What Size Hot Water System Do I Need? Australian Sizing Guide

Jun 25, 2026

Choosing the right size hot water system matters just as much as choosing the right brand or energy type. A system that is too small can run out during peak use, while a system that is too large can cost more upfront, take up extra space and store more hot water than the household needs.

The right size depends on the number of people in the home, number of bathrooms, shower habits, bath use, laundry use, dishwasher use, peak demand, energy source, tank recovery, flow rate, installation location and whether the home uses electric, gas, instant or heat pump hot water.

This guide explains how to size electric storage, gas storage, instant gas, instant electric and heat pump hot water systems before ordering.

You can compare Hot Water Systems, Electric Hot Water Systems, Instant Electric Systems, Gas Hot Water Systems, Instant Gas Hot Water Systems, Heat Pump Hot Water Systems and Installation Add-Ons online with Installed Today.

Quick answer: what size hot water system do I need?

As a general starting point:

Household	Common sizing direction
1 person	Smaller electric storage, compact option or point-of-use system where suitable
2 people	125L to 160L electric storage often compared, or smaller heat pump where suitable
3 people	160L to 250L electric storage, suitable heat pump or gas option
4 people	250L electric storage, family-sized heat pump or correctly sized gas system often compared
5 people	315L electric storage, larger heat pump or carefully sized instant gas
6+ people	Larger storage, larger heat pump or high-flow instant gas where the site supports it
Two bathrooms	Tank recovery or instant flow rate becomes more important
Homes with baths	Stored capacity matters
Airbnb or short-stay	Size for maximum guest use, not average use

This is only a guide. The right system depends on real usage, not just headcount.

Why hot water sizing matters

Hot water sizing affects comfort, running costs, installation cost and long-term performance.

If a system is too small, you may experience:

Hot water running out
Poor recovery after showers
Boost heating being used more often
Complaints from tenants or guests
Frustration during morning and evening peaks
Poor performance after adding bathrooms or family members

If a system is too large, you may experience:

Higher upfront product cost
Extra space requirements
More stored hot water than needed
Higher standing heat loss in some storage systems
More difficult access or installation
Unnecessary system capacity

The best size is the one that matches your household’s real peak demand.

Sizing by number of people is only a starting point

Many buyers start with household size, but this is not enough on its own.

Two homes with four people can use very different amounts of hot water.

For example, one four-person household may have short showers, no bath and low laundry use. Another may have teenagers, long showers, a bath, guests and high morning demand.

Sizing should consider:

Number of people
Number of bathrooms
Shower length
Shower head flow rate
Bath use
Laundry use
Dishwasher use
Morning peak demand
Evening peak demand
Guest use
Rental or short-stay occupancy
Tank recovery rate
Energy source
Timer or tariff setup
Rooftop solar timing
Installation location

The more hot water used at the same time, the more careful sizing becomes.

Storage size vs flow rate

Hot water systems are sized in different ways depending on the type.

Storage systems are usually sized by tank capacity in litres.

This includes:

Electric storage
Gas storage
Heat pump hot water
Solar storage systems

Instant systems are usually sized by flow rate in litres per minute.

This includes:

Instant gas hot water
Instant electric hot water

A 250L electric storage system and a 26L instant gas system are not directly comparable because they deliver hot water differently.

Electric storage hot water sizing

Electric Hot Water Systems are commonly sized by tank capacity.

Common electric storage sizes include:

Small tanks for low-demand homes
125L systems
160L systems
250L systems
315L systems
400L systems

Electric storage sizing depends on how much hot water needs to be stored and how quickly the system can recover after use.

125L electric hot water systems

A 125L electric hot water system may suit:

One to two people
Small units
Townhouses
Low-demand homes
Some rental properties
Properties with limited space
Homes with moderate shower use

A 125L system may not suit:

Larger families
Long showers
Homes with baths
Multiple bathrooms
High guest use
Homes that previously ran out of hot water

If your old 125L system was not keeping up, replacing it with another 125L tank may repeat the problem.

