Best Hot Water System for Granny Flats

Choosing the best hot water system for a granny flat is usually about balancing size, space, cost and daily usage. A granny flat does not normally need the same hot water system as a full family home, but it still needs enough capacity for showers, kitchen use and laundry if those fixtures are connected.

For most granny flats in Australia, the best hot water system is usually a 50L to 160L electric storage hot water system, depending on the number of occupants and daily hot water use. Where gas is available, a 16 L/min continuous flow gas hot water system may also suit some granny flats. A heat pump hot water system can be worth comparing where there is suitable outdoor space, airflow and a long-term efficiency goal, but it may be more system than a very small granny flat needs.

The right choice depends on whether the granny flat has its own bathroom, kitchen and laundry, whether it is used full-time or occasionally, and whether the system is independent from the main house.

Quick answer: what hot water system suits a granny flat?

For most granny flats, these are the common options to compare:

For many granny flats, a 125L electric storage hot water system is a practical middle ground. It gives more usable hot water than a very small 50L or 80L unit without jumping to a large family-sized tank.

You can compare options in the Installed Today hot water systems range, including electric hot water systems, instant gas hot water systems and heat pump hot water systems.

What is the best hot water system for a granny flat?

The best hot water system for a granny flat is usually one of these options:

1. 80L to 125L electric storage hot water system
   Best for small granny flats, single occupants and compact spaces.

2. 125L to 160L electric storage hot water system
   Best for full-time use, couples, rental granny flats and higher daily demand.

3. 16 L/min continuous flow gas hot water system
   Best where the granny flat has suitable natural gas or LPG supply and an external location.

4. Small or suitable heat pump hot water system
   Best for efficiency-focused setups with enough outdoor space, airflow and drainage.

For most granny flats, electric storage is the simplest starting point because it is compact, familiar and available in smaller capacities. Gas and heat pump options can work well in the right property, but they need more careful checks before ordering.

Best overall option: 125L electric storage hot water

For many granny flats, a 125L electric storage hot water system is the best overall balance of size, comfort and practicality.

A 125L system may suit:

• 1–2 people

• A granny flat with one bathroom

• Daily shower use

• Kitchen hot water use

• Occasional laundry hot water use

• Rental granny flats

• Replacing an existing 125L system

• Properties where 80L may be too small

A 125L unit gives more buffer than very small storage systems while still being compact enough for many granny flat layouts. It can be a good choice when the property is occupied full-time and the hot water system needs to handle normal daily routines.

You can browse suitable options in the electric hot water systems collection.

When a 50L or 80L electric hot water system makes sense

A 50L or 80L electric hot water system may suit very low-use granny flats, studios, detached rooms or small secondary dwellings with limited hot water demand.

It may be suitable where:

• One person uses the granny flat

• Shower use is short and light

• The granny flat is used occasionally

• The system only serves a small bathroom or kitchenette

• Space is extremely limited

• The existing unit is already a similar size

The main limitation is capacity. A 50L or 80L system can run out quickly if the occupant takes long showers, uses hot water for laundry, or has guests staying. For full-time living, many households will find 125L more comfortable.

When a 160L electric hot water system is better

A 160L electric storage system may be better for a granny flat with higher daily use.

It may suit:

• 2 people

• Rental granny flats

• Longer showers

• A separate laundry

• More regular kitchen use

• Full-time independent living

• Properties where a 125L system has run out before

• Owners wanting more hot water buffer

A 160L system gives extra capacity without moving into large family-sized tanks. It can be a safer choice where the granny flat is not just a guest space but a proper long-term residence.

The trade-off is that it may cost more, take up more space and be unnecessary for very low-use setups.

Is a 250L system too big for a granny flat?

A 250L electric hot water system is usually larger than most granny flats need, but it may suit rare cases.

It may be worth considering if:

• The granny flat houses a small family

• There are 3 or more occupants

• There is a full laundry with regular hot water use

• The existing 160L system regularly runs out

• The system also serves another nearby area

• The property has enough space and access

For most one-bedroom granny flats, a 250L system is likely to be more capacity than required. A 125L or 160L unit is usually the better starting point.

Best gas option: 16 L/min continuous flow gas hot water

A 16 L/min continuous flow gas hot water system can be a good option for some granny flats with natural gas or LPG.

Continuous flow gas systems heat water as it passes through the unit instead of storing it in a tank. This makes them compact and useful where outdoor wall space is available but ground space for a tank is limited.

A 16 L/min gas system may suit:

• One bathroom granny flats

• 1–2 occupants

• Properties already using gas

• Granny flats with suitable external wall space

• Homes with natural gas or LPG available

• Setups where a compact wall-mounted system is preferred

You can compare options in the instant gas hot water systems collection.

Natural gas vs LPG for granny flats

Gas hot water systems must match the property’s fuel type. Natural gas and LPG are not interchangeable.

Natural gas is usually supplied through a street gas connection. LPG is usually supplied through gas bottles. Some granny flats may share services with the main house, while others may have their own separate gas supply or no gas at all.

Before ordering a gas hot water system, check:

• Whether the property has natural gas or LPG

• Whether the granny flat has access to that supply

• Whether the chosen model matches the gas type

• Whether the gas pipe sizing is suitable

• Whether there is a compliant outdoor location

• Whether a licensed gas fitter has confirmed the setup

If the granny flat does not already have gas, changing from electric to gas may involve extra cost and extra licensed trade work.

