The intention of this test was to try and understand the effects of switching the geyser off for a long period every day. The test was conducted on an old Sadia 30gal (136 litre), 3Kw geyser. Hot water was used exclusively to shower twice a day, once in the morning and once in the evening. No other hot water was used during the test period. During the first two days the geyser was not switched off. After that it was switched off at about midday and then switched on again at about 10:30pm at night for two days. The temperature of the water did not seem to be noticeably lower during the evening shower on those days.
The graphs below represent the electricity used on each day:
You can clearly make out the "reheating spikes" where the thermostat switches the heating element on briefly to reheat the water after it has cooled. The longer duration heating events show when cold water entered the geyser to replace that used during a shower and required heating for up to a few hours. It is assumed that the reheating spikes that occur shortly after the water was heated after a shower, are due to mixing of hot and cold water. (We think that water near the element reaches operating temperature and thus the thermostat switches off the heating element. But not all the water in the geyser has reached that temperature yet. So the thermostat has to switch on again when the hot water near the element mixes with cooler water elsewhere in the geyser).
Day1 - Normal Use Summary Figures:
Energy used heating cold water during/after showers - 4.09kWh
Energy used by reheating spikes - 1.65kWh
Number of reheating spikes - 5
Average energy used per reheating spike - 0.33kWh
Day2 - Normal Use Summary Figures:
Energy used heating cold water during/after showers - 4.68kWh
Energy used by reheating spikes - 1.42kWh
Number of reheating spikes - 5
Average energy used per reheating spike - 0.28kWh
Day1 - Simuated Time Switch Summary Figures:
Energy used heating cold water during/after showers - 5.45kWh
Energy used by reheating spikes - 1.02kWh
Number of reheating spikes - 4
Average energy used per reheating spike - 0.26kWh
Day2 - Simuated Time Switch Summary Figures:
Energy used heating cold water during/after showers - 4.84kWh
Energy used by reheating spikes - 1.03kWh
Number of reheating spikes - 3
Average energy used per reheating spike - 0.26kWh
Conclusions that can be drawn from this test:
- For the geyser tested, 0.28kWh is used, on average, during each reheating spike, which occurred roughly every 4-6 hours.
- 5 reheating spikes occur per day when the geyser is not switched off.
- Switching off the gesyer from noon to approx. 10:30pm reduces the number of reheating spikes.
- Because the showers weren't rigorously timed to be exactly the same each day, its not really possible to say for sure whether more energy is consumed on days when the geyser is switched off as opposed to days when it is not.
- At best one can say the only energy saving one can expect from switching off the geyser for an extended period each day is a reduction in the average number of reheating spikes per day (reduced from 5 to 3.5 in this test). Which is thus a maximum saving of 0.42kWh per day. For a saving to occur, no additional energy should be used when the geyser is switched on again, other than that required to heat the cold water that replaced hot water used in any shower whilst the geyser was off. However in this test no such saving was observed, the reverse in fact.
- A more rigorous methodology will be required to conclusively establish whether any savings can be achieved by switching off the geyser each day.
The summary figures for the test are:
| Average energy used per "normal day" | 5.92kWh |
| Average energy used per "timer day" | 6.17kWh |
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