Symbol registry

What it is

The equations, where-lists and figures in these pages use the symbols below with the meaning given here, and the API field that carries the same quantity is named next to it. Reference papers keep their own notation; each method is translated into this registry once. Quantities with no natural symbol stay words, and the frame is always azimuth 0° at true north counted clockwise, elevation measured up from the horizon. Each symbol has one meaning on every page; summation indices such as j, k and n are counters, named on the page that uses them. Two units change inside some equations, and the page's table says so: θz is in radians in the Perez coefficients, and T is in K inside the module models.

Symbols

Symbol	Meaning	Unit	API	Figures
$\alpha$	Sun elevation above the horizon	°	series sun.elevation_rad (in rad)	8
$\theta_z$	Zenith angle of the sun, 90° − α (derived)	°	derived	4
$\gamma_s$	Sun azimuth, clockwise from true north	°	series sun.azimuth_rad (in rad)	8
$\gamma$	Azimuth of a direction in the sky or on the ground, clockwise from true north	°	derived
$A_p$	Azimuth the module plane faces, clockwise from true north	°	array.azimuth_deg
$\beta$	Tilt of the module plane from the horizontal	°	array.tilt_deg	3, 4, 9
$\theta$	Incidence angle: between the sun and the perpendicular to the plane	°	derived	5
$\psi$	Profile angle: the sun elevation projected into the plane across the rows	°	derived	3
$\rho$	Tracker rotation about its axis	°	derived
$P$	Row pitch, axis to axis	m	array.collector_width_m / array.gcr	3, 6, 9
$W$	Collector width along the slope	m	array.collector_width_m	3, 9
$W_{tot}$	Total row width: W plus the two inactive bands	m	derived	3
$b_{top}, b_{bot}$	Inactive bands at the top and bottom edges of the row	m	array.band_top_m, array.band_bottom_m	3
$L$	Shaded length up the rear collector from its bottom edge	m	derived	3
$F_{sh}$	Shaded fraction of the collector (the shaded length over the width the option uses)	—	derived	3
$F_{sky}$	Fraction of the sky light the modules receive between the rows	—	derived	6
$e(x)$	Elevation of the front row's top edge seen from the point x up the collector	°	derived	6
$G_h$	Global horizontal irradiance	W/m²	weather.glob_hor
$D$	Diffuse horizontal irradiance	W/m²	weather.diff_hor
$\text{DNI}$	Direct normal irradiance	W/m²	weather.dni
$F_1, F_2$	Perez circumsolar and horizon-band coefficients	—	internal	4
$E_0$	Extraterrestrial normal irradiance, S (1 + 0.033 cos 2πt/8760); S alone under the SSC settings	W/m²	derived
$S$	Solar constant: 1361 (PVsyst 8 settings) or 1367 (published and SSC settings)	W/m²	array.perez_settings
$\text{DNI}_{used}$	Direct normal irradiance the Perez model uses: (G_h − D)/sin α under the PVsyst 8 settings, else the file's DNI	W/m²	array.perez_settings
$\varepsilon$	Perez sky clearness	—	derived
$\Delta$	Perez sky brightness	—	derived
$f_z$	Altitude factor of the Perez brightness	—	array.perez_settings
$z$	Site elevation above sea level	m	site.elevation
$B$	Direct sunlight on the module plane	W/m²	series poa.beam_inc_wm2 (after the low-light threshold)
$D_{plane}$	Diffuse sky light on the module plane	W/m²	derived
$D_{circ}$	Circumsolar part of the diffuse light on the plane	W/m²	derived
$G_{gnd}$	Ground-reflected light on the module plane	W/m²	derived
$\text{albedo}$	Ground reflectivity; a word, never a symbol	—	site.albedo_monthly	4
$F(\theta)$	Incidence-angle modifier: the accepted fraction of light at incidence angle θ	—	derived	5
$b_0$	ASHRAE incidence-angle coefficient	—	iam.b0
$G$	Effective irradiance on the modules	W/m²	series glob_eff_wm2	10
$b_f$	Bifaciality factor: rear-side efficiency over front-side efficiency	—	module.bifaciality_factor
$G_{ref}, T_{ref}$	Standard test conditions: 1,000 W/m² and 25 °C	W/m², °C	module.g_ref_wm2, module.t_ref_c
$T$	Cell temperature	°C	series t_cell_c	10
$T_{amb}$	Ambient air temperature	°C	weather.t_amb
$v$	Wind speed	m/s	weather.wind_vel
$a$	Absorptivity: the fraction of the light the module absorbs	—	thermal.absorptivity
$U_c, U_v$	Constant and wind heat-loss factors	W/(m²·K), W/(m²·K) per m/s	thermal.u_c, thermal.u_v
$\eta$	Module efficiency at the operating point	—	derived
$N$	Number of modules in the system	—	derived from the stringing
$P_{nom}$	Module nameplate power	W	module.p_nom_w
$P_{mpp}(G, T)$	Maximum power of one module at irradiance G and cell temperature T	W	derived
$I, V$	Array current and voltage	A, V	series	7
$I_{mpp}, V_{mpp}$	Array current and voltage at its maximum-power point	A, V	series	11
$U, U_{mpp}$	Array voltage at the inverter terminals, and at the maximum-power point	V	derived	11
$U_{mod}$	Array voltage at the modules, before the DC cable drop	V	derived	11
$V_{min}, V_{max}$	Bottom and top of the inverter's tracking window	V	inverter.v_mppt_min_v, inverter.v_mppt_max_v	7
$P_{lim}$	AC output limit of the inverter at the ambient temperature	W	inverter.p_nom_ac_w, inverter.derate_temps_c, inverter.derate_max_p_w	7
$R_w$	DC wiring resistance of one inverter's array	Ω	dc_losses.dc_wiring_r_ohm or derived	11
$\text{AM}$	Relative air mass	—	derived
$P_w$	Precipitable water	cm	weather.precipitable_water
$M$	Spectral modifier	—	derived	10
$\varphi, \delta, \omega$	Latitude, solar declination, hour angle	°	site.latitude, derived, derived
$\Delta t$	Length of one timestep	h	weather.step_hours
$\gamma_r$	Azimuth of a direction measured from the azimuth the module plane faces	°	derived
$X$	Pitch over collector width, the inverse of the ground cover ratio	—	derived
$N_{inv}$	Number of inverters in the system	—	array.n_inverters
$I_m, V_m$	Current and voltage of one module in the single-diode models	A, V	internal
$k_B$	Boltzmann's constant	J/K or eV/K	constant
$\text{PF}$	Power factor	—	ac_losses.power_factor
$P_{ref}, P_{ref,sys}$	AC power reference of one inverter, and of the system	W	Model tab: AC power reference
$E_{before}, E_{after}, E_{row}$	Running total of the loss diagram before and after a row, and the row's value	kWh/kWp or kWh/m²	derived
$i$	Timestep index	—	—