Ref: Plumbers training handbook (No air temperature stated so would need to be calculated for each location.)

The old hot water systems

Electric storage -  an electric element heats the water water stored in an insulated tank - Electricity from off peak or continuous tariff

Gas storage - Gas heats the water stored in a tank)

Gas instantaneous - gas  heats the water just before it is used

Modern hot water systems

Electric heat pump - a vapour is compressed making it hot enough to heat water.

Solar hot water - close coupled. Tank is above the collector plates allowing a thermo-syphon to circulate the water without pumping. 

Solar hot water split system - Tank is on the ground and a small pump circulates the water. A little more expensive, but usually chosen for looks. It is probably more efficient because the faster flow allows more heat transfer.

Solar HW - close coupled                Solar HW - split system

Electric storage heater         Gas storage water heater

Heat pump is an air conditioner used in reverse. The hot side heats the water. The cold side could cool the house. In most heat pumps this "coolth" is wasted.

Boost

Many solar HW systems are provided with gas or electric boost. During wet weather solar may not provide enough heat. The extra heating can be provided by an electric or gas boost.

Electric boost - Electric heating elements  will turn on when the water is not hot enough. The heating element can be in the bottom, the middle or both. The advantage of the middle element is that it only heats up half the water.

The problem with electric boost on a solar hot water is that if you shower at night, then the electricity will turn on and heat the water. Next morning there will be nothing left for the sun to heat during the day. This is a tremendous waste of opportunity. You've bought the solar HW system, and you are still buying electricity. The boost should be left off and turned on only if a couple of wet days are expected. The heating can be turned off at the fuse box. Better if possible to have a switch in the kitchen, or somewhere convenient. This will save most of the electricity and change it's position on the chart above.

These systems need a redesign to prevent this waste. A larger  tank holding a few days hot water is the simplest answer. If it is raining for a week you probably don't need as many showers anyway.

Gas boost, the gas heats the water instantaneously just before it is used. Thus it does not waste energy in heating the water unnecessarily. That is why it has a lower CO2 emission than electric boost.

To reduce the need for any boost, see the page on this topic: 

www.energy-without-carbon.org/ReducingElectricBoost

Choosing the system

There are excellent websites for more details

http://www.climatechange.gov.au/what-you-need-to-know/appliances-and-equipment/hot-water-systems/solar.aspx  

Choosing your solar water heater / heat pump - Clean energy regulator

Comparing solar hot water systems - Clean energy regulator

Choosing a hot water system - Your Home.gov.au

Electric boost

Gas boost

Other pages on this site on hot water

Best Alignment of solar collectors

Reducing Electric Boost

Heat pump efficiency -

Usual recommended tank size

However if you put in a larger system you can reduce the electric or gas boost to almost zero.

Electricity - Continuous s​upply

• Charged at a single price all day.
• Higher price per kWh than other options.
•  Important for pump-controlled SWHs, to ensure that the pump can operate when needed
(e.g. for frost protection).
•  Important for heat pump systems because they are most efficient when they can access hot daytime air.
•  For electric boosters, can allow the booster to come on at any time of the day, increasing
reliability but possibly resulting in higher electricity bills.
• Essential for gas boosters because of their electronic ignition systems.

Calculating REC and STCs

RECs or Renewable Energy Certificates, are divided in to two types - large-scale generation certificates (LGCs) and small-scale technology certificates (STCs). They are both RECs.

The number of RECs is calculated by determining the amount of electricity the system displaces over a determined period (called a deeming period). Each REC is equivalent to 1 MWh of renewable electricity generated or deemed to have been generated. Each REC is worth $40. You will probably get $23.

Domestic or small business solar or heat pump hot water is part of the Small Scale Renewable Energy Scheme. 

For exact zone informationi use this postcode table from 

The clean energy regulator tests all systems and issues them with a number of certificates according to how much CO₂ they will save.This varies with the climate. For example a solar heater will be more efficient in The tropics than in the cooler south, so will be awarded more certificates. Australia is divided into 5 zones.

This is a typical entry in the tables you can access online.

Register of Solar Water Heaters/Heat Pumps and RECs applicable

Calculate your REC

Solar water heaters and heat pumps must be installed according the requirements of the Small-scale Renewable Energy Scheme in order for them to be eligible for Small-scale Technology Certificates (STCs). These certificates can be assigned to a registered Agent (such as a retailer or installer) in exchange for a financial benefit, such as a discount off the invoice. 

Your Agent will create the STCs in an online system called the REC Registry. After the STCs have been validated the Agent will sell them to entities (usually electricity retailers) that have a legal requirement under the Renewable Energy (Electricity) Act 2000 to buy a certain number of STCs each year.

For information about assignment of your STCs visit Installing your Solar Water Heater/Heat Pump. 

Payback time