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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

In step 3 the irradiation on site has been determined. The month of lowest irradiation is the design month for the system. In the example of Ghana, this is August (3.9 hours of full sunshine). Furthermore, it is important to realise that the total system has an efficiency that is lower than 100%. Due to charging/ discharging of the battery, a mismatch between the solar panel and the battery and some more effects, not all the power generated by the panel will reach the load. A system efficiency of 60% is indicative for a wide range of applications.

Sizing Solar

▲ Due to system losses, not all the power will be utilised. For generating 2kWh per day in the darkest month (Ghana) in total 850 Wp of solar panels is needed.

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.
Quality standardCrystalline solar panels: “IEC 61215” is mentioned on the specification sheet; Amorphous silicon solar panels: Idem, but other number: “IEC61646”