These are invariably systems based on thermosyphon, the type of systems where you have a tank up on the roof.
This is because of the way thermosyphon works.
The fact that hot water rises and that the thermosyphon process is very slow, the hot water in such systems rises to the top of the tank leaving markedly colder water at the bottom. This design is unable to heat the lower volume of water in the tank.
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In large bodies of water such as lakes for example a similar effect is noticed. Divers will witness what is known as the thermocline, a point where the water temperature suddenly drops. The same thing happens in smaller volumes of water where there is no stirring of the upper and lower layers.
What this means is that if you buy a thermosyphon system you can’t really use the full capacity of the tank. So you need a bigger tank than you would have done.
One supplier states that their 300L product has a solar delivery of 240L and their 180 L product can only deliver 160L from solar. This suggests that approximately 20% of the capacity is not usable.
This phenomenon however only applies to thermosyphon systems.
The more modern designs which are actively circulated by the use of small pumps, does not suffer from this loss of capacity. As such it is perfectly acceptable to use your existing geyser or one of the more common geyser sizes that will suit your water demand needs.
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