From the Rheem website:
"The Rheem Hybrid Electric Water Heater features heat pump technology that makes it over TWICE as efficient as standard electric water heaters"
And from their How It Works section:
"Rheem air-source heat pump water heaters work much like a refrigerator in reverse. The heat pump extracts the heat from warm air, intensifies the heat with a compressor, delivers the heat to the water, and exhausts the cooler air. Because it uses the warm ambient air temperature to do most of the work, it is a very efficient way to heat water."
There is a 40 gallon and 50 gallon version.