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SECTION 2 | SOLAR PANEL POWER THEORY
The rated power of the Solar Panel in Watts (Pmax) is derived from voltage at Vmp
multiplied by current at Imp to get the Maximum Power Point (MPP):
• Rated power in Watts, Pmax = Vmp X Imp
= 2.5A
= 17A
=
=
= 2.7A
= 21V
Fig. 2.2. I-V Curve and Ratings of a 12V PV / Solar Panel
I-V Curve for a typical 12V Solar Panel is shown in Fig. 2.2. The Open Circuit Voltage V
oc
is 21V and the Short Circuit Current I
sc
= 2.7V.
Maximum Power Point in the example curve given above is where Vmp is 17V, and the
current Imp is 2.5A. Therefore, the rated or the maximum power Pmax in watts is 17V
times 2.5A, or 42.5 Watts.
2.3 Standard Test Conditions (STC) for Specifying Solar Panels
The industry uses three standard conditions to generate the I-V curve and other
specifications for the solar panel. These STC are made up from three conditions:
•
Temperature – the temperature of the cell and not the surrounding air or metal
of the panel. STC=25oC.
•
Solar-Irradiance – The amount of light energy falling on the panel. STC=1000W/m2
•
Mass of air – The number is related to the angle of light and amount of
atmosphere STC=1.5.
When the panels are rated based on these industry standards, designers are able to
compare products.
2.4 Factors Affecting Output of Solar Cells
The amount of electric current generated by photon excitation in a Solar Cell at a
given temperature is affected by light in two ways:
• By the intensity of the incident light.
• By the wavelength of the incident rays.