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House and pool heating

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. Only after these have been taken care of one can look at something like a PV (solar electric) system. There are a few 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. For more information on this statement.

PV vs Solar Thermal vs Gas vs Heat Pump Water Heating

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.

A Background to the Technologies

Instantaneous gas water heaters use LP Gas to heat water instantaneously and therefore no geyser (hot water storage tank) is needed. 1kg of LPG “contains” 13.6kWh of energy and therefore can produce 11.5kWh of thermal energy if the gas heater has an average efficiency of 85%.

Solar PV water heating element uses solar electric panels (PV panels) to power an electrical heating element. An electrical element is close to 100% efficient. In other words, if it is a 1kW element it will consume 1kW of electricity to produce 1kW of thermal output. This power now comes from a PV panel. PV panels must be installed facing North and at a horizontal angle equal to the Latitude of the location and it is critical that there is no shading on the panel. Shading as little as 2% of the panel surface can result in more than 50% panel output reduction.

The efficiency of the PV panels is around 17% (under lab conditions with the cell at 25°C). In real life with the sun shining on the cell, it will heat up and cause a decrease in output power of about 8% (0.4%/°C with 20°C NOCT). Panel ageing will typically result in a maximum degradation of 2.5% in the first year and up to 10% in 10 years. Next you need a MPPT to ensure optimal power transfer from the panels to the element. A very good MPPT is typically around 95% efficient and therefore you lose another 5% of the available power.