Stiebel Eltron’s (stiebel-eltron-usa.com) Accelera 300 is an air-source heat pump with an 80-gallon tank that heats and stores domestic water. It is designed to heat about 50 gallons to 140°F; a 1,700 W electric backup heating element provides supplemental heating when more hot water is needed.
Heat pump performance is evaluated by the “coefficient of performance” (COP) ratings (the higher the value, the better). A typical conventional tank-type electric water heater’s efficiency may range from 0.8 to 0.95; the Accelera’s COP is 2.51, which means it uses about one-third of the energy of an electric water heater. (Note that the COP is greater with higher ambient air temperatures and higher relative humidity.)
As a side benefit, in warm climates, the water heater can be placed inside the home and it will cool the air around it, reducing the air-conditioning load. In cold climates, the unit can be placed in a basement to act as a dehumidifier. The Accelera 300 carries a 10-year warranty.