Small and efficient. One heater has kept a 1000-gallon pond free of ice in temps regularly below freezing. There were a few days of temps in the teens and single digits where thick ice eventually covered the surface of the main pool but the heater kept the small pool and waterfall flowing. Not sure how much it has affected the electricity bill so I can't give this product 5 stars. I recommend this product! (PS - this is the same heater sold at Lowe's under different packaging and it costs twice as much there.)