A watt is a watt is a watt. Doesn't matter if it is less wattage or higher wattage, a watt can only produce so much heat. We are billed by watts. Now, the amount of wattage will depend on the upper temperature you want to keep the pond. Lower wattage will take much longer than higher wattage to get it up to desired temperature, and lower wattage may not do it depending on the volume of water and how many watts the heater is putting out. More wattage will get the temp up faster. But as far as heating the water, a watt is a watt is a watt. There is no way to "cheat" the electric meter to get to a desired temperature.
