Cheapest way to heat bath/shower water

CapeTalk's Lester Kiewit speaks to Professor Thinus Booysen of the Faculty of Engineering at Stellenbosch University.

Listen below:

What's the most economical way to heat your geyser or shower water?

If you're a two-person household, it might not make sense to heat a 150-litre geyser and watch those electricity units deplete. 

It takes approximately 10 units of electricity to heat 150 litres to 70 degrees versus six units of electricity to heat it to 50 degrees. Typically, you'll need to keep it above 50 degrees to avoid legionella growth which is bacteria that grows in water, explains Professor Booysen.

One issue with a tanked water heater is that it loses energy throughout the day.

For example, if you run a geyser at 70 degrees, it loses approximately two units of energy.

What that means is the efficiency of heating is lower than 100%. So, if you use 10 units of energy for hot water, your heating is about 75% efficient. 

Boosen says LPG tends to be unsafe and is something you don't want inside your home.

Electric heaters are efficient but it puts a high strain on the grid, he says.

LPG costs come down to about R3,50 per useful kilowatt while electricity costs are complicated and range substantially because it comes down to tariffs and how much water is used:

• Low-income tariffs are about R2,40 to R2,50 Rand per kilowatt hour.
• General domestic tariffs are about R3.90 per kilowatt hour.
• Anything above 600 kilowatt hours per month is about R4.75 per kilowatt hour.
• If you're heating electricity with a water heater, costs can range from R2.50 up to R10 depending on how it's used and the geyser's temperature setting.

Two things to consider when using a geyser:

1. Mixer tap: If you have a mixer tap, you're using hot water every time you open the tap which can lose about five litres of hot water per minute.
2. Pipes: For every 10 metres of pipe between the tap and geyser, you're losing three litres of hot water every time you use it, so there's a big loss regardless of whether you're using gas or electricity. 

Alternative heating options:

Other than electric, gas and LPG options, you can try: 

• Heat pump: the price is a third of what you would spend on a 150-litre geyser in terms of running costs which can cost between R10 to R30 per day versus the geyser which can cost between R25 to R100 per day. LPG can typically cost about R35 per day.
• Direct solar heating without an inverter: this will set you back R15,000 to R20,000 and come down to R10 to R30 per day.
• Inverter-based solar heating: this allows you to use excess energy and divert it into a water heater which can typically cost around R30 to R40 a day to heat up and keep running. 

Geyser timers work.

According to Booysen, you can save as much as 28% of energy using a geyser timer, if it's used correctly. 

To use a geysery timer correctly, simply switch off the water heater before drawing water. This allows the water to run at a colder temperature saving energy and units.

Your piping might cost you.

If the piping around your geyser is done incorrectly, you can lose up to three to four units of energy per day.

Scroll up to the audio player to listen to the full conversation.