NRDC, Nov. 30, 2016, by Pierre Delforge, Christina Swanson, and Eric Weiner.
https://www.nrdc.org/experts/pierre-delforge/very-cool-heat-pump-water-heaters-save-energy-and-money

"A new NRDC study shows that water heaters that use high efficiency heat pump technology—the same process that keeps your refrigerator cold—can cut the amount of electricity used to heat water by half or more, saving significant amounts of energy and money on your household electricity bills."