In most cases, solar thermal energy systems are designed to meet 100% of a household’s energy demands for water heating during the summer months from May to September. During the winter months, the boiler will likely be used for space heating and can also heat water during that time. In this way, solar energy accounts for approximately 60% of the energy used to heat water throughout the year.
Generally speaking, the size of your solar thermal system will vary depending on the climate and overall water usage. The following guidelines can be used to estimate your system requirements:

Collector surface area
1.5 m² flat-plate collector surface per person
1 m² evacuated tube collector surface per person

As you can see, evacuated tube type collectors are more efficient given the same area. This may be something to consider if your rooftop is not very large.

Storage tank volumes
20-30 gallons per person 

Consistent Hot Water Demand throughout the Year

Since household hot water requirements remain relatively consistent throughout the year, the use of solar energy for hot water generation can be extremely cost-effective. The solar thermal system can easily be designed to meet a specific household’s energy demands for hot water usage. With a properly sized system, 50% to 65% of the annual hot water requirements would be provided by solar energy – and during the summer, 100% could be achieved, allowing the conventional heating system to be completely off during that time.