Here you need to consider a number of factors, such as the size of your house (how many rooms?), the size of your household (how many residents?), your lifestyle choices (long hot showers?) and your budget.

You also need to consider any likely changes to any of these factors in future years (growing families, possible sale of house, children leaving home etc).

Household hot water consumption can vary from 70 litres per day to several hundred, depending on usage patterns.

If you under-invest, you are likely to need excessive electric/gas boosting which will end up costing you more than your saving in capital cost.

If you over-invest, you may produce more solar heat than you can use up and will also increase your payback period.

The difference in price between a 20-tube system and a 30-tube system is approximately $300, so if there is doubt and you can afford it, it is generally better to opt for the larger system.

In Tasmanian conditions for a typical household (3 people or more) it is generally recommended to install a 30-tube system.

Remember that solar hot water cylinders are normally designed to store water for use over 2 days, rather than one, thus allowing you hot water even if the following day is cloudy.