160L electric hot water systems

A 160L electric hot water system may suit:

Two-person homes
Small families with moderate use
Units or townhouses
Homes needing more capacity than 125L
Low-to-moderate shower demand
Simple electric storage replacement

Before choosing 160L, check:

Shower length
Bath use
Guest use
Whether the system runs on off-peak
Whether hot water ran out previously
Whether the household may grow

A 160L system can be practical, but it is not always enough for high-demand homes.

250L electric hot water systems

A 250L electric hot water system is commonly compared for many family homes.

It may suit:

3 to 4 person households
One or two bathroom homes
Moderate hot water use
Rentals
Standard family replacements
Homes replacing an existing 250L tank that performed well

Before choosing 250L, check whether the old system ran out regularly. If it did, the household may need a larger tank, a different tariff setup, a heat pump option or a system with better recovery.

315L electric hot water systems

A 315L electric hot water system may suit:

Larger families
Higher-demand homes
Homes with multiple bathrooms
Back-to-back showers
Homes with baths
Some off-peak electric setups
Larger rental properties

A 315L system needs more space than smaller tanks and should be checked for access, base support, electrical setup and installation location.

400L electric hot water systems

A 400L electric hot water system may suit:

Large households
High hot water demand
Multiple bathrooms
Larger rental properties
Homes with frequent guests
Situations where a large stored volume is needed

Before choosing 400L, confirm:

Space
Access
Base support
Electrical supply
Recovery requirements
Whether heat pump hot water may be a better long-term option
Whether the household actually needs that much stored water

Bigger is not automatically better. Oversizing can add cost and complexity.

Heat pump hot water sizing

Heat Pump Hot Water Systems are also storage systems, but they heat water differently from standard electric tanks.

Heat pump sizing depends on:

Tank capacity
Recovery rate
Outdoor air temperature
Airflow
Timer settings
Boost mode
Rooftop solar timing
Household demand
Shower habits
Bath use
Number of bathrooms
Installation location

A heat pump that is too small may struggle during peak use. A heat pump that is too large may cost more and take up more room than needed.

Smaller heat pump systems

A smaller heat pump may suit:

1 to 2 person homes
Low-demand households
Units or townhouses where site suitability is confirmed
Homes with moderate shower use
Customers replacing smaller electric tanks
Properties with suitable airflow and drainage

Before choosing a smaller heat pump, check recovery time, boost settings and whether guest use or future household changes may increase demand.

250L heat pump systems

A 250L heat pump may suit:

2 to 4 person homes
Moderate family use
Homes replacing 250L electric storage
Properties with rooftop solar timing
Rentals with moderate occupancy
Households wanting efficient electric hot water

Check outdoor airflow, condensate drainage, electrical setup, noise placement and service access before ordering.

270L heat pump systems

A 270L heat pump is commonly compared by family homes.

It may suit:

3 to 4 person households
Moderate to higher hot water use
Families replacing electric storage
Homes with rooftop solar
Customers wanting efficient electric hot water
Properties with suitable outdoor installation space

Before choosing 270L, check peak demand, recovery, timer setup, noise placement and access.

315L heat pump systems

A 315L heat pump may suit:

Larger families
Homes with multiple bathrooms
Higher shower demand
Short-stay properties
Rentals with higher likely occupancy
Homes with bath use

A larger heat pump still needs suitable airflow, condensate drainage, electrical setup and space. Do not size by litres alone.

Gas storage hot water sizing

Gas Hot Water Systems may include gas storage systems that heat and store water in a tank.

Gas storage sizing depends on:

Tank capacity
Gas recovery rate
Number of people
Number of bathrooms
Shower habits
Bath use
Natural gas or LPG
Existing gas supply
Location
Valve requirements

Gas storage can suit homes already using gas storage, especially where the old system performed well and household demand has not changed.

When to stay with the same gas storage size

Staying with the same gas storage size may make sense when:

The old system provided enough hot water
Household size has not increased
Shower habits have not changed
The home still has the same number of bathrooms
The location still suits a storage tank
Natural gas or LPG supply is suitable
The customer wants a simple replacement

If the old system ran out often, reassess size before replacing like-for-like.

Instant gas hot water sizing

Instant Gas Hot Water Systems are sized by flow rate in litres per minute.

