Let’s first touch on some basics with regards to solar water heaters and heat pumps.

The Fundamentals

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. If we for example have a 2.5m2 solar panel connected to a 150L geyser this might give us 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 geyser to 35°C.

In this case we would need an electrical element to cover the shortfall. If we have a solar panel that is only half the size (1.25m2) we would only get out half the energy and an electrical element will need to do the rest.
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 be only lukewarm in the morning and an electrical element will need to heat the water to a useable temperature. For these reasons most solar water heaters rely heavily 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.

The sad truth is that most solar systems in South-Africa are undersized and will provide way less than a 50% saving on the water heating bill. We see so many houses where families of three or more are living having just a 2m2 solar panel on a 150L geyser. Most likely the company that sold it to them promised them big savings but it is simply not physically possible.

If a solar water heater gives a 50% saving on your water heating bill this will typically result in a 25% saving on your total electrical bill (geyser consumption is typically 50% of total electric bill).

Domestic hot water heat pumps work slightly different. The heat pump uses a small amount of electricity to extract a lot of energy from the surrounding air. A heat pump is therefore using the energy from the sun indirectly and so it can work day and night, winter and summer. The efficiency of a heat pump is called the COP. A COP value of 4 means that the heat pump produces four times as much thermal energy as what it uses electrically – in other words a 75% saving on the water heating bill.

Unfortunately, the COP of a heat pump is dependent on the ambient and the water temperature and so a more realistic annual COP value is 3. In other words, a 66% saving on the water heating bill.

A high efficiency heat pump like the ITS-3.6HDP takes about 2 hours to re-heat a 150L geyser, which is used in most households in South Africa. This enables you to always have hot water at a fraction of the cost no matter when or how much water you use.

A Practical Example

My neighbours are a family of 4 (husband, wife and 2 young boys) that use water moderately. Measurements we have done on their 200L geyser show an average consumption of 16kWh/day. They would like to save on their electricity bill and the most cost effective way is to install a solar water heater or a heat pump.

Let’s first look at the solar water heating option. On a 200L geyser most solar installers will put a 2.5m2 solar panel (flat plate or equivalent evacuated tube). An ITS-2.5FPC high efficiency 2.5m2 solar panel that is perfectly installed will provide a thermal output of about 7.6kWh/day (based on an average solar radiation of 23MJ/m2/day and some typical thermal pipe losses).

At an Eskom tariff of R3/kWh this will equate to a saving of R50841 over 5 years (assuming a 10% annual electrical tariff increase). A solar system like this will cost them about R19k for a retrofit system (solar panel connecting to the existing geyser) or about R25k for a complete system (solar panel plus new solar ready geyser).

With an ITS-3.6HD high quality domestic hot water heat pump working at average South-African ambient temperatures you will conservatively get a COP of 3. With a COP of 3 they would on average save 10.66kWh/day and therefore R71378 over 5 years. A heat pump like this would cost them about R25k fully installed.

Now, let’s assume the neighbours discover how incredibly nice it is to have big high flow rate shower heads and their usage goes up to 30kWh (about 400L) per day. The 2.5m2 solar panel will still give the same output and therefore a R50k saving over 5 years. The rest of the energy will need to come from Eskom. With the domestic hot water heat pump, the same ITS-3.6HD heat pump can be used. Again, working on a very conservative COP of 3 they will now save 20kWh/day and therefore R134k over 5 years!


From the examples above it can be seen that even for a family that use water conservatively the heat pump will pay for itself in just over 2 years and provide a much bigger long-term saving than a solar system. The solar panel savings calculated above also assumes that the solar collector panels are mounted Solar-North with the optimal inclination and that the family adjusted their water usage pattern to ensure the geyser is cold when the solar starts working in the morning (not pre-heated by the element). Deviating from these will decrease the systems output. With a heat pump it does not matter when and how much hot water you use.

With the high usage example, the heat pump will pay for itself in just over one year!

The life expectancy of a heat pump is about 10 years. Solar water heaters have a life expectance of 20 years but do keep in mind that it really is only the panel. Many solar water heating systems uses a controller and pump and these will reduce the system life expectancy to around 10 years. Thermosiphon solar heating systems (no circulation pump and differential temperature pump controller) should be the life expectancy “winner” if one ignores the hot water storage tank needed in all the above systems. Both solar systems and heat pumps need to be serviced annually to ensure optimal performance. Servicing is basically just cleaning the system and making sure everything is working correctly.

A solar system can provide a bigger saving than a heat pump but for that the solar system needs to be oversized and water usage patterns needs to be adjusted. Typically, you need double the volume of hot water that what you would need for a normal electrical geyser or a heat pump system. In coastal areas like Cape Town however the winter irradiation is much lower than summer irradiation. This means that even if your solar system was sized for 100% of your solar usage in summer you will have only about half the thermal output you need in the winter and the electrical element would need to do the rest. Winter is also when people use the most hot water and so a heat pump would almost always be a better solution in coastal areas like Cape Town.

The three graphs below show the saving a heat pump, a 2m2 and a 2.5m2 solar panel can provide: