Research has shown that the radiation in South Africa is sufficient to supply about all the hot water needed nationwide. A study done showed that sun power could supply as much as 99% of hot water needed in summer months between November and February.
This percentage drops gradually from March at 95% until it hits a low in July at 52% and then increases again from August. In the colder months electricity is used to top up the hot water needs. On average this equates annually to a Solar Contribution Factor of 70% and only 30% in electrically heated.
Keep in mind that even if limited in the cold winter months and on a completely overcast day, invisible ultraviolet radiation will still provide enough energy for the solar panels.