Common instant gas sizes include:

16L class
20L class
26L class

The right instant gas size depends on:

Number of bathrooms
Number of showers
Whether showers run at the same time
Shower head flow rates
Gas pipe sizing
Water pressure
Natural gas or LPG
Distance from the unit to outlets
Controller requirements
Household expectations

A larger L/min rating does not guarantee better performance if gas pipe sizing or water pressure is not suitable.

16L instant gas systems

A 16L instant gas system may suit:

Smaller homes
One-bathroom homes
Low-to-moderate demand
One shower at a time
Some units or townhouses where gas setup suits
Customers replacing a similar existing unit

A 16L system may not suit homes wanting multiple showers at once.

20L instant gas systems

A 20L instant gas system may suit:

Small to medium households
One to two bathroom homes with moderate use
Homes replacing an existing 20L unit
Properties with suitable gas pipe sizing
Homes wanting continuous flow gas hot water

Before choosing 20L, check gas type, gas pipe sizing, water pressure and shower habits.

26L instant gas systems

A 26L instant gas system may suit:

Larger homes
Two-bathroom homes
Higher hot water demand
Families wanting stronger continuous flow capacity
Homes replacing an existing 26L unit
Properties with suitable gas pipe sizing and water pressure

A 26L system should be confirmed by a licensed gas fitter because gas supply and water pressure matter.

Instant electric hot water sizing

Instant Electric Systems are usually sized by flow rate, application and electrical capacity.

Instant electric may suit:

Hand basins
Small sinks
Kitchenettes
Point-of-use applications
Remote fixtures
Compact spaces
Low-demand areas

Instant electric may not suit whole-home hot water unless electrical capacity, flow rate and temperature rise are carefully assessed.

Point-of-use sizing

Point-of-use systems should be sized around the exact fixture.

Check:

Fixture type
Flow rate
Desired temperature
Incoming water temperature
Distance from power supply
Electrical capacity
Water pressure
Usage frequency
Access
Compliance requirements

A hand basin and a shower have very different hot water needs.

Whole-home instant electric sizing

Whole-home instant electric is more demanding.

Before choosing it, confirm:

Number of bathrooms
Shower flow rates
Whether multiple outlets run at once
Temperature rise
Electrical capacity
Switchboard capacity
Circuit requirements
Water pressure
Installer advice

For many whole-home applications, electric storage or heat pump hot water may be more practical.

Sizing for 1 person

A one-person home may suit:

Smaller electric storage
A compact electric option
A smaller heat pump where site suitability is confirmed
Point-of-use instant electric for selected fixtures
Gas or instant gas only where the property already has suitable gas

Do not oversize automatically. If usage is low, a large storage system may be unnecessary.

Sizing for 2 people

A two-person household may compare:

125L electric storage
160L electric storage
Smaller or medium heat pump
Gas storage where already installed
Instant gas where gas supply suits
Instant electric for selected point-of-use needs

Check shower length, guest use, bath use and off-peak setup before choosing.

Sizing for 3 people

A three-person household may compare:

160L electric storage for moderate use
250L electric storage for more comfortable storage
Medium heat pump hot water
Gas storage where the home already uses gas
20L instant gas where gas supply and water pressure suit

Sizing depends heavily on showers and bathrooms.

Sizing for 4 people

A four-person household may compare:

250L electric storage
Family-sized heat pump hot water
Gas storage where already using gas
20L to 26L instant gas where gas supply supports it

Before ordering, check whether two showers are used at once, whether there is a bath, and whether hot water runs out during peak times.

Sizing for 5 people

A five-person household may need:

315L electric storage
Larger heat pump hot water
Larger gas storage where suitable
26L instant gas where gas pipe sizing and water pressure support it

Five-person homes can vary widely. Long showers, teenagers, baths and laundry use can change the best size.

Sizing for 6 or more people

Large households should be assessed carefully.

Options may include:

Larger electric storage
Larger heat pump hot water
Gas storage with suitable recovery
Correctly sized instant gas
Custom hot water design where demand is very high

Check:

Number of showers per day
Whether showers overlap
Bath use
High-flow shower heads
Guest use
Laundry demand
Recovery rate
Gas pipe sizing
Electrical supply
Site suitability

Do not undersize a large household to save upfront cost.

Sizing for one bathroom

One-bathroom homes may have simpler hot water needs, but usage still matters.

