The average Melbourne house creates approximately 12tonnes of CO2/yr from its energy use. Hot water accounts for approximately 20% of this. The good news is that approximately 3/4 of this hot water can be made carbon free from the sun via a Solar Hot Water unit. There are 2 broad main types; flat plate and evacuated tube solar. Both types have systems that work well. The information you really want to get, is to find out the yearly percentage solar contribution, then you can judge the efficiency of one against another. Anthing 70% or higher is good. (Meaning 70% of the hot water is generated from the sun over the year.)
Also to be considered is whether the system is thermosiphon with the tank above the panels, or split system with the tanks on the ground. We tend to favour thermosiphon. It is the older technology, but it is more elegant. There are no pumps to break down or consumer energy, and it simply works on the fact that hot water in the panels is lighter and so rises into the tank, while the coldest water in the tank falls into the panels to get heated and rise back into the tank. Neat. Solar hot water will also require a booster (typically gas) to give the water any extra heat needed if the day was not sunny enough to make the water >60 degrees.
Over the last couple of years the Sanden Heat pump system has also been becoming popular for sustainable homes. Many of our clients desire to ditch the gas and become an all electric household. (See a discussion of this in Heating & Cooling.) While this can still be done with solar hot water, it typically means the booster is an electric resistance element in the hot water tank, which is a very inefficient way of making hot water. This increases the carbon intensity of the solar hot water system in operation, making it significantly less green than when it is boosted with gas. The Sanden system, while electric, uses a much more efficient heat pump technology, to make the water hot. (Currently the most efficient generally available in the market with a COP of 3.4 at zero degrees.) The timer for its heating operation can also be set to co-incide with the middle of the day, which combines well with photovoltaic energy production. So if you have a PV array, the power it produces, at least in part, also heats the water. Of course the most efficient electric option would be to combine solar hot water with a Sanden unit as the booster, but this would be an expensive way to make hot water.
So which hot water to use? Our general advice with current technology, is to use solar hot water if you are still connected to gas, but to use the Sanden Heat pump if going all electric.
Note: We have no links to Sanden, other than having used the product, and there are other heat pump hot waters on the market. When assessing one heat pump hot water against another, ask for their graph of COP vs. temperature if they have one. If they only have one COP number for you it is probably the efficiency of the system at 20 degrees C, which is the industry test. This is useful information, but you also want to know how efficient they are at lower temperatures, when they are making hot water during winter, or at night.