What Size Hot Water System Do I Need?

Jun 6, 2026

Choosing the right size hot water system depends on the number of people in your home, number of bathrooms, shower habits, bath use, kitchen use, laundry use, energy source and the type of system you are buying. Storage hot water systems are sized by tank capacity in litres, while instant gas hot water systems are sized by flow rate in L/min.

At Hot Water Outlet, you can compare hot water systems online, including electric hot water systems, heat pump hot water systems, gas hot water systems, instant gas hot water systems, instant electric hot water systems and common valves, pipework and fittings.

Quick Answer: What Size Hot Water System Do I Need?

The right hot water system size depends on whether you are choosing a storage system or an instant system.

As a general guide:

Choose a smaller storage system for 1 or 2 people with lower daily hot water use.
Choose a medium storage system for 3 people or smaller families with moderate hot water use.
Choose a larger storage system for families of 4 or more, especially where showers happen back-to-back.
Choose an even larger storage system or carefully matched recovery rate for families of 5 or more, homes with baths or higher peak demand.
Choose an instant gas system by flow rate in L/min, not tank size.
Choose a heat pump by tank capacity, household demand, outdoor space, airflow and noise rating.
Choose instant electric only for suitable point-of-use applications where the electrical requirements and flow demand match the product.

For most homes, the best starting point is to check the current hot water system size. If the current system keeps up well, a similar size may be practical. If it often runs out, compare a larger capacity, better recovery rate or different system type before ordering.

Hot Water System Sizing at a Glance

Household Type	Storage Hot Water Sizing Focus	Instant Gas Sizing Focus	Main Thing to Check
1 person	Compact storage may suit	Lower-flow instant gas may suit where gas is available	Shower habits, bath use and space
2 people	Small to medium storage may suit	Lower to medium flow may suit	Back-to-back showers and laundry use
3 people	Medium storage often worth comparing	Medium flow often worth comparing	Bathroom count and peak use
Family of 4	Medium to larger storage often needed	Medium to higher flow may suit	Showers, bath use and simultaneous demand
Family of 5+	Larger storage or strong recovery usually needed	Higher-flow instant gas may be required	Multiple bathrooms and peak demand
Granny flat	Compact independent system may suit	Lower-flow gas or point-of-use may suit	Distance from main system and energy source
Rental property	Size for likely tenant demand	Size for bedrooms and bathrooms	Avoid undersizing
Renovation	Size for the finished layout	Size for future bathrooms and fixtures	Do not size from the old layout only

This table is a starting point only. The right system still needs to match the property, current setup and product specifications.

Storage Systems vs Instant Systems: Why Sizing Is Different

Hot water systems are not all sized the same way.

Storage systems hold heated water in a tank. These systems are measured by tank capacity in litres.

Storage systems include:

Instant systems heat water as it flows through the unit. They do not store a large tank of hot water.

Instant systems include:

Instant gas systems are usually compared by flow rate in L/min. Instant electric systems are selected by intended use, flow demand and electrical requirements.

This means you should not compare a 160L storage tank directly against a 26 L/min instant gas system as though they are the same measurement. Litres and L/min measure different things.

What Affects Hot Water System Size?

Hot water system size depends on more than the number of people in the home.

Important sizing factors include:

Number of occupants
Number of bathrooms
Number of showers per day
Shower length
Whether showers happen back-to-back
Whether two showers run at the same time
Bath use
Kitchen hot water use
Laundry hot water use
Dishwasher or washing machine connection
Current system size
Whether the current system runs out
Energy source
Available space
Recovery rate
Flow rate for instant gas
Outdoor space and airflow for heat pumps
Required valves and fittings

A home with 3 people and two bathrooms may need stronger hot water performance than a home with 4 people and one bathroom if the two-bathroom home has more simultaneous use.

How Many People Live in the Home?

The number of people is one of the easiest sizing starting points, but it is not the full answer.