Consider:

Number of people
Shower length
Bath use
Whether the bathroom has a high-flow shower
Whether the kitchen and laundry also use hot water
Whether guests stay often
Existing system performance

A smaller system may suit one bathroom if demand is low, but not if multiple people shower back-to-back.

Sizing for two bathrooms

Two-bathroom homes need more careful sizing.

Check:

Whether two showers run at once
Shower head flow rates
Bath use
Household size
Morning peak demand
Evening peak demand
Tank recovery
Instant gas flow rate
Gas pipe sizing
Water pressure

A two-bathroom home does not always need the largest system, but simultaneous use must be considered.

Sizing for homes with baths

Baths can use a lot of stored hot water.

If the home has a bath, check:

Bath size
How often it is used
Whether showers are used after the bath
Tank capacity
Recovery time
Heat pump boost settings
Whether the old system ran out

Homes with baths often need more careful storage sizing than homes with showers only.

Sizing for high-flow shower heads

High-flow shower heads increase hot water demand.

They can affect:

Tank run-out time
Instant gas flow requirements
Instant electric suitability
Heat pump recovery
Running costs
Peak demand

Before upsizing the hot water system, consider whether shower flow rate is part of the issue.

Sizing for off-peak electric hot water

Off-peak or controlled-load electric hot water can require more stored capacity because the system may only heat during certain times.

Before choosing a size, check:

Heating window
Household usage pattern
Tank capacity
Boost heating availability
Whether hot water runs out before reheating
Tariff setup
Installer advice

A tank that works on continuous supply may not suit the same home on restricted heating times.

Sizing for rooftop solar homes

Rooftop solar can work well with electric or heat pump hot water where timing is set up properly.

Before choosing a size, check:

Solar system size
Daytime household usage
Timer settings
Tank capacity
Recovery rate
Boost settings
Controlled-load setup
Electrical configuration
Installer advice

Do not assume solar pairing happens automatically. Setup matters.

Sizing for rentals

Rental properties should be sized for likely occupancy, not just the current tenant.

Check:

Number of bedrooms
Number of bathrooms
Likely tenant count
Past hot water complaints
Maintenance access
System simplicity
Energy source
Replacement cost
Warranty terms

A system that is too small can lead to tenant issues.

Sizing for Airbnb and short-stay homes

Short-stay homes should be sized for maximum guest use.

Check:

Maximum guest count
Number of showers
Bath use
Back-to-back showers
Laundry between bookings
Recovery rate
Guest simplicity
Maintenance access
Noise placement for heat pumps
Reliability

Short-stay guests may use more hot water than the owner expects.

Sizing for renovations

A renovation is the right time to size hot water properly.

Before choosing, confirm:

Final number of bathrooms
Shower locations
Bath use
Kitchen and laundry locations
System location
Pipe runs
Energy source
Electrical capacity
Gas pipework
Heat pump airflow
Drainage
Future servicing access

Do not leave hot water selection until after plumbing and electrical layouts are locked in.

Sizing for units and townhouses

Units and townhouses often have space, access and compliance constraints.

Check:

Existing system type
Cupboard space
Indoor drainage
Body corporate rules
Electrical supply
Gas availability
Noise restrictions
Access for removal
Replacement dimensions
Whether heat pump airflow is suitable

In some units, a like-for-like electric storage replacement may be the most practical option.

Sizing and brand choice

Brand matters, but sizing and system type matter more.

Installed Today’s homepage-linked hot water brands include:

Compare brands by tank size, flow rate, dimensions, warranty, energy source and installation requirements.

Rheem sizing considerations

Rheem Hot Water Systems are commonly compared across electric, gas and heat pump categories.

When sizing Rheem systems, check:

System type
Tank capacity or flow rate
Household size
Number of bathrooms
Energy source
Gas type where relevant
Installation location
Warranty terms

Choose the Rheem system that suits the household, not just the most familiar model.

Rinnai sizing considerations

Rinnai Hot Water Systems are commonly compared across instant gas, electric and heat pump options.

When sizing Rinnai systems, check:

Instant gas flow rate where relevant
Gas type
Gas pipe sizing
Water pressure
Tank capacity where applicable
Household demand
Installation location

Rinnai instant gas should be sized around real simultaneous use and site suitability.

