Depending on your region and size of system solar can provide between 50-80% of your domestic hot water needs. A properly sized system will provide almost all of a home's hot water in the summer months.  The following table provides a rough guide based on your location's climate and number of people.  

• The tank sizing is the minimum suggested capacity for the solar dedicated tank, so using a larger tank is acceptable. 
• Sizing is based on providing 90% of summer domestic hot water supply.
• Assumes an equator pointing collector installed +/- 10^o of the latitude angle.