More people usually means:

More showers
More laundry
More kitchen use
Higher chance of back-to-back hot water demand
More chance of running out of stored hot water
Greater need for recovery rate or higher instant gas flow

However, usage habits matter. A couple with long showers and regular bath use may need more hot water than a three-person household with short showers and low laundry demand.

Use the number of people as a guide, then check how the home actually uses hot water.

How Many Bathrooms Does the Home Have?

Bathroom count is one of the most important sizing factors.

A one-bathroom home usually has lower peak demand because only one shower or bath is likely to be used at a time.

A two-bathroom home may need more capacity or flow rate because two showers could run close together or at the same time.

A home with three or more bathrooms needs careful sizing, especially if it is a larger family home, rental property or renovation.

When comparing size, ask:

Will two showers run at once?
Will showers happen back-to-back?
Is there a bath?
Will the bathroom count increase after renovation?
Has the old system been keeping up?

If bathroom demand is increasing, do not simply replace the old system with the same size without checking suitability.

Shower Habits Matter

Showers are usually one of the biggest daily hot water uses in the home. A household with short showers may need a smaller system than a household with long showers, even if both have the same number of people.

Consider:

How many showers are taken each day
Whether showers happen in the morning or evening
Whether showers happen back-to-back
Whether teenagers or adults take longer showers
Whether two showers may run at the same time
Shower rose flow rate
Whether guests often stay

If your current system runs out during morning showers, it may be undersized for your routine.

Bath Use Can Change the Size You Need

Baths can use a large amount of hot water at once. If your home has a bath that is used regularly, sizing should account for that demand.

Bath use matters most for:

Families with young children
Homes with large baths
Renovations adding a bath
Homes where showers are used soon after the bath
Smaller storage systems that may empty quickly

If your home has a bath, avoid choosing a very small storage tank unless you are confident it suits your usage.

Kitchen and Laundry Use Also Count

Hot water is not only used in bathrooms. Kitchens and laundries can also affect sizing.

Consider:

Kitchen sink use
Dishwasher connection where relevant
Washing machine hot water use
Laundry tub use
Cleaning routines
Whether kitchen or laundry use overlaps with showers

A home where hot water is used in the bathroom, kitchen and laundry at the same time may need a stronger system than a home where hot water use is spread out.

Current System Size Is a Useful Starting Point

If you are replacing an old hot water system, the existing unit is a useful guide.

Check the label or data plate for:

Brand
Model
Tank capacity
Flow rate
Energy source
Natural gas or LPG type
Power rating
Pressure rating
Manufacturing date
Product code

Then ask one important question: has the current system been keeping up?

If the current system works well:

A similar capacity or flow rate may be practical.
The same system type may be worth comparing first.
The existing location may already suit the replacement.

If the current system runs out:

Do not replace with the same size automatically.
Compare a larger tank, better recovery rate or different system type.
Check whether household demand has increased.

If the current system is oversized:

A smaller system may suit if fewer people now live in the home.
Compare current usage rather than old household size.

What Size Electric Hot Water System Do I Need?

Electric hot water systems are usually storage systems, so they are sized by tank capacity in litres.

Electric storage sizing depends on:

Number of people
Number of bathrooms
Shower habits
Bath use
Kitchen and laundry use
Recovery rate
Current tank capacity
Whether the old system runs out
Available space
Power requirements

Electric storage may suit:

Homes already using electric hot water
Properties without natural gas or LPG
Like-for-like tank replacements
Units, townhouses and family homes
Rental properties
Homes where a heat pump location is not suitable

The right electric hot water size should provide enough stored hot water without being unnecessarily oversized.

Electric Storage Sizing Mistakes to Avoid

Common mistakes include:

Choosing the smallest tank because it is cheaper
Replacing with the same size even though the old tank runs out
Choosing a larger tank without checking dimensions
Ignoring recovery rate
Forgetting power requirements
Forgetting valves and fittings
Not checking whether a heat pump may be worth comparing

If the home already has electric storage and you want a straightforward replacement, compare electric systems first. If energy efficiency is a priority and the location suits, compare heat pump options as well.