Dux sizing considerations

Dux Hot Water Systems are commonly compared for electric storage, gas and broader hot water replacement.

When sizing Dux systems, check:

Tank capacity
Household use
Bathrooms
Existing system size
Whether the old system ran out
Dimensions
Installation location

A Dux replacement should be chosen around demand, not just the size of the old tank.

Vulcan sizing considerations

Vulcan Hot Water Systems may suit customers comparing value-focused storage hot water options.

When sizing Vulcan systems, check:

Tank capacity
Household size
Bathrooms
Shower demand
Installation location
Warranty terms
Total replacement pathway

A lower upfront price should still be balanced against correct sizing.

Stiebel Eltron sizing considerations

Stiebel Eltron Hot Water Systems are commonly compared for compact electric, instant electric and premium electric options.

When sizing Stiebel Eltron systems, check:

Point-of-use or whole-home application
Flow rate
Temperature rise
Electrical capacity
Switchboard suitability
Fixture type
Water pressure
Installation space

Instant electric sizing depends heavily on the specific application.

iStore sizing considerations

iStore Hot Water Systems are commonly compared by customers wanting heat pump hot water.

When sizing iStore systems, check:

Tank capacity
Household demand
Rooftop solar timing
Timer setup
Boost mode
Outdoor airflow
Condensate drainage
Noise placement
Installation access

iStore may suit electric storage upgrades where heat pump site requirements are met.

Enviroheat sizing considerations

Enviroheat Hot Water Systems may suit customers comparing heat pump options.

When sizing Enviroheat systems, check:

Tank capacity
Household use
Outdoor airflow
Condensate drainage
Electrical setup
Timer settings
Recovery expectations
Access

The site still needs to suit the heat pump, not just the tank size.

Aquatech / Hydrotherm sizing considerations

Aquatech / Hydrotherm Hot Water Systems are worth comparing as alternative heat pump options.

When sizing Aquatech or Hydrotherm systems, check:

Tank size
Recovery
Household demand
Airflow
Drainage
Electrical setup
Noise placement
Installer advice

Confirm suitability before ordering supply only.

Bosch sizing considerations

Bosch Hot Water Systems are commonly compared for gas hot water and continuous flow systems.

When sizing Bosch systems, check:

Flow rate
Gas type
Gas pipe sizing
Water pressure
Ignition requirements
Household demand
Installation location

Bosch gas options should be matched carefully to the property’s natural gas or LPG setup.

Supply-only sizing checks

Supply-only ordering can suit customers who already have their own licensed installer arranged.

Before ordering supply only, confirm:

Correct system type
Correct size
Correct gas type where relevant
Suitable electrical supply
Suitable gas pipe sizing where relevant
Suitable water pressure
Suitable installation location
Suitable drainage
Correct dimensions
Access for delivery and installation
Whether old unit removal is arranged
Whether disposal is arranged
Whether valves or add-ons are needed

Supply-only product pricing generally does not include installation, valves, old unit removal, disposal, electrical work, gas work, pipework changes, drainage, access upgrades or compliance upgrades unless clearly selected or included.

Supplied-and-installed sizing checks

Supplied-and-installed options may be available on selected products in eligible Brisbane and Gold Coast service areas where shown.

Before ordering supplied and installed, check:

Service area eligibility
Existing system type
Current system size
Household demand
Product suitability
Installation inclusions
Valve requirements
Gas work
Electrical work
Pipework changes
Drainage
Old unit removal
Disposal
Access
Compliance upgrades

Do not assume every site requirement is included. Check the selected product and installation options carefully.

Installation Add-Ons and sizing

Installation Add-Ons may be relevant where a hot water replacement needs extra selected work or components.

Add-ons may relate to:

Valves
Disposal
Electrical work
Gas work
Pipework changes
Drainage requirements
Access issues
Other installation-related needs where available

The right size hot water system still needs the right installation pathway.

Common mistake: choosing the same size automatically

Replacing the old system with the same size can be fine if the old system worked well.

But it may be wrong if:

The old system ran out
Household size increased
A new bathroom was added
Shower habits changed
A bath was added
The home became a rental or Airbnb
The old system was poorly sized from the start

Always reassess before replacing like-for-like.

