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Join Date: Aug 2007
Location: Manor, TX
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The 18w is under perfect conditions and that's assuming the panel is actually 18w and not rounded off. For the panel to work perfectly it needs to be pointed at the sun with no obstructions. Laying it flat on the roof, behind a window, clouds, dirt, bird crap will all make the output go down.
Then you have the voltage issue. Many panels put out full power at a voltage above what you charge your batteries at. For instance, that panel may put out 18w at 18v which is 1a. If your charging at 14v with a PWM style charge controller you are essentially just flipping a switch off and on rapidly to regulate voltage. For simplicity let's say that's .8a, couple that with non optimal conditions and you will be lucky to keep your battery charged. For larger setups, there are charge controllers that use MPPT or Maximum Power Point Tracking which helps make up for the loss and allows you to use higher voltage charging sources to reduce the size of wire you have to use.
If your roof is somewhat flat, there are http://www.amazon.com/Uni-Solar-PVL-68-PowerBond-112-Inch-15-5-Inch/dp/B002MWDI44/ref=sr_1_1?ie=UTF8&qid=1380196363&sr=8-1&keywords=adhesive+solar+panel that would provide some more power.
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