What Size Heat Pump Hot Water System Do I Need?

Heat pump hot water systems are also storage systems, so they are sized by tank capacity. However, heat pump sizing also depends heavily on location.

Heat pump sizing depends on:

Tank capacity
Number of occupants
Number of bathrooms
Shower habits
Bath use
Current electric storage size
Outdoor space
Airflow
Noise rating
Product dimensions
Electrical requirements
Delivery access
Required valves and fittings

A heat pump may suit:

Homes replacing old electric storage
Households wanting efficient electric hot water
Properties with suitable outdoor space
Homes with enough airflow around the unit
Families needing stored hot water
Renovations where location can be planned early

A heat pump with the right tank size may still be unsuitable if the home lacks outdoor space or airflow.

Heat Pump Sizing Mistakes to Avoid

Common mistakes include:

Choosing by tank capacity only
Ignoring airflow requirements
Ignoring noise rating
Choosing a location too close to bedrooms or neighbours
Choosing a system that does not fit the available space
Forgetting delivery access
Choosing a small tank for a high-demand household
Assuming every electric storage system can be replaced with a heat pump

Heat pump sizing is about both capacity and location suitability.

What Size Gas Storage Hot Water System Do I Need?

Gas hot water systems can include gas storage systems that heat and store water in a tank.

Gas storage sizing depends on:

Tank capacity
Recovery rate
Natural gas or LPG
Number of people
Number of bathrooms
Shower habits
Bath use
Kitchen and laundry use
Current system performance
Product dimensions
Connection positions

Gas storage may suit:

Homes already using gas storage
Properties with natural gas or LPG
Like-for-like gas replacements
Homes wanting traditional stored hot water
Rental properties where gas storage already works well

For gas storage, recovery rate is important. A smaller gas storage system with strong recovery may perform differently from an electric tank with the same capacity. Always check product specifications.

Gas Storage Sizing Mistakes to Avoid

Common mistakes include:

Choosing the wrong gas type
Ignoring recovery rate
Choosing tank size only by litres
Replacing with the same size even though the old system struggled
Forgetting natural gas and LPG are not interchangeable
Not checking dimensions or connection positions
Forgetting required valves and fittings

If you are choosing gas, confirm whether the property uses natural gas or LPG before ordering.

What Size Instant Gas Hot Water System Do I Need?

Instant gas hot water systems are sized by flow rate in L/min. They heat water as it passes through the unit and do not store a large tank of hot water.

Instant gas sizing depends on:

Required flow rate in L/min
Natural gas or LPG
Number of bathrooms
Number of occupants
Shower demand
Whether multiple outlets run at once
Bath use
Kitchen and laundry use
Controller requirements
Gas supply suitability
Product dimensions
Connection positions

Instant gas may suit:

Homes with natural gas or LPG
Properties wanting continuous flow hot water
Homes replacing older instant gas units
Homes moving from gas storage to continuous flow where suitable
Townhouses and homes where a compact wall-mounted unit is preferred

The right instant gas size depends on peak demand. A lower-flow system may suit a smaller one-bathroom home. A higher-flow system may be needed for larger homes or multiple bathrooms.

Instant Gas Flow Rate Guide

Flow Rate	Common Use Case	Main Thing to Check
16 L/min	Smaller homes, lower demand or one-bathroom properties	Gas type, shower demand and outlet use
20 L/min	Moderate household demand	Bathrooms, shower habits and simultaneous use
26 L/min	Larger homes, families or higher peak demand	Gas supply, bathroom count and product suitability

This is only a general guide. The selected flow rate should match the home’s actual use and gas supply.

Instant Gas Sizing Mistakes to Avoid

Common mistakes include:

Choosing the cheapest low-flow model for a family home
Choosing natural gas when the property uses LPG
Choosing LPG when the property uses natural gas
Ignoring the number of bathrooms
Ignoring simultaneous outlet use
Forgetting controller compatibility
Forgetting gas supply suitability
Comparing L/min directly against storage litres

Instant gas sizing is not about tank capacity. It is about flow rate and peak demand.

