PVsyst 8 settings

Method

The Perez equations run with the inputs and constants below. The solar constant is 1361 W/m². The direct normal irradiance is computed from the file's global and diffuse horizontal values; the file's direct normal column is not used. The direct sunlight and the circumsolar part are both zero while the sun is less than 2° above the horizon. Before it enters the sky clearness, the direct normal irradiance is capped at 0.9 of the extraterrestrial irradiance. The sky brightness uses the Kasten 1966 air mass and an altitude factor that decreases with the site elevation. The circumsolar coefficient has no floor: under some overcast skies it is negative, which makes the circumsolar part negative and raises the uniform part.

Settings

Setting	Value
Solar constant $S$	1361 W/m²
Eccentricity factor $e_t$	$1 + 0.033\cos\frac{2\pi t}{8760}$, with $t$ in hours
Direct normal irradiance $\text{DNI}_{used}$	$(G_h - D)/\sin\alpha$ from the file's global and diffuse horizontal values; zero when $\alpha < 2°$
Cap on the direct part of the clearness $c_\varepsilon$, as a fraction of $E_0$	0.9
Air mass $\text{AM}$	Kasten 1966
Altitude factor $f_z$	$e^{-1.30 \times 10^{-4}\,z}$, with $z$ the site elevation in m
Floor of the circumsolar coefficient $F_{1,min}$	none; negative values are kept
Circumsolar cutoff elevation $\alpha_c$	2°
Coefficient table	Perez 1990, all-sites composite, 8 clearness bins

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

$$E_0 = 1361\,\left(1 + 0.033\cos\frac{2\pi t}{8760}\right) \tag{1}$$ $$\text{DNI}_{used} = \begin{cases} \dfrac{G_h - D}{\sin\alpha} & \alpha \ge 2° \\ 0 & \alpha < 2° \end{cases},\qquad B = \text{DNI}_{used}\,\max(0,\ \cos\theta) \tag{2}$$ $$\varepsilon = \frac{(D + \text{DNI}_\varepsilon)/D + 1.041\,\theta_z^3}{1 + 1.041\,\theta_z^3},\qquad \text{DNI}_\varepsilon = \min\!\left(\text{DNI}_{used},\ 0.9\,E_0\right) \tag{3}$$ $$\Delta = \frac{D\,\text{AM}\,f_z}{E_0},\qquad \text{AM} = \frac{1}{\sin\alpha + 0.15\,(\alpha + 3.885)^{-1.253}},\qquad f_z = e^{-1.30 \times 10^{-4}\,z} \tag{4}$$ $$F_1 = f_{11} + f_{12}\,\Delta + f_{13}\,\theta_z,\qquad F_2 = f_{21} + f_{22}\,\Delta + f_{23}\,\theta_z \tag{5}$$ $$D_{iso} = D\,(1 - F_1)\,\frac{1 + \cos\beta}{2} \tag{6}$$ $$D_{circ} = \begin{cases} D\,F_1\,\dfrac{\max(0,\ \cos\theta)}{\max(\sin 5°,\ \sin\alpha)} & \alpha \ge 2° \\ 0 & \alpha < 2° \end{cases} \tag{7}$$ $$D_{hb} = D\,F_2\sin\beta \tag{8}$$ $$D_{plane} = D_{iso} + D_{circ} + D_{hb} \tag{9}$$

where

Symbol	Meaning	Unit
$E_0$	the extraterrestrial normal irradiance at time $t$	W/m²
$t$	the time from 1 January 00:00	hours
$\text{DNI}_{used}$	the direct normal irradiance the model uses	W/m²
$G_h$	the global horizontal irradiance	W/m²
$D$	the diffuse horizontal irradiance of the weather file	W/m²
$\alpha$	the elevation of the sun above the horizon	degrees
$B$	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	—
$\text{DNI}_\varepsilon$	the direct normal irradiance that enters the sky clearness	W/m²
$\theta_z$	the zenith angle of the sun, 90° minus $\alpha$	radians
$\Delta$	the sky brightness	—
$\text{AM}$	the relative air mass of Kasten 1966	—
$f_z$	the altitude factor of the sky brightness	—
$z$	the site elevation	m
$F_1$	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. PVsyst 8 help, Transposition model
4. Analysis of PVsyst 8.1.5 outputs shows this convention in its results.

API

array.perez_settings = pvsyst_8

Symbols follow the symbol registry.