Heat pumps are designed to work off air temperature.  They use a minimal amount of electricity and work like air-conditioners in reverse.

The heat pump is usually mounted outside the house where it comes into contact with free-flowing air.  Hot air is drawn into the heat pump, where it then passes over a refrigerant.  The refrigerant boils and evaporates, heating up a condenser coil which heats water in the tank – and warm water then flows to the taps.

Dual function heat pumps heats water, cools water down, or heats and cools water at the same time.  This leads to twice as much efficiency.

And while it isn’t convenient to climb into the ceiling to turn down the thermostat, in a smart green home, the adjustment is easily made on a smartphone/computer.