solar_azimuth *

Solar azimuth angle (degrees).

airmass *

Relative (not pressure-corrected) airmass values. Must be >=0 (unitless).

perez_model Optional 1990 Composite 1988 Composite Sandia 1988 USA 1988 France 1988 Phoenix 1988 El Monte 1988 Osage 1988 Albuquerque 1988 Cape Canaveral 1988 Albany 1988

Name of the Perez coefficient set to use.