A Backround To The Technologies
Solar water heaters use the radiation from the sun to generate heat. The size of the solar panel will determine how much energy can be collected from the sun. So, if we for example have a 3m2 solar panel connected to a 150L geyser this might give us 150L of 60°C water at the end of a warm sunny day but, during cooler days with less sunshine, it might only be able to heat the 150L to 35°C.
In this case we would need an electrical element to heat the water further. If we have a solar panel that is only half the size (1.5m2) we would only get out half the energy and an electrical element will need to do the rest.
Also if we assume we have 150L of 60°C water at the end of a warm sunny day and we use half the hot water in the evening, the water will more than likely be cold in the morning and so if someone would like to shower in the morning, an electrical element again will need to heat the water to a useable temperature.
From the above it should be clear that solar water heaters do rely on electrical elements to provide hot water at all times.
A high efficiency properly sized solar system (double the geyser size compared to a normal electrical heating installation) installed in a place like Johannesburg could provide almost all the hot water required if the user is willing to use hot water only in the evening or only in the morning. If you however install this same system in Cape Town you would typically not be able to get more than a 60% saving. The reason for this is that Gauteng has a very constant solar irradiation level all year round while Cape Town has very high levels of summer irradiation and very low levels in winter.
However, the sad truth is that most solar systems in South-Africa are undersized and will provide much less than a 50% saving on the water heating bill. We see so many houses where families of 3 or more are living having just a 2m2 solar panel on the roof connected to a 150L geyser. Most likely the company that sold it to them promised them big savings but it is simply physically impossible.
Please also note that if a high efficiency properly sized solar water heater gives a saving of 50% on your water heating bill this will not result in a 50% saving on your total electrical bill unless the geyser is the only electrical device in your house.