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Step 6 Choice of solar panels

The concept of Watt-peak (Wp)

The performance of the solar panel is expressed in units of Watt-peaks (Wp). This is the power that is generated under 'standardised' sunshine of 1000 Watts per square meter. This international standard is comparable to the solar irradiation of a sunny bright day in July in The Netherlands. A crystalline silicon solar panel (see below) of one square meter will supply about 100-150 Wp. The daily performance (in Wh, say Watt-hour) is calculated by multiplying this number with the amount of hours of full sunshine (see step 3). This number is a calculation unit to express the amount of light per day, because also diffuse light will contribute to the performance of the solar panels. So also if the sun does not shine, it can contribute to the hours of full sunshine. Of course, in that case, it will take longer than an hour to come to an hour of full sunshine.

Capacity of solar panels

The type and amount of solar panels follow from the previous step, where the type of pump has been determined. Two conditions need to be fulfilled:

The voltage should be within the window that is required by the controllerThe system voltage is determined by the number of solar panels that is connected in series. A panel with 36 solar cells has a ´nominal voltage´ of 12V (because this number of cells is required to charg 12V batteries). So, to obtain a nominal voltage of 48-72V, 4 to 6 of this type of panels should be connected in series.
Sufficient peak powerThe total power of all panels should be in accordance with the design scheme of step 5.

Types of solar panels

Characteristicdescription
Material solar cell99% of the solar cells is made of silicon. We distinguish crystalline silicon (blue or black wafers) and amorphous silicon (dark brown strips on glass). Most commonly used are crystalline solar cells. Poly crystalline: Blue speckled; mono crystalline: evenly dark.
Amount solar cells For crystalline (both mono-crystalline and poly crystalline) silicon, the number of 36 is holy. 36 cells are needed to charge a 12V battery. Usually panels contain 4x9, sometimes 6x6 cells. For 24V systems, there are large panels with 72 cells.
For solar pumps the number of cells per panel is less critically. It is important that the voltage of the array (ray of panels) will fit within the voltage window of the pump controller
Quality standardCrystalline solar panels: “IEC 61215” is mentioned on the specification sheet; Amorphous silicon solar panels: Idem, but other number: “IEC61646”