Saving Power at Home & Work

Geyser Blanket Test

The intention of this test was to try and understand the effects of installing a geyser blanket on an ordinary geyser.


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. A data logging electricity meter was installed on the meter to measure its electricity consumption over a period of two weeks.  After the first week, the geyser blanket was installed so that before/after energy consumption could be compared.

Three days of the week, namely; Tuesday, Wednesday and Thursday of successive weeks were compared.   The geyser blanket was installed on the Saturday between the two weeks.  Those particular days were selected to use for comparison purposes because they were completely “normal” days in terms of weekly routine with no additional hot water requirements other than showering.

Unfortunately a cold front arrived in Johannesburg the day after the geyser blanket was installed.  The ambient temperature was thus quite a lot colder in the second week. But the lower ambient temperatures would make the geyser consume slightly more energy in those conditions, so if this test found  savings were achieved, they would be a conservative result (i.e. expected savings under stable conditions would be probably be better than those achieved in this test).

The geyser blanket used was a common product, widely available, consisting of a 50mm layer of fiberglass wool with a layer of white foil on the outer surface.  The kit also included lengths of pipe insulation which were installed on the hot water pipes.


The graphs below represent the electricity used during each week:

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).

Week 1, No Blanket – Summary Figures:

Average daily energy used heating cold water during/after showers       – 4.63kWh
Average daily energy used reheating hot water                                    – 1.38kWh
Average time between heating events                                                 – 164 Min.


Week 2, Blanket Installed – Summary Figures:

Average daily energy used heating cold water during/after showers       – 5.54kWh
Average daily energy used reheating hot water                                    – 1.06kWh
Average time between heating events                                                 – 229 Min.


Conclusions that can be drawn from this test:

  • Due to the lower ambient temperature more hot water was used showering in the second week.
  • Even though the ambient temperature was quite a lot colder, less electricity was used reheating hot water after the geyser blanket was installed (1.06kWh vs 1.38kWh per day). In this case a reduction of 0.32kWh per day was achieved, on average.
  • The average time between heating events increased after the blanket was installed from 164 minutes to 229 minutes, (these figures include heating of cold water after a shower).

The summary figures for the test are:

Reduction in energy used to reheat water  0.32kWh per day
(23% improvement)
Proportional increase in average time between heating events  40%

Leave a Reply

Your email address will not be published. Required fields are marked *

15 + seven =

This site uses Akismet to reduce spam. Learn how your comment data is processed.