Going Green – Solar Water Heating

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Solar Flat Plate Collector

Solar Water Heating Options

My next step in going more off grid – which should have been my first step – is to install a more efficient form of water heating to replace my existing electric geyser. My current setup is a Kwikot 150litre geyser with 3kW element controlled via a Geyserwise controller.

I’ve got the system set to heat water to 55 degrees Celsius at 3 different times of day:

  • 4:00am to 5:30am to ensure there is hot water for the morning showers and for dishwashing first thing in the morning.
  • 4:30pm to 6:00pm to make sure there is hot water for my kids to have a shower before dinner time.
  • 8:00pm to 9:30pm to reheat the water so the adults have hot water to shower before bed time.

Quite honestly, I have no idea what this saves me in terms of actual electrical consumption as I’ve had the geyser running this way for long before my decision to go partially off-grid. The timings have just been changed slightly to shift my peak loads around a bit and “flatten” my load profile during the day. This will help when I eventually get around to installing solar PV panels.

My options for more efficient forms of water heating are as follows:

  • Heat pump
  • Evacuated tube type solar collector
  • Flat panel type solar collector

Each of these systems has it pro’s and con’s

Heat Pump

Heat Pump


  • Uses less energy than a resistive element heater. In ideal conditions you can expect a CoP of 4, which in effect means 1kW of consumed power provides 4kW of heat.
  • Fairly simple installation with very little modification required to existing plumbing.


  • A heat pump is at is most efficient when the ambient temperature is high. The lower the ambient temperature, the less efficient the heating. In really cold weather, there is a chance that water is not heated sufficiently.
  • Heat pumps require electricity to run. This does not help if you are trying to go off-grid or reduce dependence on the grid.

Solar Evacuated Tube Collector

Evacuated Tube Solar Collector


  • Very efficient at heating water even in low ambient temperature.
  • Round collector tubes allow passive tracking meaning that heating starts earlier in the day.
  • Little or no grid power required for operation.
  • Tubes can be replaced individually if damaged.
  • Little or no maintenance required.


  • In our climate, the evacuated tubes can sometimes be too efficient, heating water to above boiling point if left unchecked. This could lead to damage of the geyser and water wastage.
  • For a retrofit to an existing geyser, and electrically operated pump is required. This could be solar powered, so isn’t really and issue in terms of consumption, it just adds more complication to the plumbing.
  • Plumbing becomes a bit more complicated than that of a heat pump.
  • Water is only heated during the day, so after the first use at night, cold water enters the geyser and the water begins to cool. Supplemental heating may be required to ensure there is hot water first thing in the morning.

Solar Flat Plate Collector


  • Fairly efficient at heating water even in low ambient temperatures.
  • Low cost and proven simple technology.
  • Little or no maintenance required.
  • Little chance of heating water to boiling point.


  • Not as efficient as evacuated tube collectors.
  • As with the evacuated tubes, a pump is required to circulate water during heating.
  • Plumbing is a bit more complicated than that of a heat pump.
  • As with the evacuated tube system, water is only heated during the day and supplemental heating may be required overnight to ensure hot water is available in the morning.

My Choice of Water Heating

Having weighed up the pro’s and con’s of the various systems, I rejected the use of a heat pump. This is mainly due to the fact that the system, although very energy efficient, is still wholly dependent on electricity.

This leaves me with a choice between an evacuated tube system and a flat plate collector system.

Based purely on initial cost, I have decided to opt for the installation of a flat plate collector system coupled to my existing geyser.

The installed cost for the flat plate system is around R12,000 compared to the evacuated tube system at R18,000. Based on my research into the two systems, I don’t believe the evacuated tubes to be 30% more efficient than the flat plate collectors, so I can’t justify spending 30% more.

Solar Water Heating System Design

My first consideration with the design was aesthetics – I did not want to have a water tank on my roof coupled to the solar collector. I have chosen to retrofit a pumped system to my existing geyser in the ceiling void. This is also a bit of a cost saving as I get to reuse some existing equipment.

The next problem to address is usage patterns. In order to get the most out of a solar water heating system, some lifestyle adjustments may need to be made. For example, instead of running a bath (which we seldom do) use the shower (which we already mostly do). Shower or bath in the early evening while the water is still hot, etc., etc.

In my case, it would be easier squeeze blood from a stone than to get my family to change their routines!

The only way to overcome this is to increase hot water storage. As a rule of thumb, whatever works for you with a conventional water heating system needs to be doubled to ensure you have hot water when you need it.

For me, this means adding an additional 150 litre geyser to increase my total water storage to 300 litres.

This also means the installation of an additional flat plate collector to cope with the larger water storage.

The way the system will work is that the two geysers, existing and new, will be connected in series with the flat plate collectors also connected in series and the system forming a complete loop. Water is used from the new geyser and refilled with hot water from the existing geyser, this way ensuring that the water stays hot for longer. Only in exceptional cases should the geyser element be required to heat water.

What this also means is that the installation, in my opinion, becomes fairly complicated and a lot more expensive.

My initial cost has now jumped from R12,000 to R23,000! An evacuated tube system of similar capacity would be in the region of R27,000, so I’m still better off in terms of initial cost.

Solar Water Heating Energy Cost Saving

My current monthly energy consumption is in the region of 1,000kWh. My current water heating system accounts for around 375kWh of this amount. This is based on an average consumption of 12.5kWh per day as currently measured.

Working out the potential cost saving gets a little complicated based on our electricity supply tariff from the Ekurhuleni Municipality. The tariff is based on a sliding scale as follows:

  • 0 – 600kWh charged at R 1.08/kWh
  • 600 – 700kWh charged at R 1.83/kWh
  • 700+kWh charged at R 5.13/kWh

This gives me a bill as follows:

  • 600 units for R 648.00
  • 100 units for R 183.00
  • 300 units for R 1,539.00

Total bill for 1,000 units comes to R 2,370

So, in theory, if my geyser is completely solar powered, my energy consumption would reduce to 625kWh per month. I’ll assume some bad days when the element is needed for backup and only reduce my consumption by 300kWh per month.

This would mean that my bill should reduce by R 1,539.00 per month giving me a payback period on the R23,000 system of 15 months.

In my opinion, a complete no-brainer. Time to get going with the installation. Stay posted for more info on the actual installation.