What size hot water system do I need?

Every family and home is different, so you need to make sure you choose carefully when deciding what size hot water system to install.

Hot water heater sizes start at around 25L and go up to around 400L. In between, there are lots of different options. It’s not simply a case of saying X number litres per family member. There are a number of different important factors. So, check out this comprehensive guide to help you decide what size hot water heater is right for you and your family.

Why the right size is important

There are two main reasons why correctly sizing your hot water heater is important. The first is that, if your hot water heater is too small for your needs, you will continually run out of water. On the other hand, if your hot water system is too large for your needs, you are going to spend an unnecessary amount of energy heating it. This is because your hot water system will keep maintaining the heat of your water, even when you are not using it. Your system has to work harder to keep a larger amount of water hot. So, it’s best to find a tank size that comfortably covers your needs but doesn’t go too far beyond them.

Factors to considerhot water tank

When deciding what size hot water heater is right for your needs, the following factors will need to be taken into account:

  1. How many people will be using the hot water heater?
    • You should obviously count the number of people living in your home full time, but also consider how often you have visitors, such as relatives coming to stay.
  2. How many showers or other outlets will the heater be attached to?
    • If you only have one shower attached to your hot water heater, then the heater may have more time to heat more water for subsequent showers, which means a smaller heater may be suitable.
    • If, however, the heater needs to service a number of bathrooms at the same time, it will need to have a larger amount of water initially on demand.
  3. The water usage habits of your family.
    • You should consider things such as whether your family takes long showers, but also at what times of the day your family uses hot water.
    • For example, if everyone in your family showers in the morning, you may need a larger water tank than if some showered in the morning and some in the evening.
  4. The type of shower heads you use in your bathrooms.
    • A shower head can have a big impact on your hot water consumption.
    • This includes both the size of the head and the pressure at which it passes water.
    • Some shower heads are water-saving, which will significantly reduce the demand on your hot water system.
  5. Gas or electric?
    • Whether you choose a gas or electric system will have an impact on how large the tank will need to be – read on to find out how.


Gas or electric?

Whether your hot water system is powered by electricity or natural gas will make a significant difference to the size of the tank you will need.

Gas hot water systems are able to heat their tanks at a faster rate. This means that, generally, the system can heat new water entering the tank quickly enough to service hot water outlets soon after water has been taken from the tank. An electric system, by contrast, needs to have greater reserves of pre-heated water to service hot water outlets when demand is high.

However, a small gas system will not be able to provide the same number of hot showers if they are all taken at the same time (for example if three or four bathrooms are used at once).

Another thing to consider is that gas systems are generally cheaper to run than electric systems. You might therefore be able to save yourself some money by buying a smaller (and therefore cheaper) gas system that is also kinder to your energy bill over the longer term. However, you also need to check that your home has an available gas supply or can be connected to the natural gas grid.


What size hot water system do I need?

The following tables will give you a rough idea of how to calculate the right sized tank for you and your family. Bear in mind that the ‘peak usage’ is the rate at which water is consumed in your home at those times when hot water is most in demand (i.e. in the mornings, when the whole family showers before going to work or school).

The peak usage takes into consideration the factors noted above, such as the number of showers attached to the system, the type of shower heads used, and whether the whole family uses the hot water system within a short period of time. For example, if your whole family showers in the morning using three different showers, your peak usage will be high.

Electric hot water system

Number of people Peak usage Tank size
5 – 8 High 400
Low 250
3 – 6 High 315
Low 160
2 – 4 High 160
Low 125
1 – 3 High 125
Low 80


Gas or LPG hot water system

Number of people Peak usage Tank size
5 – 8 High 340
Low 170
3 – 6 High 260
Low 160
2 – 4 High 160
Low 135
1 – 3 High 135
Low 90

Dux hot water systems are available through your local, authorised agents here at Australian Hot Water

How to make the right decision

To make the right choice regarding your new hot water system, you should take the time to analyse your family’s current water usage habits. Make sure you do it over a large enough period to get accurate data – for example 2 weeks of regular work and school hours.

You can note how many showers are taken in which periods of the day, as well as how long these showers generally last. Note any times that the hot water runs out and correlate this with the number of showers and shower lengths.

When you gather this data, make a note of the type of shower heads you are using, the size of your current tank, as well as the average cost of your energy bill over a two week period.

With this information you should be able to come to a well-grounded decision about what size hot water system is best going to suit your needs and save you money in terms of efficiency.