Principal
A solar hot water system heats through placing water inside thermal solar collectors, and positioning the thermal solar collectors in an optimum direction (IE north) and an optimum pitch (IE 30 degrees). If your solar collectors are either not in exactly the right direction, or at the perfect pitch some reduction in efficiency will result.
Storage
Tradtionally Solar storage tanks were located on the roof itself, although the latest technology units are generally 'split systems' meaning the tank is located at ground level. By only having the panels on the roof means a more aesthetically pleasing job, lower maintenance costs, higher efficiency & no need for roof strengthening.
Efficiency
A Solar hot water system in Australia will be generally between 60-95% efficient depending on location & hot water system quality. (A standard Titanium Solar system is around 80% efficient in Adelaide, Sydney, Brisbane & Perth) (Darwin, Cairns etc closer to 100%)
Back up power
On days where solar radiation is low, all solar hot water systems depend on a back up energy source this is usually either Gas or Electricity. With a Titanium Solar system boosting is automatic, so you never need to remember to switch anything on or off.
Low solar radiation
Low solar radiation is usually experienced during the winter months when you have the compounding issues of fewer daylight hours & thick cloud cover. (A Titanium Solar will on average be around 40% efficient in winter).