The Eskom situation in South Africa has forced many to turn to PV (solar electric) systems. Most of the inverters used in these systems have mobile phone app connectivity allowing the user to monitor solar power levels, battery state and consumption graphs. When users look at the consumption graphs, they are usually shocked to find that one appliance in the house is using about half of the total available power, causing significant strain on the battery and inverter capacity. The geyser’s power consumption is so high, in fact, that many PV solar installers will simply advise the client not to connect it to the PV system and to rather leave it on Eskom.

At ITS we are so inundated with calls of this nature that we have decided to try and shed some light on which water-heating technology will be the most suitable in combination with a PV system. Due to complaints from sales people that my articles are too technically orientated, I have decided to, for my fellow engineers, move the more technical parts into footnotes.

As a case study, let’s assume a three-bedroom house with a family of four that uses 28kWh (Eskom units) on average per day. The PV system is a typical 5kW inverter with 4kW of PV panels and 10kWh of battery storage. The house has a standard 200L geyser with a 4kW element in.

What Are The Options?

  • Leave the existing geyser connected to Eskom. Heating 200L will require more than 14kWh (electrical units) per day. To achieve this through Eskom (if available) will cost more than R15k per year (>R93k over 5 years with 10% annual increase). This is a very costly way of doing it, and does not allow you to take advantage of excess solar energy collected by your PV panels during the day.

  • Connect the existing geyser to the PV system. The 4kW element on the 5kW inverter will leave too little capacity on your inverter to accommodate other appliances in the house. One would therefore have to upgrade to something like an 8kW inverter, or change the geyser element to 2kW, resulting in double the time to get the geyser to temperature. No matter what the element size is, the energy required to heat the geyser will still be 14kWh and the PV panels will supply an average of around 18kWh per day. This hardly leaves energy for anything else. To compensate for the geyser consumption, one will have to add about 3.3kW of extra PV panels to the system, which means you will almost need double the ideal, no-shade, north facing, roof space. Suitable roof space is a problem on most houses and now you will need more than 34 sqm of it and an MPPT that can handle 7kW of PV. During overcast days there will not be enough solar energy, and you will need to rely on Eskom (or a generator) to bring the tank up to temperature. A standard 5kW generator will not suffice, as the geyser element will use 4kW and there will only be 1kW available for the rest of the appliances. The additional cost of heating the geyser in this way is about R25k excluding installation.

  • Install a Solar PV water heating element on the geyser. This is very similar to the above case, except that the geyser now has its own MPPT and dedicated PV panels, and is therefore not putting extra load on the inverter. You will still need to add about 3.3kW of extra PV panels which means you will have to find another 16 sqm of no-shade, north facing roof space. Should you have extra PV power available on the water heating system it can’t be used for the other purposes in the house. During overcast days there will not be sufficient solar energy and you will need to rely on Eskom to bring the tank up to temperature. The additional cost of heating the geyser in this way is about R25k excluding installation. Installation cost will be a bit more than the previous option.

  • Install an Instantaneous gas water heater. Using LP Gas to heat 200L of water per day will require about 0.9kg of LPG per day. At R38/kg it is R12491/year. Besides the fact that at current pricing gas heating is even more expensive than electrical heating, gas shortages and delivery issues should also be taken into consideration. The cost of a basic gas water heater that can do 20L/min is about R10k excluding installation. We estimate that the installation cost will be more or less the same as the PV heating element.

  • Install a Solar Thermal Geyser. To get the full 14kWh of thermal energy required you would need a 30-tube evacuated tube collector or 2 x 2.5sqm flat plate collectors. This requires about 5 sqm of non-shaded, north facing, roof space. For solar water heating to work optimally, you will need about double the water storage capacity, so you will need to add a 200L geyser to pre-feed the existing 200L geyser. During overcast days there will not be enough solar PV energy and you will need Eskom once again. The cost of a flat plate collector system with an extra 200L geyser is about R25k excluding installation. Installation cost will be slightly more than a PV heating element as an additional 200L geyser need to be installed.

  • Install a Domestic hot water heat pump on the existing geyser, and power it from the PV system. An ITS-3.6HD heat pump will use about 4.6kWh to heat the 200L tank and needs about 1kW of inverter capacity. This will leave 4kW of capacity on the inverter for the house, and enough PV power to almost fully cover the rest of the household consumption. During overcast days there will not be enough solar PV energy and you will need Eskom (or a generator). A standard 5kW generator should be enough as the heat pump will only use 1kW and there will still be 4kW available for the rest of the appliances. The cost of heating the geyser in this way will be about R16k excluding installation. The installation costs will be similar to that of a PV heating element.

Conclusion and Additional Comments

A domestic hot water heat pump is hands down the best option. It will provide the greatest saving, quickest return on investment, minimize the roof space required and makes complete off-grid (with the help of a generator) possible.

With the PV and thermal solar options above, the family would also need to adjust their hot water usage habits, preferably using the bulk of the hot water either in the morning or in the evening, to minimize the need for heating outside sunshine hours. In coastal areas like Cape Town, the solar systems will perform poorly during the winter months, resulting in high heating costs if a heat pump is not used.

All of the above systems should be serviced/cleaned at least once a year. The life expectancy of a heat pump is about 10 years while solar PV panels often comes with a 20-year performance guarantee. The electronics and batteries of the PV systems however have a life expectancy that is in line with the heat pump.

About The Author

Riaan Honeyborne has a degree in electronic engineering and worked for some of the world’s leading companies like Nokia specialising in RF design. In 2006, after years of experimenting at home with solar water heating systems, he founded ITS Heat Pumps & Solar. Apart from his role as managing director, he heads up the product design team and oversees all the production plants globally. Riaan is passionate about renewable energy technology and is often seen in the lab working on new ideas and product improvements. His house is still used as a real-life testing facility and he has almost every product ITS offers installed.