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.


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.

In addition, one should be able to estimate energy production from a PV system for financial analysis or for sizing a PV system. Energy production from a PV system can be estimated by multiplying the peak rating of a PV module or array by the number of hours irradiance of 1000 W/m² is incident on it. The later period is known as peak sun hours and is numerically equal to radiation computed using GHI for a horizontal surface or radiation computed using PoA for other plane configuration. Therefore electrical energy production of a PV array or module is equal to its peak rating multiplied by PoA radiation. PoA under the given project is measured using c-Si reference cells tilted at 20 degrees and facing the North. Therefore, electrical energy production from c-Si technologies can be readily estimated using the available data. For other technologies, the GHI data should be adjusted to the technology which requires consideration of spectral mismatch and adjustment to required plane of array configuration. Users are recommended to contact us for advice for such cases.