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The hot water system (geyser) in most homes is the single largest consumer of electricity and is typically responsible for around 50% of the total electric bill. Clients often ask how they can reduce their electricity bill or even get away from Eskom by going off-grid and the first step is to make the hot water system energy efficient, change to energy efficient lighting and maybe get a gas top stove.
There are a number of different technologies that are commonly used to make the hot water system more energy efficient but for most households it will either be a solar water heating or a heat pump water heating system that will work the best and provide the quickest return on investment.
The cost of electricity, stability of the national grid and the awareness to sustainable living have forced many consumers to turn to renewable energy technology for heating water. The national building regulations also require that at least 50% of water heating be done with something other than an electrical element. As with many products on the market, one can find a considerable number of contradicting claims with regards to the different technologies. Many of these claims are nothing more than marketing jabber and opinions and have no facts behind them.
Domestic hot water heat pumps use a small amount of electricity to extract a lot of energy from the surrounding air. A heat pump also uses the energy from the sun, although only indirectly, allowing the unit to work day and night, winter and summer. The efficiency of a heat pump is called COP (coefficient of performance). A COP value of 4 means that the heat pump produces four times the amount of thermal energy as it uses electrically – in other words a 75% saving on the water heating bill.
The COP of a heat pump is dependent on the ambient temperature and the water temperature and so, in a practical domestic hot water system using a well-designed heat pump, a realistic annual COP value is 3 – in other words a 66% saving on the water heating bill.
A family of 4 that use water moderately uses about 200L of hot water per day. To heat 200L of water from 15°C to 60°C requires 10.4kWh. The standing loss of a B-rated 200L tank is 1.6kWh/day in the lab but with installation piping included it is typically around 3.6kWh/day. Therefore the total energy requirement for this family’s hot water is 14kWh/day. If they are paying R2/kWh for electricity their water heating is costing them R840/month. Assuming that water heating typically makes up 50% of the total electrical bill we are looking here at a family that spends about R1680 per month on electricity. Let’s now look at what the different water heating technologies can do for them.
A domestic hot water heat pump working at average South-African ambient temperatures will conservatively provide an annual average saving of 66% on whatever system it is connected to. The monthly Eskom bill for keeping this system at 60°C will therefore be R280. Assuming a 10% annual electrical tariff increase they will save R41609 over the next 5 years on their water heating bill.
The life expectancy of a heat pump is estimated at 10 years but we know of systems that are running for more than 20 years and still going strong. Solar thermal and solar PV heating systems are often stated to have a life expectancy of 20 years, keeping in mind that it really is only the panels that is expected to last that long. All PV heating systems and some solar water heating system use a controller, 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 life expectancy “winner” if one of course ignores the hot water storage tank needed for all except the instant gas heating system.
A domestic hot water heat pump will provide the best saving in this example. An ITS 4.7kW heat pump installation will cost you about R5k more than a 2.4m2 flat plate installation, but will result in a R9k better saving after 5 years.
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, so a heat pump would almost always be a better solution in areas like Cape Town.
Get in touch with us today for a price on a hot water heat pump for your home.
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