Small systems in detached private homes are typically designed such that they largely reach a full supply outside the heating periods, so that the boiler can be shut down in the summer. Around 60 % of annual hot water requirements can be covered by solar energy in this way. Larger solar fractions, i.e. if a large proportion of water must be heated by solar energy in spring, autumn, or in winter, give rise to a surplus in the summer which cannot be used. The solar system is then no longer operating as effectively as possible. In other words, an increasing solar fraction reduces the efficiency of a solar system. For systems in multiple dwellings or social institutions in which the auxiliary heating cannot be switched off because of tenancy laws or other provisions, current solar systems are designed with a solar fraction of up to 30 %.

There are no simple methods to calculate the yield of a solar system precisely. The number of parameters which determine the performance of a system is too large, and includes not only the changeable, non-linear characteristic of the weather but also the dynamic processes in the system itself. Although there are rules of thumb, such as around 1-2 m² of collector area per person and 50 l storage content per m² of collector area, these apply at best for small systems in detached or semi-detached houses.

In larger systems, computer simulation is the only way to investigate the influences of ambient conditions, user behaviour, and of various components on the operational state of the solar system.

Solar systems can also be used for heating wherever heat is required in the summer or where solar energy can be used for cooling in summer. These systems can then also make an appreciable contribution to building heating in the spring and autumn.

A further use of solar systems for auxiliary heating is in the field of low-energy houses. There, the fraction of heating energy occupies the same order of magnitude as the hot water supply.

In buildings with current thermal insulation standards, designing solar systems with the option of seasonal storage for heating purposes, also in winter, is inadvisable. This results in very large collector areas and, at the time, high surplus energy in the summer, i.e. in systems with very poor efficiency and consequently very high solar heat prices.