Hybrid Heat Pump Energy Efficient Water Heating

This type of water heater uses a compressed gas to produce heat, instead of a resistive electrical heating element.  They can save over 60% of the energy consumed by a standard electric water heater.  This works out to a savings of $320 a year on average.  The cost is about double that of a standard water heater, with a payback on average of 3-5 years.  With an expected lifespan of 13 years, it could save you around  $3000 over its service life.

There are some issues with noise and installation location issues, so do your research before you buy one.

It’s generally easier to move something than to make something. Putting that principle to use, heat pump water heater (HPWH) technology uses electricity to move heat from one place to another instead of generating heat directly.
To understand the concept of heat pumps, imagine a refrigerator working in reverse. While a refrigerator removes heat from an enclosed box and expels that heat to the surrounding air, a HPWH takes the heat from surrounding air and transfers it to water in an enclosed tank. A low-pressure liquid refrigerant is vaporized in the heat pump’s evaporator and passed into the compressor. As the pressure of the refrigerant increases, so does its temperature. The heated refrigerant runs through a condenser coil within the storage tank, transferring heat to the water stored there. As the refrigerant delivers its heat to the water, it cools and condenses, and then passes through an expansion valve where the pressure is reduced and the cycle starts over.

Water heating energy usage comparisons

Sources: 
American Council for an Energy-Efficient Economy  
U.S. Environmental Protection Agency and the U.S. Department of Energy
General Electric