Solar geysers: everything you need to know to make the switch and save electricity

‘I always tell my clients the first thing they should do is put their geyser onto solar, because that uses 40% of your electricity. Some people would say 50%, but we average it over the year at just over 40%, because in winter, you’re gonna get less [heat],” says Marc Miller, CEO of the Cape Town-based Waterloo Plumbing. 

Getting into solar needn’t be an all-or-nothing proposition. 

Although not exactly cheap, a modular approach, implemented over time, is still far more accessible than installing everything at once. 

https://www.dailymaverick.co.za/article/2023-02-19-there-are-affordable-ways-to-use-solar-power-to-brighten-up-those-load-shedded-days/

What is a solar geyser?

A household solar set-up is typically made up of photovoltaic solar panels that convert sunlight into electrical energy, an inverter that turns it into usable electricity for your household, as well as a battery to store the energy for later use. 

However, a solar geyser, also generally referred to as a solar water heating system, does not have a battery or an inverter or a typical photovoltaic solar panel. 

It works with solar collectors, which take sunlight, ultraviolet or infrared rays, transform those into heat and use them to heat the water directly. 

There are two main types of solar collectors to choose from: a flat-plate collector, or a series of glass evacuated tubes. These are the ones often seen on roofs, with a geyser tank above them. 

Flat-plate vs glass evacuated tubes

Both follow a similar principle, in that they collect heat and transfer it to the water. Which one you go with is primarily dependent on which direction your roof faces and how much sunlight it gets.

A flat-plate collector has pipes that run up and down underneath a glass panel which collects the heat and transfers it to the pipes to heat up the water, explains Miller. As the cold water heats up, it rises naturally towards the tank above the panel, filling it with hot water supply for the house. 

The glass evacuated tube system doesn’t have water running through its cylindrical tubes. Rather, the tubes are evacuated and a brass rod is inserted. When the rod heats up, it transfers that energy into a manifold at the top, to which all the rods are attached. The water is heated as it runs through the manifold.

Which option is right for you?

The sun rises in the east and sets in the west, and we're in the southern hemisphere. Hence, the solar panel on the roof should be oriented towards the north to get maximum exposure to sunlight, because as Miller explains, “that’ll get you five and a half hours of sun on a full day”. In that case, where the roof is oriented towards the north and is able to get the optimal amount of sunlight, he advises that a flat-plate collector would be the right option.

For homes where a north orientation is not possible, then the glass evacuated tubes, which are far more efficient at collecting energy from ultraviolet rays, would be the right option. “When you don’t have a north-facing roof, then you put those tubes on a west-facing roof, and they absorb ultraviolet rays.”

He doesn’t recommend placing the tubes on a north-facing roof because they are too efficient at collecting heat for the kind of sunlight exposure South African homes get from the north. “Sometimes they heat up to 90°C, and when that happens your geyser basically becomes a bomb that can take your roof off,” he warns. 

To avoid this worst-case scenario, the set-up will typically come with a safety valve, so that when the water reaches 90°C, the geyser will let some steam out and some cold water in. However, if those tubes were placed on a north-facing roof during the South African summer, the water would be likely to reach those maximum temperatures regularly, and lead to significant water waste as the system lets off steam and mixes in more cold water to cool it down, explains Miller. 

“And if you had to put a flat-plate panel on a west-facing roof, you wouldn’t have enough hot water, because it’s not efficient enough. So it’s a flat plate on the north-facing roof and tubes on a west-facing roof.”

Should you install a brand-new solar geyser or retrofit a panel to your existing geyser? 

Unlike a new system, where the geyser is placed above the panel so that the hot water naturally rises to it, a retrofitted panel would likely mean that the geyser, which would be your existing normal electric geyser, is below the panel, between the roof and the ceiling, or fitted on a wall. 

In this case, you would then also need to install a pump to push the hot water into the geyser. 

“To give you an idea of how much retrofitting might cost in comparison to a new system:

“If you’re thinking at a 200-litre tank, you’re looking at about R30,000 for a brand-new complete installation, including a panel and a tank. If you want to retrofit your old geyser, you’re looking at about R22,000, and that would cover the fitting, the panel and the pump, basically everything,” says Miller. 

Pros and cons

Due to the system’s dependence on the sun, Miller advises that homeowners always keep their Eskom connection as back-up, mainly because in winter, when there might be minimal sun exposure, it is likely that far less heat will be collected. 

So, unlike a gas geyser, the solar geyser would be less effective for a home that is completely off-grid. 

https://www.dailymaverick.co.za/article/2022-11-06-choose-the-right-size-gas-geyser-to-save-on-electricity-and-money/

That said, considering the amount of sun South Africa gets, Miller says that homeowners would still reap significant savings over time from solar heating, in addition to the convenience of hot water even during extended rolling blackouts and other potential power outages.  

“If you take into consideration the 40% saving on your electricity bill over the year, it takes approximately three years to pay back the cost of a retrofitted system in savings, and about four to five years for a brand-new system to basically pay for itself,” says Miller.

Last, he stresses that should you go the solar route for water heating, “it has to be installed by a qualified solar installer. So, just because one is a plumber, doesn’t mean they can install solar. They’ve got to do an added [qualification] to install solar. You’ve also got to make sure you get a certificate of compliance for your system. It must be compliant by law.” DM/ML

Disclosure: This article has been edited from the original version, to correct an error in the technical explanation about solar water heating.