For a typical solar water heating system it will generate 40% and 60% of the households in the course of more than a year of
hot water shower, hot tap and so on, which is sometimes referred to as a solar fraction of 40% - 60%.
Of course, the amount of hot water generated also depends on the time of year:
In the summer, a large enough solar hot water system can produce most or all of the required hot water.
In spring and autumn, the solar water heater can significantly reduce the amount of energy used for heating water, pre-heating the water in the hot water tank, then the boiler requires less running.
In the winter, the solar water heater usually make only a small contribution because if weak sun and the days are shorter in winter.
It is of course possible to produce a greater proportion of the hot water by installing more panels. However, due to the solar water heater in summer, if adding more panels, it may result in summer panels produce more hot water than required.
Therefore, from a certain point, added the additional panel will contribute less than panel before, and add more panels, ultimately become uneconomical.
Since the solar radiation is much weaker in the winter, and because a large amount of energy is required to heat a typical UK house. SWH is not normally used for space heating in the UK.