New answers tagged landscaping
40 watts at 12 vac is about 4 amp which leaves plenty of headroom on the 12ga cable (which is normally rated for 20 amps (with heat loss and safety factors included). But what if your wire gets cut? Shorted? the worst case max current that a 300 watt transformer can put out (if it really is a 300 watt not a 310 watt or 315 watt) is 25 amps which is flirting ...
A google search for "solar powered fish pond filters" shows many sources. Since you didn't mention the size of your pond or geographic area I didn't make any specific suggestions. But there is a wealth of info and products out there.
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