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What energy savings can a solar water heater achieve?
Solar water heaters generate heat through the solar collector with the heat transferred through the solar collector and stored in the water vessel or tank.
This solar thermal process is substituting the electricity used in heating water through an electrical resistance element. The performance of a solar water heater is governed by the South African National Standards. SANS 6211 measures the energy collected and stored during a day, using 6 representative sample days of different solar irradiation. This represents a sample performance of the typical weather in South Africa over a period of a year.
SANS 6211 results in a ‘Q’ test result. Dividing the ‘Q’ by 3.6 results in the deemed saving equivalent in kWh’s as calculated at 20MJ irradiation, which is the average across South Africa. A maximum performance is stipulated as 10MJ per 50 liters of water per day under SANS 1307, and a minimum performance of 5MJ per 50 liters.
The minimum and maximum Q factors allowed are:
Min Output – “Q”
Max Output – “Q”
Min Output – ‘kWhs’
Max Output – ‘kWhs’
The ‘Q’ factor is therefore extremely important indicator of the performance of the solar water heating system chosen. Much in the same way as kilometers per liter of diesel or petrol it provides a measure of performance, but it is not the only criteria for choosing a SWH system. With the purpose of a Solar Water Heater being to heat hot water and save electricity, the efficiency of the solar water heater is an important factor in choosing the SWH system. In reality no SWH system can be 100% efficient due to weather, but some systems come close.
To calculate the efficiency of a system, take the Q test figure at 20MJ and divide by the Q factor for 100% theoretical efficiency. For example:
Size of System in (litres)
Electrical Consumption from Cold to Hot (kWh)
Q factor required for 100% Efficiency
Q Factor on SWH system (illustrative)
Efficiency Rating in %
Manufacturers should be able to produce ‘Q’ test results according to SANS 6211 for their products.