The geyser is the big quiet cylinder in the roof. Why does everyone say it is the one that costs?
The geyser
The geyser is the big quiet cylinder in the roof. Why does everyone say it is the one that costs?
The geyser is a kettle the size of a bath. A common household geyser holds 150 litres and keeps it at about 65 degrees. Heating that tank once from cold, a 45 degree lift, takes 150 times 4.2 times 45 kilojoules, which is 28,000 kilojoules or 7.9 units. At R2.71 a unit that is about R21 for one full heat.
The knowhow
It does not reheat the whole tank every day. It replaces what you draw. Every litre of hot water that leaves through a tap is a litre of cold water coming in at the bottom, and the element heats that litre by 45 degrees. So the geyser's cost is really the cost of the hot water you use, plus a little to make up for heat that leaks through the cylinder overnight.
And so
That is why hot water sits at the top of the bill. Eskom's residential calculator and studies of South African homes put water heating at between a third and a half of a household's electricity. The shower, tomorrow, is where most of it goes.
Keep this
The geyser does not heat a tank, it heats every litre you draw.
Now try it with your own numbers. Find the litres on your geyser's label, or ask whoever installed it. Enter the tank size, the thermostat setting, and your rate. The result is the cost of one full heat from cold, and the units used for every 10 litres of hot water you draw.
Specific heat capacity of water, 4.186 kJ per kg per kelvin. | Eskom Distribution, Residential calculator assumptions page (water heating share of household use). | Lawrence Berkeley National Laboratory, Residential Water Heating Demand Side Management, South Africa, 2024.
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My tries: every time you save, your numbers, what they came to, and your note are kept here so you can compare later.