Heat pumps are a highly efficient way to heat (and cool) homes because they scavenge and concentrate heat from the surrounding air; for every unit of electrical input, a heat pump produces three units of heat. The AirTap heat pump from AirGenerate channels those same efficiencies into heating water. The device, which can be retrofitted to new or existing heater tanks, draws about 6 amps, nearly 2.5 times less energy than a standard electric water heater. (AirTap's efficiency goes up as the air temperature rises.) And while it takes longer for AirTap to bring cold water up to temperature than a gas or electric heater, all that cool, dry air it produces is a welcome side benefit in summer. In winter, just vent that air outside.
About $700; AirGenerate