Symbols used in photovoltaics and in solar energy engineering

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Units and symbols used in solar energy engineering

"In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual."
(Galileo Galilei)

Thin film products - facade integration, Thyssen solartec

Thin film modules - integration into facade
(source/copyright: Thyssen-solartec).

Some important photovoltaic terms and symbols used in solar energy engineering and in photovoltaics are presented in the tabels below. Detailed information can be found in "IEC/TR2 61836, Solar photovoltaic energy systems - Terms and symbols". For detailed definitions please see also references at the bottom of the page.

Units and symbols used in photovoltaics

Specific heat	c	J / kg K
Thermal conductivity	k	W / m K
Extinction coefficient	K	-
Index of refraction	n	-
Absorptance	α	-
Emittance	ε	-
Reflectance	ρ	-
Density	ρ	kg / m³
Transmittance	τ	-

Altitude ( 0 do +/- 90° )	α	°
Surface tilt angle ( 0 do +/- 90° tilt toward equator + )	β	°
Azimuth of surface ( 0 to + 360° - CW from N positive )	γ	°
Declination ( 0 to +/- 23.45° )	δ	°
Incidence angle ( 0 to + 90° )	i	°
Zenith angle ( 0 to + 90^o )	Θ_z	°
Latitude ( 0 to +/- 90^o, N from equator + )	Φ	°
Hour angle ( -180° to + 180° , noon 0°, afternoon positive )	ω	°

Global irradiance	G	W / m²
Beam irradiance	G_b	W / m²
irradiance	G_d	W / m²
Global irradiation*	H	J / m²
irradiation*	H_b	J / m²
irradiation*	H_d	J / m²

*NOTE: The unit Wh/m² (kWh/m²) is often used in technical practise in Europe. Please look for further information and additional symbols in suggested literature.

Units and symbols - interesting web sites
	The NIST Reference on Constants Units and Uncertainty - This site addresses three topics: fundamental physical constants, the International System of Units (SI), which is the modern metric system, and expressing the uncertainty of measurement results. Both essential information and background information are given for each topic. Languages: Webmaster's choice - SI units explained in detail.
	convert-me.com - Here you will find interactive calculators for many measurement systems both commonly used like metric and U.S. Avoirdupois and quite exotic like Ancient Greek and Roman. Languages: Webmaster's choice - Conversion calculator for SI units, british units etc.
	Eric Weisstein's World of Science - Great web site; physical and other scientifical terms explained in detail. Languages: Webmaster's choice - Physical terms explained in detail.

More information and some useful tools

IEC: Solar photovoltaic energy systems - Terms and symbols; IEC/TR2 61836.
Units and Symbols in Solar Energy; Solar Energy, vol.71, number 1, 2001.
TCAEP: The Constants and Equation Pages.
English weigths and measures.