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Basics: Solar Panels
POWER: The amount of power that a solar panel provides is indicated by the wattage (W). The higher the wattage, the more powerful the panel. Amps (current) and volts are also important because:
W = A x V
Solar panels suitable for 12V battery charging generally have voltages rated at around 16-17V. The lower the voltage, the higher the Amps, and the faster current flowing out of the panel, and the faster a battery will charge.
PANEL RATINGS: panel ratings are performed under test conditions of 1000W/m sq and 25oC. Although all 10W panels will give 10W per hour under these test conditions, they won’t all perform the same under other conditions.
PANEL OUTPUT: A 10W panel will give 10W over an hour under standard test conditions. In the UK allow around 4 hours equivalent sunshine in summer and 1 in winter. Ie In Winter a 10W panel will give 10W over a whole day, whereas in summer it will give 40W. These are fairly conservative figures – some companies use up to 6 hours in summer. You can do the same calculations with the Amps (people are often more familiar with Amps).
TYPES OF PANEL: There are two main types of solar panel: amorphous and crystalline. In general, amorphous perform better than crystalline under low light conditions and don’t suffer as much power loss in hot temperatures. However, in good conditions, the efficiency of amorphous panels is lower, and they are physically larger than crystalline panels of the same wattage. Rollable, folding and flexible panels are generally amorphous, whereas crystalline panels tend to be aluminium framed and glass fronted.
MOUNTING: Crystalline framed panels have aluminium frames with mounting holes in, so you can screw them onto something. They need to be raised up by around 10mm to allow the air to circulate underneath. Flexible panels generally have holes in the corners to use for mounting. Some can be glued down, sewed on, velcroed on etc.
The output from a solar panel will be greatest when it faces into direct sunlight. In the UK that means it should be mounted facing south at an angle of around 45 degrees. Perhaps more important is to make sure that there is no shading over the panel.
BYPASS DIODES: If part of a solar panel is in shadow then output from the whole panel goes down, unless there are bypass diodes between the cells to isolate them.
BLOCKING DIODES: These prevent power from going back into the panel from the battery at night. The larger framed panels tend not to have blocking diodes fitted, whereas the smaller portable panels generally do have them fitted. If you use a charge controller you don’t need a blocking diode, because the charge controller performs that function (amongst other things).
CABLING: Some panels have built-in cables coming out of the box on the back (the junction box). In this case you need to be aware of the connectors on the end – can you connect to them? Where there are no cables, you need to wire the cable into the junction box, usually just a case of striping it back a little and screwing it in.
VOLTAGES: Solar panels for 12V battery charging are usually rated at 15-17V. Those for 24V battery charging are rated at around 34V.