A roof mounted solar hot water system relies on sunshine, and expensive boosting is then required in order to provide the total hot water needs of a family.  This means that the actual heating costs will vary considerably.

Most suppliers of roof mounted solar hot water systems don't properly explain the proper sizing of a system, because in many cases people should be installing a larger, more expensive system to enjoy the full benefits of reduced heating costs.

Central Solar Systems installed roof mounted solar heating systems for more than 25 years, and provided its customers with information to assist in determining the correct system size.  The following is an extract from this information.

Because of the way the technology works, a Quantum heat pump hot water system is very easy to size and the savings on hot water heating costs are consistent. 

Sizing Considerations

The amount of money a solar hot water will save you is determined by many factors, the main ones being:

• The number of people in the house and their hot water usage pattern

• The storage capacity of the solar hot water cylinder

• The orientation of the roof relative to north, and the pitch (slope) of the roof

• The number of solar panels installed

• The efficiency of the solar panels

The commonwealth and state governments have published guidelines for sizing solar hot water systems. These guidelines are very straightforward.

• The sun provides heating for a limited number of hours during the day. Therefore, just like a large capacity electric hot water system that is connected to the night rate electricity tariff, a solar hot water storage tank should be sized to be able to provide stored hot water for the times that the sun isn’t shining. For example, each night and when the weather isn’t the best. Anything less than 300 litres would never be installed on night rate tariff for a family of four or on a three bedroom home. Similarly there is no logical reason to install anything less than a 300 litre solar hot water system for a family of four or on a three bedroom home.

• A spa bath or other large user of hot water should be counted as 1-2 extra people depending on frequency of use.

• The amount of water heated by the sun, and the optimum energy savings, are governed by the number of panels. The government recommends 1 panel per 2 to 2½ people. However, this only applies if the roof has a good pitch and is facing between north-east and north-west. Otherwise a stand or additional panel will be required. If the roof is facing east or west, a third panel will generally provide more heating capacity than if a stand was used to face the system north.

• Putting a 300 litre tank with a single panel virtually halves energy savings. A large amount of boosting will be required. 

• Putting a tank smaller than 300 litres with two panels will waste panel heating capacity and reduces the amount of hot water in reserve for periods of poor weather. More boosting will be required, that is, it will cost you more to operate. 

• Most solar hot water systems have an inbuilt electric element which is controlled automatically by a thermostat. This is used to top up the water temperature during colder days, when it is raining, or to provide additional hot water. It is usual to install a switch in an easily accessible location inside the home, to allow you to switch the electric heating on and off as needed. In some cases it may be more practical to have a timer on this switch.

• The amount of water usage per person can be reduced if water saving measures are also used. For example, AAA shower roses will significantly reduce hot water consumption.

There is not an easy answer to this question. One supplier's sizing software appears to significantly overstate the solar contribution of their products compared to the RECs allocation. Maybe they have assumed that a significant amount of hot water will be used between 8am and 4pm, when the sun has energy to immediately reheat the used water to some extent. This is not a typical hot water usage pattern for the average family. It is more typical for the majority of hot water to be used between 5pm and 8am, when the sun will not be providing any significant energy to immediately reheat the water.