A typical solar water heating system in the UK will produce between 40% and 60% of the hot water a household uses over the course of a year for showers, hot taps etc. This is sometimes referred to as a solar fraction of 40%-60%.
The amount of hot water produced will, of course, depend on the time of year:
It is of course possible to produce a greater proportion of hot water by installing more panels. However, as SWH produces most hot water in summer, adding more panels can lead to a situation where in summer the panels produce more hot water than required. Thus, from a certain point on, every additional panel that is added will contribute less usable hot water than the panel before, and adding more panels eventually becomes uneconomic.
Because the solar radiation is much weaker in winter, and because a large amount of energy is required to heat typical UK houses, SWH is normally not used for space heating (radiators) in the UK.