 ### Solar Energy Related Quantities

Below is a list of some fundamental definitions of solar power and energy which will help users while browsing through the website.

• Irradiance represents the solar power intensity incident on a surface. Its unit is W/m² or kW/m².
• Spectral irradiance represents the solar power density of a particular wavelength of light. It defines the number of photons of a given wavelength incident on a surface. Its unit is W/um.m²
• Radiation, Irradiation and Insolation represent the solar energy intensity. It is the solar power received by a surface over a given period of time. Unit is Wh/m²
• The Solar constant represents the solar power intensity just outside the atmosphere. I0 = 1.367 kW/m².
• Global horizontal irradiance (GHI) is the total amount of solar power received on a horizontal surface
• Global horizontal insolation (GHI) is the total amount of solar energy received on a horizontal surface
• Direct normal irradiance (DNI) is the portion of GHI that comes in a straight line from the sun
• Diffuse horizontal irradiance (DHI) is that portion of radiation that arrives at the surface from indirect paths.
• Plane of Array Irradiance (PoA) is the solar power received on a surfaced tilted at an angle and oriented towards a given direction. Normally, the tilt angle is tilt angle of a PV array and the orientation is the orientation of the PV array.
• Plane of Array (PoA) radiation is the solar energy received on a surfaced tilted at an angle and oriented towards a given direction.

#### Why do we need GHI, PoA and Temperature?

A PV cell is the most elementary device that exhibits the photovoltaic effect (i.e. the direct non-thermal conversion of radiant energy into electrical energy). A PV module is the smallest complete environmentally protected assembly of interconnected cells. A PV string is a circuit of one or more series-connected PV modules. A PV array is an assembly of electrically interconnected PV modules, PV strings or PV sub-arrays comprising all components up to the d.c. input terminals of the inverter or other power conversion equipment or d.c. loads. It does not include the PV array foundation, tracking apparatus, thermal control, and other such components.

Solar photovoltaic devices convert solar energy into electrical energy. The output of a PV cell/module is given by solar power input multiplied by the PV cell/module efficiency. Incident power can be estimated using empirical or physical models or measured using calibrated equipment. But, PV modules characteristics, such as efficiency and electrical ratings, are defined at Standard test conditions (STC) which is a standard set of reference conditions used for the testing and rating of photovoltaic cells and PV modules. The standard test conditions are the following: (a) PV cell temperature of 25°C. (b) Irradiance of 1000 W/m². (c) Light spectrum corresponding to an atmospheric air mass of 1.5. It means that a module of given rating, known as peak rating, is “X” Wp at STC only and will vary if irradiance and temperature vary.

Therefore for sizing of PV strings to inverter or other balance of system components it is very important to understand impact of temperature and irradiance on the electrical characteristics such that down time or damage can be prevented. Contractors may readily use information of cell temperature available from the website to do necessary calculation or contact us for support.