Just like a conventional, electrically heated geyser, solar water heaters require a storage hot water cylinder. This allows water to be heated by the sun during the day and stored for use that night or the following morning. It also provides an electrical backup element to heat the water should there be insufficient solar radiation or in case of higher usage.
There are various types of hot water cylinders being used in the South African market. Following are some of the types of cylinders and their characteristics:
Vitreous enamel – These are the most commmonly used hot water cylinders in South Africa due to low manufacturing cost. The inner tank is lined with a glass enamel in order to separate the steel tank from the water. As the hot water cylinders expand and contract with heating and cooling, this enamel lining is not sufficiently elastic to absorb this movement. Hairline cracks develop in the lining and the water comes into contact with the steel and corrosion begins to occur. 
For this reason these tanks are fitted with one or two soft metal sacrificial anodes. These operate in the same manner as the sacrifical anode on an outboard motor, corroding preferentially and thereby saving the steel tank. These anodes are meant to be replaced every 1 to 3 years, depending on the water charactersitics. Once the anode is corroded away, corrosion begins to accelerate in other parts of the tank, particulalry the joints. We have come accross plumbers with many years in the industry who were not even aware of the anode and the need to periodically replace it. The picture shows an anode inside a tank that has almost completely corroded away.
Stainless Steel – Obviously more expensive than enamelled tanks, stainless steel is also prone to corrode, but at a much slower rate. These tanks are therefore also fitted with sacrifical anodes which have to be periodically checked and replaced to ensure the lonevity of the tank.
Copper – Just as copper pipes have been used for decades as water pipes due to the ability of copper to withstand corrosion, so copper hot water cylinders have a very low rate of corrosion (except with low pH levels). This means that typically, copper hot water cylinders last for many years. Due to the high price of copper, these tanks are generally very expensive and due to the high temperature and pressure of solar geysers, thicker copper has to be used resulting in prohibitively high cost of tanks.
Fibre Glass – Hot water cylinders with both an inner and outer tank of glass fibre are fairly popular. Over an extended period of expansion and contraction, the tanks can split and companies making these tanks have tried altering the design to avoid splitting.
PEX – Although Hills Solar often do retrofit solar systems to different types of tanks, generally they use the PEX lined Solartherm tanks when they install a full system. PEX is a cross-linked polyethylene that is used to line the inside of the tanks. This PEX is quite elastic and so can withstand considerable expansion and contraction without cracking. 
The water inside the tank is permanently separated from the steel, minimising the chance of corrosion. Made by WE Geysers in Krugersdorp since 1997, these hot water cylinders are able to withstand the temperatures of solar heated water as well as the diverse water characte
ristics of South African water. WE Geysers give a 10 year, maintenance free guarantee on their geysers which are made to ISO-9001/2008 standards and are SABS approved. Hills Solar supply these geysers with their ESKOM rebated 150, 200 and 300 liter systems. They are avilable in horizontal and vertical models.