Helping You To Become Energy-Smart

Solar Water Heating Basics

Solar collector

The most energy intensive appliance in the home is the hot water cylinder or geyser. It is also the most expensive to run. The good news is that solar water heaters can replace up to 100% of the electricity used to heat water, and solar water heating should be considered as one of the first steps in energy and cost savings, providing a better return on investment than other renewable energy saving or generating technology in the South African environment.

The SESSA information provided on solar water heating will help the consumer to decide on which type of system is best for them, will enable them to calculate the potential savings in electricity and costs and will enable them to avoid pitfalls and disappointments as they embark on their journey towards cleaner, cheaper and more reliable energy. The information provided is impartial and is provided as a representative body of the South African solar water heating industry and does not favour a ny particular products or manufacturers.

The amount of electricity consumed in heating water is fundamental to understanding the overall financial equation. The simple formula is to take the volume of water to be heated, the temperature of the water when cold and when hot, called here the 'temperature differential', and the amount of power used to heat the volume by the temperature differential.

Volume ×Temperature Differential ÷ 860 = kWh (electricity used)

The cold water in Johannesburg is as low as 10 ºC in winter and as high as 22 ºC in mid summer with an average of 16 ºC. The typical hot water thermostat setting in the electric geyser is 60 ºC giving an average temperature differential of 44 ºC.

For example:

100 Liters × 44 °C ÷ 860 = 5.11kWh

If the price of electricity is R1.65 per kWh the cost of heating the tank of 100 litres from cold to hot is 5.11kWh × R1.65 = R8.44

Solar water heaters generate heat through the solar collector with the heat transferred through the solar collector and stored in the water vessel or tank. This solar thermal process is substituting the electricity used in heating water through an electrical resistance element. The performance of a solar water heater is tested by the South African National Standards. SANS 6211 measures the energy collected and stored during a day, using 6 representative sample days of different solar irradiation. This represents a sample performance of the typical weather in South Africa over a period of a year.

SANS 6211 results in a 'Q' test result. Dividing the 'Q' by 3.6 results in the deemed saving equivalent in kWh's as calculated at 20MJ irradiation, which is the average across South Africa. A maximum performance is stipulated as 10MJ per 50 liters of water per day under SANS 1307, and a minimum performance of 5MJ per 50 liters.

The minimum and maximum Q factors allowed are: 

Volume100 litres150 litres200 litres250 litres300 litres
Min Output - “Q” 10 15 20 25 30
Max Output - “Q” 20 30 40 50 60

The minimum and maximum electrical savings output in kWh is therefore:

Volume100 litres150 litres200 litres250 litres300 litres
Min Output - ‘kWh’s’ 2.77 4.16 5.55 6.94 8.33
Max Output - ‘kWh’s” 5.55 8.33 11.11 13.88 16.66

The ‘Q’ factor is therefore extremely important indicator of the performance of the solar water heating system chosen. Much in the same way as kilometers per liter of diesel or petrol it provides a measure of performance, but it is not the only criteria for choosing a SWH system. With the purpose of a Solar Water Heater being to heat hot water and save electricity, the efficiency of the solar water heater is an important factor in choosing the SWH system. In reality no SWH system can be 100% efficient due to weather, but some systems come close.

To calculate the efficiency of a system take the Q test figure at 20MJ and divide by the Q factor for 100% theoretical efficiency.

For example:

Size of System in (litres)Electrical Consumption from Cold to Hot (kWh)Q factor required for 100% EfficiencyQ Factor on SWH system (illustrative) Efficiency Rating in %
150 7,67 27,62 23 83%
200 10,23 36,83 32 87%
250 12,79 46,04 38 83%
300 15,34 55,25 46 83%

Despite advertising claims that solar water heaters will save 40% of a home's electricity bill this is a misleading comment. It depends purely on what percentage of the home electricity bill is spent on heating water. It may be as low as 10% or as high as 60% of the total home electricity bill. The amount of electricity saved in heating water depends on the size and the efficiency of the solar water heater. A solar water heater can only save a percentage of the amount of electricity used in heating the volume of water in the tank.

Taking the Q factor and dividing by 3,6 provides the equivalent electrical saving. Taking the current rate of electricity and multiplying by the kWh savings will provide the average financial saving daily.

For example – (using the same illustrative system as before):

Size of System (litres)Cost Including Installation (Illustrative approx.)Approximate Payback Period
150 R15, 000 3 years 4 months
200 R20, 000 3 years 2 months
250 R25, 000 3 years 6 months
300 R30, 000 3 years 5 months

All solar water heaters do the same thing. They collect heat from sunlight and irradiation through the solar collector and transfer that heat to water which is then stored in a vessel (the geyser or tank). The solar collector is generally a flat plate collector or an evacuated tube collector. More basic systems may be a coil of black pipe within a box or similar. The levels of efficiency are determined by quality and size of the collector. Although there are over 140 solar water 'high pressure' systems tested and passed by the South African Bureau of Standards (SABS), and more that have not passed the rigorous testing process, they all set out to do the same thing. That is, to heat water from the sun and to store the hot water.

High Pressure and Low Pressure Systems

The pressure of the water determines the type of overall system you require. If 'high pressure' water comes out of your tap, you will need a 'high pressure' solar water heating system. If the water flows with no pressure, only using gravity, it will generally be a 'low pressure' system. Most homes with an existing electric geyser will require a 'high pressure' solar water heating system. Lower income homes that have no existing hot water boiler are generally fitted with 'low pressure' solar water heaters.

