Your apartment does not have an independent geyser, therefore hot water is generated by a central heat pump in the building. This heat pump uses energy to generate this heat and circulate the hot water around the building. The building then recovers this energy costs by charging for that hot water based on how much you use.

The rate being charged is R 0.13 per litre, this rate is cost effective compared to individual geysers. On average a geyser accounts for approximately 40% of the electricity bill in a household. As there is no geyser in the apartment, electricity usage is dramatically reduced within your apartment but is offset against the cost of hot water usage.

Cost comparison between geyser and hot water charge from central heat pump.

Geyser

There are 2 components that make up the cost of the geyser.

Please note that there are certain variables that influence the cost, amount of water, outside temperature, size of geyser, insulation, temperature of geyser and type of geyser.

In this scenario I am going to use a standard 200 l geyser with a normal usage of 100 litres per day.

1: Electricity required without using water.

Geysers, just like kettles, lose heat through their walls. When they are turned off the water inside will slowly cool down to the same temperature as outside the geyser. When a geyser is turned on it automatically switches itself on and off to keep that temperature the same – like a kettle being switched back on once the water gets too cold. Fortunately your geyser has a lot of insulation so that is doesn’t lose heat too quickly and even more luckily for us working out this calculation, modern geyser’s are even required to test how much they lose.

If you have one of these modern geysers it may have an energy category on the side with a letter from A to G. If you’re geyser is rated A it is very efficient and very green, all the way down to G which is very inefficient. These categories depend on a test and little formula from SABS/SARS.

Standing Losses = F + xV0.4

Don’t worry, we’re not going to dive into that… instead I’ve put the results of that calculation in the table at the bottom.

I have a 200L tank that is category E – so my geyser uses 3.0kWh per day, just keeping itself warm – regardless of whether I use ANY hot water. If you don’t have an energy rating on your geyser, it’s probably pretty low down. Very old ones and ones stored outside will be much worse even than these figures – but to be on the safe side, just assume they are category G — e.g. so if I have an old 200L geyser, it probably uses a little more 4.1kwh/per day staying hot. Luckily I’ve got a more efficient one than that and only use 3.0 units/day.

Current rate per kWh is R 3.50. So that would equate to R 10.5 per day, or R 315 per month.

2: Electricity cost related to usage.

The first step is figuring out how much hot water you use. Most of your hot water is probably used when showering or bathing.

Using data from the App, I can work out that I use around 100L of hot water per day, that’s from showering, bathing the kids, washing up etc. My hot water is set to 60 degrees, I can see that on my geyser. That’s all I need to know for this bit. Using a bit of science the amount of energy I use up in the hot water is:

• 100 Litres of water is the same as 100 kilograms of water

• Hot water is 60 degrees, and cold water is around 15 degrees – so I want to heat it up by 45 degrees (60 minus 15)

• The ‘specific heat capacity’ of water is 4.2 KJ/kg/Degree — don’t worry if you don’t understand what this is, but we need it to convert the degrees-C and kilograms of water into how much energy we need (it’s the same for all water everywhere, as long as it’s not turned into steam or ice!)

• The calculations goes: 100 x 45 x 4.2 = 18,900 Kilo Joules of energy

• We normally see energy in our electricity meter and bill as kilo-watt hours, rather than kilo-joules, so we have to divide by 3,600 (the number of seconds in an hour) to convert them — so 18,900 KiloJoules is 5.25 kilowatt-hours (18900 ÷ 3600 = 5.25)

So basically I use 5.25 electricity credits every day heating up water, this would be added to the amount for keeping the water hot while not in use (standing losses shown above).

Combining the numbers

To work out how much your geyser uses you need to just add the two numbers that we’ve worked out:

5.2 kwh/day for actual hot water (down the drain!) + 3.0 kwh/day for keeping it hot (in my airing cupboard!) = 8.2kwh/day

So looking at a month, we need to multiple by the number of days – 8.2 x 30 days = 246 kwh/month

And finally to convert that to Rand, we need to multiply by the cost per unit of electricity. City of Cape Town prepaid tariff is R 3.50 per kWh for the first 600 kWh and then R 4.27 per kWh after the 600 kWh within the month. For sake of example I will use the lower rate.

So finally I take 246kwh/month and multiply by 3.5 to get R 861. I spend R 861 each month on hot water, an average of R 28.70 per day.

Fixed rate per litre from heat pump

As this is a fixed rate based on usage only, there is an easy calculation.

100 litres per day multiplied by R 0.13 = R 13 per day

3000 litres per month = R 390 per month.

This would equate to a saving of R 471 (54%) per month if we I use 100 litres of hot water per day.

If I had to use 200 litres per day, a geyser would cost R 47.25 per day vs R 26 per day on the fixed rate, therefore a R 637.50 (45%) saving per month.