Common mistake: choosing the biggest system available

Bigger is not always better.

Oversizing can increase:

Product cost
Space requirements
Installation complexity
Heat loss in some storage systems
Difficulty with access
Unnecessary stored hot water

Size the system around demand, not fear.

Common mistake: ignoring recovery rate

Tank size is not the only factor.

Recovery rate affects how quickly the system reheats after use.

A smaller system with good recovery may suit some homes, while a larger tank may be needed where reheating windows are limited.

Common mistake: ignoring flow rate

For instant gas and instant electric systems, flow rate matters.

A unit must be able to supply enough hot water for the intended outlets, and the property must have suitable gas, electrical and water supply to support it.

Common mistake: ignoring gas pipe sizing

Instant gas sizing is not just about choosing 16L, 20L or 26L.

Gas pipe sizing, gas type and water pressure all affect performance.

Common mistake: ignoring electrical capacity

Instant electric and heat pump systems need suitable electrical setup.

Switchboard capacity, circuits, timers and isolators may need checking.

Common mistake: ignoring heat pump location

Heat pump sizing and performance depend on location.

Airflow, condensate drainage, noise placement and access all matter.

Common mistake: sizing for average use instead of peak use

Hot water systems need to handle peak demand.

Think about:

Morning showers
Evening showers
Bath nights
Guest stays
Laundry days
Short-stay turnover days
Multiple showers running at once

Sizing for average use can leave the home short during real peak times.

Common mistake: DIY hot water sizing and installation

Hot water sizing affects plumbing, electrical, gas, pressure, drainage and compliance requirements.

DIY installation can create:

Water leaks
Electrical risks
Gas leaks
Carbon monoxide risk
Fire risk
Scalding risks
Poor performance
Warranty issues
Insurance problems
Non-compliant work
Property damage

Use licensed professionals for installation and technical suitability checks.

Final verdict: what size hot water system should you choose?

The right hot water system size depends on household demand, system type, energy source, number of bathrooms, shower habits, bath use, recovery rate, flow rate, electrical supply, gas supply, installation location and future use.

For simple electric storage replacement, many households compare 125L, 160L, 250L, 315L and 400L options depending on use.

For heat pump hot water, tank size must be matched with airflow, drainage, recovery, timer settings and household demand.

For instant gas, 16L, 20L and 26L classes should be matched to bathrooms, simultaneous use, gas pipe sizing and water pressure.

For instant electric, size should be based on the exact fixture, flow rate, temperature rise and electrical capacity.

Start by comparing Hot Water Systems, Electric Hot Water Systems, Instant Electric Systems, Gas Hot Water Systems, Instant Gas Hot Water Systems, Heat Pump Hot Water Systems and Installation Add-Ons.

The best size is not the smallest, largest or cheapest option. It is the system that reliably matches your household’s real hot water use and suits the installation site.

Long-tail FAQs

What size hot water system do I need for 2 people?

Many two-person homes compare 125L to 160L electric storage or a smaller to medium heat pump where the site suits. Actual usage, shower habits, bath use and tariff setup matter.

What size hot water system do I need for 4 people?

Many four-person homes compare 250L electric storage, family-sized heat pump hot water, gas storage or 20L to 26L instant gas where gas supply and water pressure suit.

Is a 125L hot water system enough?

A 125L system may suit one to two people with moderate use, but it may be too small for longer showers, baths, guest use or larger households.

Is a 250L hot water system enough for a family?

A 250L system can suit many 3 to 4 person households, but it depends on showers, bathrooms, bath use, recovery rate and whether the old system ran out.

Do I need 315L or 400L hot water?

Larger systems may suit bigger households, multiple bathrooms, high shower demand or off-peak setups, but they need more space and may cost more upfront.

What size heat pump hot water system do I need?

Heat pump sizing depends on tank capacity, household demand, recovery, timer settings, boost mode, airflow, condensate drainage and installation location.

What size instant gas hot water system do I need?

Instant gas sizing depends on bathrooms, simultaneous shower use, gas type, gas pipe sizing, water pressure and flow rate. Common classes include 16L, 20L and 26L.