I just went with the 800 watt . JH800 Titanium Heater, 800W, 12.5" I have it in my g'house pond that is 750 gallon's .
Been keeping the water at 60*-70* even when it get's down to 5* with wind chill -12* .
I keep a big peice of plastic over the whole pond to help keep the heat from exscapeing .too
I love it !
bonnie