All I've been able to find is high-wattage tank heaters (1000 and 1500 watts).

But I want something lower wattage to keep several 15- and 25-gallon plastic tubs thawed out during the day. My electrical set up from the barn is not sufficient to handle six 1500 watt heaters (nor is my income sufficient to pay for that kind of power usage). Tubs will be insulated, so all they need is a little something, not a 1500 watt something...

Tubs/buckets with built-in heaters are not an option -- I have no faith in the longevity of such contraptions, and they are too expensive to replace very often.