Hot water is a crucial part of Australian life, from showers and baths to dishwashers and washing machines. But how do you know what hot water system size is best for your needs?

With so many hot water options on the market, you would be excused if you find it all a little overwhelming. Keep reading to find out more and if you would like further help, give Ryan Old Plumbing a call on 0488 856 880.

What are the different hot water systems available?

There are two different types of hot water systems and four different types of hot water service (confusing we know)

Hot water system types

  • Storage hot water system: also called tank systems.
  • Instantaneous hot water system: otherwise known as instant, tankless or continuous flow hot water systems.

Hot water service

What are the daily hot water demands for homes?

The average daily hot water needs of different households according to Energy Rating include:

Number of people Daily hot water demand (in Litres) Approximate size of storage tank needed (in Litres)
1-2 120 160 – 250
3-4 200 250 – 330
5-6+ 300 400+

What factors do I need to consider?

There are a range of factors to consider when looking at a hot water system and the size you need:

  • Would you like storage or an instant hot water heater?
  • How much space do you have to house a hot water system?
  • Do you have a preferred hot water brand?
  • How many people will be using the hot water heater?

One person generally uses about 50L of hot water a day.

  • When will everyone be using hot water? Will it all be at the same time, or at different times of the day?
  • How many showers are completed a day? When do they occur? E.g. morning, night, both?
  • How many hot water outlets do you have?
  • Do you wash your clothes in hot or cold water?
  • Do you have a dishwasher? How often do you use it?

How do you size a hot water system?

There are a few things to sit and look at when sizing a hot water system, including:

  • Storage hot water heaters: These hot water units are sized according to how many people are in your household. Remember, the more people that live in your house, the larger the storage hot water system will need to be.
  • Solar hot water systems: Just like storage hot water heaters, and sized in accordance with how many people like at your house.
  • Instant hot water units: These hot water systems are sized in accordance with how many hot water outlets you have in your home. The more outlets you have, the bigger the system you need.

water heating unit sizes

There are a few other points you should consider, such as:

  • If you choose a hot water system that is too big, you will be paying for water that is being heated and stored and you aren’t using it.
  • If you choose a hot water heater that is too small, you will run out of hot water regularly.
  • If you decide on an instant hot water system, and it is too small, you will not be able to get hot water from multiple hot water taps at once.

What size hot water system do I need?

The following is a guide only – we always recommend speaking to a qualified hot water professional such as Ryan Old Plumbing for further information.

Electric storage hot water systems

Off-peak Peak rate
Number of people Capacity (Litres) Number of people Capacity (Litres)
1-3 160 1 25
2-4 250 1-2 50
3-6 315 2-3 80
5-8 400 3-5 125

Natural gas and LPG hot water systems

Storage Continuous flow
Number of people Capacity (Litres) No. of outlets at one time Flow rate (Litres/minute)
1-3 90 1 16
2-4 130 2 20
3-5 170 2-3 24
4-6 200 3+ 32
5-9 260

Solar hot water systems

Number of people Hot water required

(Litres per day)

Approximate tank size Collector area (m2)
1-2 120 180 2
3-4 200 300 4
5-6 300 440 6

Information contained in the above three tables courtesy of Sustainable Energy Authority Victoria.

Heat pump hot water systems

Less than 200L (Small) 200 – 300L (Medium) 300L+ (Large)
Small home or apartment Average-sized home Large home
1-2 bedrooms, 1 bathroom 3 bedrooms, 1 bathroom 4+ bedrooms, 2+ bathrooms
Maximum of 2 people Maximum of 3 people 4+ people
Not generally suited to off-peak tariffs May not be suited to off-peak tariffs, some can run on restricted peak tariffs though Restricted hours and off-peak tariff are possible.

Information for the above table courtesy of Energy Rating.

Suggested hot water system options

In general, a solar hot water system is the most efficient option currently available on the market and the cheapest to run. If this is not an available option to your household though, a few other choices include:

  • Small household: 1-2 people. Small gas storage hot water system OR a gas or electric instant hot water system.
  • Medium household: 3-4 people. Heat pump hot water system OR continuous flow or storage gas hot water heater.
  • Large household: More than 5 people. Large heat pump hot water system, gas storage hot water heater or multiple continuous flow hot water systems (not that economical).

A family of four

For reference, if you are a family of 4 with 2 adults and 2 children, you would approximately need:

  • Electric hot water system: A 250 – 315L hot water storage tank for off-peak or a 125 – 160L tank for an instant system.
  • Gas hot water system: A storage tank of around 135 – 170L, however, there are alternative instantaneous options available too.
  • Solar hot water system: Around 4mᒾ of solar collector area and a 300 – 360L tank.
  • Heat pump hot water system: A 270 – 315L tank.

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