Whether a solar water heating system will provide any real benefit is purely a function how much energy the volume of water in question requires and how much energy the collector can deliver and when.
The energy required to heat for example, a 200L geyser by 40 degrees Celsius is 9.13kWh.
Systems currently listed on the Eskom web site show products with energy delivery ratings from 3.74kWh to 9.23kWh for 200L systems.
This rating is known as the Q factor. It is produced from SABS test measurements and indicates the heat output from an insolation level of 16MJ or 4.44kWh/m2/day of sunlight energy.
This means that the 3.74kWh product will provide 41% of the energy required whereas the 9.23kWh product will deliver 101% of the energy required to achieve a 40-degree temperature rise in the 200L of water.
Many systems available on the subsidy scheme are, in my opinion, under powered some of which are even S ABS Mark approved. Unfortunately, mark approval in the case of Solar Water Heating equipment does not indicate that the product is fit for purpose.
The minimum output energy rating required for approval being only 2.5kWh regardless of the volume of water into which the energy is being delivered. Typically in a 200l geyser 2.5kWh is just enough to keep the water hot. It will not provide sufficient energy to reheat the water used.
Make sure you select a product with a High Q Factor and compare it to the required to heat the volume of water in question.