What Size Instant Electric Hot Water System Do I Need?

Instant electric hot water systems are selected differently from storage systems and instant gas systems.

Instant electric sizing depends on:

Intended use
Flow demand
Electrical requirements
Number of fixtures
Whether it is point-of-use
Product capacity
Connection requirements
Licensed electrical requirements

Instant electric may suit:

Compact point-of-use hot water
Hand basins
Kitchenette applications
Specific small fixtures
Remote low-demand fixtures
Situations where storage is not practical

Instant electric is not automatically suitable for whole-home hot water. Always check the product specifications and electrical requirements.

Instant Electric Sizing Mistakes to Avoid

Common mistakes include:

Assuming instant electric can replace any storage tank
Ignoring electrical requirements
Using a point-of-use system for whole-home demand
Choosing by price only
Not checking flow demand
Not checking product suitability
Forgetting licensed electrical work requirements

Instant electric can be useful in the right application, but it must be matched carefully.

What Size Hot Water System for 1 Person?

A one-person household may suit a compact system, but usage still matters.

For one person, consider:

Shower length
Bath use
Kitchen use
Laundry use
Whether guests often stay
Whether the current system is oversized
Available space
Energy source

A compact electric storage system, suitable instant gas system, or point-of-use instant electric system may suit different one-person homes. A heat pump may be worth comparing if the property has regular use and suitable outdoor space.

What Size Hot Water System for 2 People?

A two-person household usually needs more hot water than one person, especially if showers happen close together.

For two people, consider:

Back-to-back showers
Bath use
Laundry frequency
Kitchen use
Number of bathrooms
Current tank size or flow rate
Whether the current system runs out

Small to medium electric storage, suitable heat pump, gas storage or lower to medium flow instant gas may suit depending on the property.

What Size Hot Water System for 3 People?

A three-person household usually needs a medium-sized system or carefully matched instant gas flow rate.

For three people, consider:

Number of bathrooms
Number of showers per day
Bath use
Laundry use
Whether multiple outlets run at once
Current system performance
Energy source

Medium electric storage, suitable heat pump, gas storage with suitable recovery or instant gas with appropriate flow rate may suit.

What Size Hot Water System for a Family of 4?

A family of 4 usually needs stronger hot water performance than smaller households.

For a family of 4, consider:

Number of bathrooms
Back-to-back showers
Bath use
Laundry use
Kitchen demand
Morning and evening peak use
Whether the current system runs out

A medium to larger storage system, correctly sized heat pump, gas storage with suitable recovery or medium to higher flow instant gas may suit.

What Size Hot Water System for a Family of 5 or More?

A family of 5 or more usually needs larger capacity, better recovery or higher flow rate.

For larger families, consider:

Multiple bathrooms
Two showers running close together
Teenagers or long showers
Bath use
Heavy laundry use
Peak morning and evening demand
Current system performance

Larger electric storage, larger heat pump, gas storage with strong recovery or higher-flow instant gas may be worth comparing.

What Size Hot Water System for a Unit?

Units often have space limits and may have body corporate requirements.

For units, consider:

Existing system type
Available cupboard or outdoor space
Number of occupants
Number of bathrooms
Body corporate rules
Delivery access
Power or gas availability
Product dimensions

Compact electric storage is common in many units. Instant electric may suit specific point-of-use applications. Heat pumps or instant gas may suit some units only where location, airflow, gas type and building rules allow.

What Size Hot Water System for a Townhouse?

Townhouses can vary from compact two-bedroom layouts to larger family homes with multiple bathrooms.

For townhouses, consider:

Number of occupants
Number of bathrooms
Garage, courtyard or cupboard space
Energy source
Body corporate or complex rules
Heat pump airflow if relevant
Gas type if relevant
Delivery access

Townhouses may suit electric storage, heat pump, gas storage or instant gas depending on the current setup.

What Size Hot Water System for a Granny Flat?

A granny flat may need its own independent hot water system, especially if it is detached or has its own bathroom, kitchen and laundry.