High Pressure Types

For the consumer the variety of choices to be made may be confusing. The first question is whether the homeowner is happy to have the complete solar water heating system sitting on their roof. This will include the tank and the solar collector, frame and accessories. If the answer is "NO" a split system will be required, where the tank is inside the home (generally in the roof) and only the solar collector is on the roof. Aesthetically this may be more attractive, and permissible by some governing bodies in developments, while also having the tank on the roof may not be.

There are numerous configurations within high pressure solar water heating systems. Indirect and direct, split and integral, pumped or thermo-syphon are the main ones, but unless one is interested in the inner workings of an engine, their relevance is not that important. The important questions, other than cost and efficiency, is whether the system has electrical back up (most do), and whether the system can operate in a non-coastal area, and is therefore freeze resistant.

The Department of Energy incentive program administered by Eskom from 2008 to April 2015 has been terminated. During the period some 120,000 HP SWH benefited from rebates, which were linked to the performance of each SWH.

At the same time the price of electricity has increased by at least 200%, depending on area and tariff structures. There is no clarity as to whether rebates or incentives will ever be reoffered, and if they are on what terms.

The reason to embrace solar water heaters should never have been decided upon because of a rebate, rather it should always be on the basis that the end user will save electricity, save money, and reduce carbon emissions. Any rebate or incentive should be viewed as a ‘cherry on the cake’ rather than the reason to go solar.

With the inevitable ongoing price increases in electricity in South Africa, the savings in 2015 over 5 years going forward are greater than if a SWH had been purchased with a rebate in a period from 2008 to 2014. 

Waiting for rebates or incentives (which may never occur) is not the reason to delay embracing energy efficiency today.

There are many energy saving tips published. Some are misleading others are just wrong. For example many people believe they will save electricity if they turn their electrical geyser 'off' in the morning and back 'on' when they get home in the evening.

As a geyser is insulated, if it is turned off, the heat loss from the tank will be minimal over a few hours. When it is turned back on in the evening the electricity will replace the heat lost. In comparison if the heat loss is topped up through the day, it will be within a fraction of a percent of the same electricity consumed in the evening if the geyser was off through out the day. The reality is that you only need to turn an electrical geyser off if you go away for a few days.

A solar water heater's tank generally has a thicker layer of insulation than an electrical geyser. This is necessary to offset heat losses when the tank is sitting outside and exposed to the elements. Geyser blankets fitted to insulated electrical geyser tanks will, over a long period of time, save energy and the payback time on the investment is considerable.

The minimum thermostat setting for an electrical geyser or solar geyser with electrical backup should be set at 55 °C. Setting the thermostat at above 65 °C is not recommended for safety purposes, but the difference in heat loss (losing electricity through lost heat) is minimal. The greatest heat loss will occur from pipes that are not insulated, particularly if they are integral to the solar system.

Heating water is extremely energy intensive and the decision to go with a solar water heater in replacement of electricity in part or in whole is by far the most important decision. You need to act NOW!

Despite legislation the building industry continues to ignore the energy saving requirements for new buildings. Solar water heaters are the biggest savers of electricity, and even if a solar water heater was not installed by the builder, a solar ready geyser or solar geyser could be installed in preparation for when the new owners arrive at very similar cost. At that time a solar water heater of the consumer's choice could be installed and the energy incentive rebate claimed on the whole system.

Architects still tend to design homes with the hot water tank in the roof, mainly for historical reasons when the electric geyser relied on gravity to feed the building's taps. With a high pressure geyser it can be located anywhere, and importantly, it can be installed vertically which generally results in an improvement in performance over horizontal. In Europe, vertical hot water tanks are fitted in airing cupboards, where linen and towels are stored.

Architects, and in particular builders tend to install smaller electrical geysers due to price. In all homes other than a 2-person apartment, 200 litres should be the minimum size, and in the future should be 'solar ready'. Other energy efficiency measures such as the use of heat pumps for heating water should be carefully reviewed prior to specification and installation. Eskom and the Department of Energy have achieved disappointing results for domestic heat pumps, particularly in the Highveld. Energy incentives have consequently been scrapped.

In the commercial sector where larger quantities of water are being heated and stored, rather than drawn off as the water is used throughout the day (as in a domestic environment), large heat pumps offer considerable opportunities for energy saving of up to 65-70% of electricity.

Solar thermal in the commercial sector is in its infancy and offers huge opportunities for businesses to save money and reduce carbon emissions. As the price of solar thermal technology falls, much in the same way as in solar electric through solar photovoltaic technology, and the price of electricity continues to rise, the benefits of going solar will become increasingly attractive. 

Already large scale solar thermal, which for convenience one can describe as 5 000 liters of hot water and above, is providing businesses with an investment where capital invested can be paid off in as little as 5 years or less. With a lifespan of 25 years and more, and potential savings for 60 years subject to maintenance, solar thermal competes with the expected lifespan of a power station. 

In South Africa SOLTRAIN has been promoting awareness and the benefits of going solar thermal for commercial applications for a few years. Technical expertise and knowledge for understanding the complexities and engineering requirements of large scale solar thermal in South Africa has and continues to be supported by the Austrians and Germans, both leaders in the field of renewable energy. 

New initiatives in South Africa in conjunction with Soltrain and SESSA are leading to greater awareness, education and training for companies, finance directors, engineers, quantity surveyors, architects and energy consultants.

Buying a Solar Water Heater