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Solar & Heat Pumps

What is a Heat Pump?
  • An air to water heat pump is an energy efficient product that typically draws 1/3 of the electricity that would normally be required of a standard resistance heater. This is achieved by moving heat from one source to another. Heat is collected by a reverse refrigeration process and used to heat water by the Heat Pump.
  • In laymen terms, for approximately every 3 units of energy transferred to heat the water, only 1 unit of electrical energy is consumed. Therefore for every 1 kW of input power you will get approximately 3 kW of useful output power. Actual results may vary from application to application.
How do they work?

Heat is extracted from the heat source by the evaporator, which is at a low temperature. In it, the heat is transferred to a working fluid, changing it from liquid to gas. A compressor then increases the gas pressure, causing temperature to rise. The hot gas flows to the condenser, where heat is given off as the working fluid changes back from gas to liquid. The working fluid which is still at a high pressure is then passed through an expansion valve, which lowers the pressure. In doing so, the fluid becomes cold and returns to its liquid state before once again entering the evaporator. In refrigeration and air-conditioning applications, the process is reversed, supplying useful cooling instead.

A heat pump is essentially a vapour compression cycle, similar to an air conditioning unit. However, instead of the cycle being used for air cooling purposes with the associated heat as a by-product, a heat pump utilises the heat recovered and generated in the cycle to heat water.

Heat Pump vs Solar

The example:
A family of 4 use water very conservatively. Measurements we have done on their 200L geyser show an average consumption of 16kWh/day. Let’s first look at the solar water heating option. On a 200L geyser most solar installers will put a 3m2 solar panel (flat plate or evacuated tube). A top of the range high efficiency 3m2 solar panel that is perfectly mounted, will provide a maximum thermal output of 7kWh/day (based on the South-African national average solar radiation of 23MJ/ m2/day).

With a 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. A correct heat pump would cost them about R13,500 fully installed and with the Eskom rebate already deducted.


A second family of 4 as an example, are more liberal with their water use. They are using an average of 30kWh per day on their 200L electrical geyser. Since a 200L geyser can only store 200L of hot water, connecting a bigger solar panel on will not necessarily result in a bigger saving. But they do have people in the house during the daytime and so we can go for a bigger 4m2 high efficiency panel.

This solar panel will give an average of 9.5kWh/day and therefore a saving of about R33,400 over 5 years. The 4m2 solar system will be about R2500 more than the 3m2 system pricing given above.

Should the second family decide to rather go for a domestic hot water heat pump, we can use exactly the same unit as the first family example. Again working on a very conservative COP of 3 then the second family will save 20kWh/day and therefore R70,400 over 5 years.

In conclusion:

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. Variances in the facing and inclination of the solar collector panels will decrease the systems output.

With the second family, the heat pump will pay for itself in about 1 year and 5 months, while the solar system will take about 4 years. The life expectancy of both the solar system and the heat pump is very similar and is estimated at 10 years but in both cases we know of systems that are running for more than 25 years and still going strong.


Based on the above, I would therefore recommend going with a Heat Pump Vs Solar. 

  1. It is a cheaper installation.
  2. The payback period is much quicker.
  3. Hot water any time of day.
  4. A safer installation whereby one does not need to work on the roof.
  5. No mounting of solar panels on the roof thereby eliminating leaks through the securing devices attaching the solar panel or geyser.
  6. Approximately 10-12% more savings over a 12 month period compared to solar.

If you have a 100L – 300L geyser, you will qualify for an Eskom rebate of R3668

If you have a 301L – 500L geyser, you will qualify for an Eskom rebate of R4320

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