HEAT PUMP HOT WATER UNITS
The basics - Heat pumps work by taking heat out of the air to heat water and expelling cold air afterwards. Generally good units are able to operate in cold conditions and still extract heat from the air. Similar to reverse cycle heaters, with a small amount of energy input, they can generate a multiplied amount of heat output due to the ability to draw heat from the air. (Air-conditioners work in a similar fashion except rather than take heat from the outside air, they take heat from the inside air and expel the cold air internally). They are also established technology, as they use the same technology as air-conditioners.
Common better heat pumps in Australia include the Sanden, Reclaim and Steibel Eltron units.
Efficiency - Heat pumps are made up of a condenser unit and a storage tank much like a traditional electric or gas storage hot water tank. However, they run on between a fifth to a third of the energy of a traditional electric resistance storage hot water unit. So they are much more energy efficient. They are also preferable to gas continuous flow units in the medium and longer term as gas is a fossil fuel that is burnt to release the greenhouse effect gas carbon dioxide, while electricity is increasingly being generated from renewable sources (Victoria has a target for 50% of its electricity from renewable resources by 2030).
The efficiency of heat pump hot water units can be determined by the STCs available for that unit. The higher the STCs available, the more efficient the unit. STCS and performance are measured by different zones, able to be checked here. Melbourne is zone 4.
The choice of different models and their STCs are available from downloading the spreadsheet on the Australian Government Clean Energy Regulator website.
Complement renewables - Due to the feed in tariffs being less than half of the price consumers purchase electricity at, the payback period of PV panels is shortest when occupants consume all of the power generated on site. A storage hot water unit that can be heated up during the day is a perfect complement to PV panels. And heat pumps are the most efficient storage hot water unit.
The Sanden and Reclaim units are split system units with the compressor separate from the tank. The Steibel Elton unit is a combined round tank tube unit of 690mm diameter with compressor on top.
Note Sanden and Reclaim heat pumps use Carbon Dioxide as the refrigerant. Others such as Steibel Eltron currently use the much higher Global Warming Potential refrigerant R410 (although they will change to a lower GWP refrigerant late in 2020). Note the GWP of the refrigerant is not a problem if there are no leaks during the life of the unit or at the end of life, the unit is disposed of correctly and the refrigerants not released to the atmosphere.