Our heat pump water heaters are designed from the ground up to rely on the heat pump, not on the back-up element because that is the most efficient way. Our “50-gallon” heat pump water heater is actually 58 gallons and our 80-gallon model is, well, 80 gallons, because large is the best for a heat pump water heater. Large capacity means the water you are using, as much as possible, was heated the most efficient way — with the heat pump.