For granny flats, consider:

One or two occupants
Bathroom use
Kitchen use
Laundry use
Distance from main house system
Whether the system will be shared or independent
Available space
Energy source

Compact electric storage, instant gas where gas is available or point-of-use instant electric may suit different granny flat layouts.

What Size Hot Water System for a Rental Property?

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

For rentals, consider:

Number of bedrooms
Number of bathrooms
Likely number of occupants
Whether tenants have complained about hot water
Existing system type
Warranty information
Product availability
Required valves and fittings

Avoid undersizing rental hot water systems. A cheaper small unit may create tenant complaints if it cannot keep up.

What Size Hot Water System for a Renovation?

Renovations should be sized for the finished home, not the old layout.

For renovations, consider:

Future number of bathrooms
New bath or larger shower
Kitchen changes
Laundry changes
Granny flat additions
System relocation
Energy source changes
Tank size or flow rate after renovation
Required valves, pipework and fittings

If you are adding a bathroom or bath, the old hot water system size may no longer be suitable.

Should You Choose a Bigger Hot Water System?

A bigger system may be useful if the current system runs out or household demand is increasing.

A bigger system may be worth comparing when:

More people now live in the home
A bathroom has been added
The household uses a bath regularly
Showers happen back-to-back
Two showers may run at once
The current system runs out
A larger rental property needs family-level hot water
Renovations will increase demand

However, bigger is not always better. An oversized system may cost more than needed, take up more space and be unnecessary for the household.

Should You Choose a Smaller Hot Water System?

A smaller system may suit if your household has reduced demand.

A smaller system may be worth comparing when:

Fewer people now live in the home
The old system was oversized
The property is a small unit
The home has low shower and laundry demand
Space is limited
The current system is much larger than needed

Do not choose smaller by price alone. The system still needs to supply enough hot water for everyday use.

What Happens If a Hot Water System Is Too Small?

An undersized hot water system can create daily problems.

Signs the system may be too small include:

Hot water runs out during showers
The last person often gets cold water
Bath use empties the tank
The system struggles after laundry or kitchen use
Tenants or family members complain about hot water
Instant gas cannot keep up with multiple outlets
Recovery is too slow for the household

If your current system has these issues, compare a larger tank, stronger recovery rate, higher instant gas flow rate or different system type before replacing it.

What Happens If a Hot Water System Is Too Large?

An oversized system can also be a poor fit.

A system may be too large if:

The household is small
The old unit was sized for previous occupants
A large tank takes up unnecessary space
Product cost is higher than needed
The home does not use much hot water
A smaller system would comfortably meet demand

The goal is not to buy the biggest system. The goal is to buy the right system.

Recovery Rate: The Sizing Factor Many People Forget

Recovery rate is how quickly a storage hot water system can heat more water after hot water has been used.

Recovery rate matters for:

Families
Back-to-back showers
Bath use
Gas storage systems
Electric storage systems
Homes with heavier peak demand
Rental properties

Two systems with similar tank capacity may perform differently if recovery rates are different. Always check product specifications before ordering.

Flow Rate: The Key Sizing Factor for Instant Gas

Instant gas systems are sized by flow rate in L/min.

Flow rate matters for:

Number of bathrooms
Number of hot water outlets
Whether two showers may run at once
Family size
Peak demand
Shower flow rate
Kitchen and laundry use

A 16 L/min system may suit some small homes. A 20 L/min system may suit moderate demand. A 26 L/min system may suit larger homes or higher-demand households. The right size depends on the actual property.

Space and Dimensions Matter

The right size also has to fit.

Before ordering, check:

Height
Width
Depth
Weight
Delivery access
Doorway or side access
Cupboard, garage or outdoor space
Wall mounting position for instant gas
Outdoor clearance for heat pumps
Drainage
Connection positions

A system with the right capacity can still be the wrong choice if it does not fit the location.

Gas Type Matters for Gas Sizing

If choosing gas storage or instant gas, confirm the gas type before ordering.

