Getting into hot water

Continuing the ‘what’s it cost’ theme, I thought I’d write a quick post about how much hot water costs…

It turns out a normal kitchen sink, at a normal hot-water temperature costs almost exactly One Rand (in 2014). Here’s how we get to that…

Just multiply the LITRES of water by the TEMPERATURE that it’s increased by (in Celsius) and then divide that number by 860*. Cold water is normally around 10 degrees – so just take away 10 from the temperature of your water to get the temperature change. This gives you the number of kilowatt-hours (or energy units on your prepaid meter) – so now you just multiply that by the amount you pay for each unit. For most people this will be somewhere between R1.00 and R1.50

A sink is normally around 10 litres – if you don’t believe me try it with a measuring jug sometime!
Normal hot water is 65 degrees – whereas cold water from the tap is about 10 degrees, so the temperature INCREASE is 55 degrees

10 x 55 = 550 litre-degrees
550 / 860 = 0.639 kwh

Eskom Homelight users pay R1.62 per kilowatt hour for an extra unit
0.639 x 1.62 = 1.03c

That’s pretty close to a Rand if you ask me 🙂

* – for any geeks out there, that’s because we’re multiplying by the specific heat capacity of water (around 4200 Joule per kilogram-degree, for liquid water, most of the time… don’t ask!), then dividing by 3,600,000 – the number of Joules in a kilowatt hour; which is basically the number of seconds in an hour times 1000.


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