a 15 watt solar panel will keep the caravan/ motorhome battery up to scratch when the unit is not being used, but stored, and when you come to use it all will be fine. 15W is not enough if you are using the unit without additional charging, unless it is mid summer with little load but lighting.
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This is what solar does best of all. You must size your solar panel to the right size for the battery - otherwise you risk overcharging the battery and destroying it.
If your solar panel is too big you would need to add a solar charge controller to your system which unnecessarily bumps up the cost.
As a rough rule of thumb, take the size of your battery in amphours. Then divide the amphour rating by five. This is the maximum size of solar panel (in watts) that you should consider buying for use as a solar battery charger.
For example: if you have a 75Ah battery, divide 75 by 5 to give you the number 15. 15 watt is the largest solar panel you should buy for basic battery charging.
You can then either prop the solar panel up in the window of your caravan, or fix it to the roof of the caravan.
Incidentally, during the spring, summer and early autumn months you'd find that a 12-15 watt solar panel would give you all the energy you'd need for caravan lighting so if that is all you use your battery for, you've got a complete system and all for less than
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