Gas systems may be:

Natural gas
LPG

Natural gas and LPG systems are not interchangeable.

Before ordering gas hot water, check:

Whether the property has natural gas
Whether the property uses LPG bottles or bulk LPG
Current system label
Gas meter or LPG bottle setup
Product listing gas type
Licensed gas fitter advice if unsure

Choosing the wrong gas type can cause delays and extra costs.

Valves, Fittings and Accessories Can Affect the Order

The hot water system may not be the only product required.

Depending on the system and replacement setup, you may need:

Tempering valves
Pressure relief valves
Duo valves
Non-return valves
Isolation valves
Copper pipework
Fittings
Controller accessories
Thermostats
Elements
Connection parts

Browse valves, pipework and fittings if you need hot water accessories or connection parts.

Always confirm compatibility before ordering.

Hot Water System Size Checklist

Before ordering a hot water system, check:

Current system type
Current brand and model
Current tank capacity or flow rate
Whether the current system runs out
Electricity, natural gas or LPG
Number of people in the home
Number of bathrooms
Number of showers per day
Shower habits
Bath use
Kitchen hot water use
Laundry hot water use
Whether outlets are used at the same time
Desired system type
Tank capacity if storage
Flow rate in L/min if instant gas
Recovery rate
Electrical requirements if instant electric
Heat pump airflow if relevant
Heat pump noise rating if relevant
Gas type if choosing gas
Product dimensions
Connection positions
Required valves and fittings
Delivery access
Warranty information
Product specifications

Compare Hot Water System Sizes Online

Hot Water Outlet makes it easier to compare hot water systems by size, system type, brand, energy source and product specifications.

Start with:

FAQs About Hot Water System Size

What size hot water system do I need?

The right size depends on household size, bathrooms, shower habits, bath use, kitchen use, laundry use and system type. Storage systems are sized by tank capacity in litres. Instant gas systems are sized by flow rate in L/min.

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

A small to medium storage system may suit many two-person households, depending on shower habits, bath use and laundry demand. If choosing instant gas, compare flow rate based on bathroom count and peak use.

What size hot water system do I need for 3 people?

A medium storage system or suitable instant gas flow rate may suit many three-person households. The right choice depends on bathrooms, back-to-back showers, bath use and current system performance.

What size hot water system do I need for a family of 4?

A family of 4 usually needs a medium to larger storage system, correctly sized heat pump, gas storage system with suitable recovery or instant gas system with enough flow rate for peak demand.

What size hot water system do I need for a family of 5?

A family of 5 usually needs larger capacity, stronger recovery or a higher-flow instant gas system, especially where there are multiple bathrooms or back-to-back showers.

What size electric hot water system do I need?

Electric hot water systems are usually sized by tank capacity in litres. The right size depends on household demand, recovery rate, current tank size, number of bathrooms and whether the current system runs out.

What size heat pump hot water system do I need?

A heat pump is sized by tank capacity, but location also matters. Check household demand, outdoor space, airflow, noise rating, product dimensions and electrical requirements before ordering.

What size instant gas hot water system do I need?

Instant gas systems are sized by flow rate in L/min. Smaller homes may suit lower-flow systems, while larger homes or multiple bathrooms may need higher-flow models.

Is a bigger hot water system better?

Not always. A bigger system may help if the current unit runs out, but an oversized system can cost more and take up more space than needed. The best size matches household demand.

Can I install a hot water system myself?

No. Hot water systems involve plumbing, gas and electrical work that should be completed by appropriately licensed tradespeople. Hot Water Outlet sells supply-only hot water systems and accessories online, and may help organise quotes from reputable installers where appropriate.

Find the Right Size Hot Water System

The right hot water system size should match your household demand, energy source, available space and replacement setup. Before ordering, compare tank capacity, flow rate, recovery rate, gas type, dimensions, connection positions, warranty information and required accessories.

Browse hot water systems online with Hot Water Outlet to compare electric, heat pump, gas, instant gas, instant electric and hot water accessories by brand, size and product specifications.

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