All you require is an unshaded roof facing roughly north (any aspect between north west and north east will give good results). Even an Easterly facing roof can work without too much loss of efficiency.

The solar collector should be mostly unshaded by large trees and buildings, especially between the hours of 9 am to 3 pm.

If your roof aspect is very limited, there are always solutions. An offset frame can be installed, or in some cases, it is possible to face part of the system in one direction, part in another.

(Some houses do get too much shade for adequate solar gain. If your house has limited solar access for some reason, then consider installing a hot water system driven by a heat pump technology. These are extremely efficient and will reduce your hot water bills by a similar amount.)

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.