NREL SSC settings

Method

The Perez equations run with the inputs and constants below. The solar constant is 1367 W/m², with no correction for the Earth's distance from the sun. The direct normal irradiance is the file's direct normal column, and it enters the sky clearness without a cap. The sky brightness uses the Kasten 1966 air mass with the constant 3.9° in place of 3.885° (4), and no altitude factor. The circumsolar coefficient is floored at zero. The coefficients are those of the 1990 table with seven digits, and a sky clearness equal to a bin edge takes the lower row. Two rules act outside the equations. While the sun is less than 2.5° above the horizon, the diffuse light on the plane is a uniform sky, $D\,(1 + \cos\beta)/2$, with no circumsolar part and no horizon band. This holds also on a lit timestep whose sun stands at or below the horizon, which the NREL SPA (SSC application) sun model gives between sunrise at −0.8333° and the horizon; such a timestep receives no direct light. The light reflected by the ground is the albedo times $(\text{DNI}\sin\alpha + D)(1 - \cos\beta)/2$, so it is computed from the file's direct and diffuse columns; it is zero below 2.5° and whenever $D$ is zero.

Settings

Setting	Value
Solar constant $S$	1367 W/m²
Eccentricity factor $e_t$	1
Direct normal irradiance $\text{DNI}_{used}$	the file's direct normal column
Cap on the direct part of the clearness $c_\varepsilon$, as a fraction of $E_0$	none
Air mass $\text{AM}$	Kasten 1966, with 3.9° in place of 3.885°
Altitude factor $f_z$	1
Floor of the circumsolar coefficient $F_{1,min}$	0
Circumsolar cutoff elevation $\alpha_c$	none
Constant of the zenith term of the sky clearness	5.534·10⁻⁶ per degree cubed, on the zenith angle in degrees (1.041 per radian cubed)
Sun elevation below which the sky is uniform	2.5°
Ground-reflected light	the albedo times $(\text{DNI}\sin\alpha + D)(1 - \cos\beta)/2$; zero below 2.5° and when $D = 0$
Coefficient table	Perez 1990, all-sites composite, 8 clearness bins, seven digits

The math

1. Extraterrestrial irradiance
2. Direct sunlight
3. Sky clearness
4. Sky brightness
5. Circumsolar and horizon-band coefficients
6. Uniform part
7. Circumsolar part
8. Horizon-band part
9. Diffuse sky light on the plane

$$E_0 = 1367 \tag{1}$$ $$\text{DNI}_{used} = \text{DNI},\qquad B = \text{DNI}_{used}\,\max(0,\ \cos\theta) \tag{2}$$ $$\varepsilon = \frac{(D + \text{DNI}_\varepsilon)/D + 5.534\cdot 10^{-6}\,\theta_{z,d}^3}{1 + 5.534\cdot 10^{-6}\,\theta_{z,d}^3},\qquad \text{DNI}_\varepsilon = \text{DNI}_{used} \tag{3}$$ $$\Delta = \frac{D\,\text{AM}\,f_z}{E_0},\qquad \text{AM} = \frac{1}{\sin\alpha + 0.15\,(\alpha + 3.9)^{-1.253}},\qquad f_z = 1 \tag{4}$$ $$F_1 = \max\!\left(0,\ f_{11} + f_{12}\,\Delta + f_{13}\,\theta_z\right),\qquad F_2 = f_{21} + f_{22}\,\Delta + f_{23}\,\theta_z \tag{5}$$ $$D_{iso} = D\,(1 - F_1)\,\frac{1 + \cos\beta}{2} \tag{6}$$ $$D_{circ} = D\,F_1\,\dfrac{\max(0,\ \cos\theta)}{\max(\sin 5°,\ \sin\alpha)} \tag{7}$$ $$D_{hb} = D\,F_2\sin\beta \tag{8}$$ $$D_{plane} = D_{iso} + D_{circ} + D_{hb} \tag{9}$$

where

Symbol	Meaning	Unit
$E_0$	the extraterrestrial normal irradiance	W/m²
$\text{DNI}_{used}$	the direct normal irradiance the model uses	W/m²
$\text{DNI}$	the direct normal irradiance of the weather file	W/m²
$B$	the direct sunlight on the module plane	W/m²
$\theta$	the angle between the sun and the perpendicular to the module plane	degrees
$\varepsilon$	the sky clearness, which selects the row of the coefficient table	—
$D$	the diffuse horizontal irradiance of the weather file	W/m²
$\text{DNI}_\varepsilon$	the direct normal irradiance that enters the sky clearness	W/m²
$\theta_z$	the zenith angle of the sun, 90° minus $\alpha$	radians
$\theta_{z,d}$	the zenith angle of the sun	degrees
$\Delta$	the sky brightness	—
$\text{AM}$	the relative air mass of Kasten 1966 as coded in SSC	—
$f_z$	the altitude factor of the sky brightness	—
$\alpha$	the elevation of the sun above the horizon	degrees
$F_1$	the circumsolar coefficient	—
$F_2$	the horizon-band coefficient	—
$f_{11} \ldots f_{23}$	the six coefficients of the clearness row, from the published table	—
$D_{iso}$	the uniform part of the diffuse light on the plane	W/m²
$\beta$	the tilt of the module plane from the horizontal	degrees
$D_{circ}$	the circumsolar part of the diffuse light on the plane	W/m²
$D_{hb}$	the horizon-band part of the diffuse light on the plane	W/m²
$D_{plane}$	the diffuse sky light on the module plane	W/m²

References

1. Perez, Ineichen, Seals, Michalsky & Stewart 1990, Modeling daylight availability and irradiance components from direct and global irradiance, Solar Energy 44(5) 271–289
2. Kasten 1966, A new table and approximation formula for the relative optical air mass, Archiv für Meteorologie, Geophysik und Bioklimatologie B14 206–223
3. Dobos 2014, PVWatts Version 5 Manual, NREL/TP-6A20-62641
4. NREL SAM Simulation Core (SSC) source code, release 2026.7.3, github.com/NatLabRockies/ssc: shared/lib_irradproc.cpp, function perez()

API

array.perez_settings = ssc

Symbols follow the symbol registry.