According to data made available by the South African Weather Bureau, the total solar radiation normal to a plate surface orientated true north for the Cape Town area = 2017kWh per m² per year.
Our panels are tested to be 65 - 70% efficient.

Therefore a collector with a apperture of 1m² will annually put (2017 x .65 ) 1311.05 kWh into the hot water cylinder.

Currently Eskom sells electricity at ± 50c / kWh.
Based on this figures above the annual savings are as follows:

Example:
200 litre copper system with 2.9 m² collector @ ± R10600
Installation cost @ ± R2500
Total cost = ± R13100

In this case the payback period is 7 years, but one must keep in mind that for a new house a hot water cylinder must be purchased in any case which means that the actual cost would only be for the collector panel and the installation thereof.
Generally the payback time of a solar thermal system varies. It usually takes between 3-7 years depending on the type of system installed and the usage patterns of the client.

Another thing to keep in mind is that the price of electricity is increasing constantly and these increases will occur more frequently in the future as Eskom needs to upgrade its infrastructure to accommodate the ever increasing demand for electricity.

Summary:

There are many ways to provide hot water. However, the evidence in support of solar power is overwhelming. If one considers that the global trend is turning toward renewable energy sources, and that this will soon be legislated, it seems foolish to use conventional heating methods. This is a short-sighted view, and will only save money in the very short term. We are extremely fortunate in this country to have such a massive amount of sunshine available throughout the year. Why not use it? No matter which way you look at it, solar water heating saves money, and the environment!