Air Generate manufactures a water tank add-on system called AirTap. It converts heat from the air using it to heat the water in your tank. Once installed you can unplug the electricity or gas currently used to power your water heater. And it requires no other purchases or upgrades.
AirTap is naturally more efficient because it takes advantage of heat in the air. The air pump in AirTap, which is powered by electricity, is a claimed 2.5 to 3 times more efficient using only 1 unit of energy from electricity and 3 from the air surrounding it.
They sell for about as much as a new water tank, but cannot be used to replace an old tank. You still need both. But the reported energy savings, if true, make it a worthy investment.
-You can unplug your gas line or the electricity currently running to your tank.
-No need to retro fit new gas lines and other changes which are common with a tankless system upgrade.
-Fairly inexpensive and highly efficient.
-Heats the water to the same degree, no loss in temperature.
-Its efficiency may be dramatically affected by the temperature in the house.
-We have seen no independent reports about AirTap, only what has been provided by the company.
I was recommended one by a plumber who had installed several, but other than that we know of no one else who has used them. All in all it seems like a good option if using a tankless system is not practical.