Can a granny flat use instant electric hot water?

Instant electric hot water may suit some very small point-of-use applications, such as a hand basin or small kitchenette, but it is not always the best option for a whole granny flat with a shower.

Whole-home instant electric systems can require significant electrical capacity, and not every property is suitable. For most granny flats with a bathroom and kitchen, a small electric storage system is usually a more practical starting point.

If you are considering instant electric for a full granny flat, check the electrical requirements carefully before ordering.

Can a granny flat use heat pump hot water?

A heat pump hot water system can work for some granny flats, but it depends on space, airflow, drainage and budget.

Heat pumps use electricity to draw heat from the surrounding air and transfer it into the water. They can be more efficient than standard electric storage systems in suitable installations, but they need the right location.

A heat pump may suit:

• A granny flat used full-time

• Owner-occupied setups

• Properties with suitable outdoor space

• Homes with solar power

• Long-term efficiency-focused buyers

• Sites with good airflow and drainage

• Setups where the higher upfront cost makes sense

A heat pump may not suit:

• Very small or occasional-use granny flats

• Tight side paths

• Enclosed courtyards

• Poor airflow locations

• Areas close to bedroom windows

• Properties with no suitable drainage

• Buyers wanting the lowest upfront replacement cost

You can browse available options in the heat pump hot water systems collection.

Electric storage vs gas vs heat pump for granny flats

Here is the simple comparison:

For most granny flats, electric storage is the simplest and most practical first comparison.

Should a granny flat have its own hot water system?

In many cases, yes. A granny flat often works best with its own dedicated hot water system, especially if it has a separate bathroom, kitchen or laundry.

A dedicated system can help:

• Avoid long pipe runs from the main house

• Reduce heat loss between buildings

• Give the occupant independent hot water

• Make sizing easier

• Avoid overloading the main house system

• Simplify future maintenance or replacement

• Suit rental or separate living arrangements

Some properties may connect the granny flat to the main house hot water system, but this is not always practical. Long pipe runs can waste water and heat, and the main system may not be sized for the extra demand.

What if the granny flat shares hot water with the main house?

If the granny flat shares hot water with the main house, the main system must be sized for both dwellings.

This can create issues if:

• The main house system is already near capacity

• The pipe run to the granny flat is long

• Hot water takes too long to arrive

• Multiple showers happen at the same time

• The granny flat is rented or used full-time

• The existing system was only sized for the main home

In some cases, installing a separate hot water system for the granny flat may be more practical than trying to make the main house system serve both areas.

A licensed plumber should assess pipework, pressure, distance and compliance before changing the setup.

Best hot water system for a rental granny flat

For a rental granny flat, reliability and flexibility matter. Occupants may change, and water use can vary.

A rental granny flat may suit:

• 125L electric storage for most 1–2 person setups

• 160L electric storage for more comfort and flexibility

• 16 L/min instant gas where gas is already suitable

• Heat pump hot water where long-term efficiency is the goal and location suits

For rentals, avoid choosing the smallest possible system unless the usage is genuinely low. A system that is too small can lead to complaints about running out of hot water.

Best hot water system for a one-person granny flat

A one-person granny flat may suit:

• 80L electric storage for low use

• 125L electric storage for more comfortable daily use

• 16 L/min instant gas where gas is available

• Small heat pump only where space and budget suit

If the occupant takes short showers and uses minimal hot water, 80L may be enough. If they want more buffer, or the granny flat is used full-time, 125L is often a better comparison.

Best hot water system for two people in a granny flat

For two people, a 125L or 160L electric storage system is usually a stronger starting point.

A 125L system may work where showers are moderate and hot water use is predictable. A 160L system may be better where showers are longer, two people shower close together, or the granny flat has a laundry.

For gas-connected granny flats, a 16 L/min continuous flow gas system may suit many 1 bathroom setups, provided the gas supply and installation location are suitable.

Best hot water system for a granny flat with laundry

A laundry can increase hot water demand, especially if the washing machine uses hot water regularly.

A granny flat with a laundry may need:

• 125L electric storage for moderate use

• 160L electric storage for more regular use

• A correctly sized gas continuous flow system where gas is available

• Careful pipework and drainage checks

If the granny flat has a bathroom, kitchen and laundry, do not size it like a tiny studio. Treat it as a small independent home.

Best hot water system for a granny flat with one bathroom

A one-bathroom granny flat commonly suits:

• 80L electric storage for low use

• 125L electric storage for everyday use

• 160L electric storage for two people or higher use

• 16 L/min gas continuous flow where suitable

The number of people matters more than the number of bathrooms. One bathroom can still have high demand if two people live there full-time.

Space and access considerations

Granny flats can have tight service areas, narrow side paths, small courtyards and compact laundries.

Before ordering a hot water system, check:

• Available floor space

• Wall space if choosing gas

• Outdoor clearance if choosing heat pump

• Access for delivery

• Doorway width

• Side path width

• Drainage

• Electrical supply

• Gas supply if applicable

• Distance from bedrooms and neighbours

• Whether the old system can be removed easily

A larger system may look better on paper, but it must physically fit and be serviceable.

Outdoor vs indoor hot water for granny flats

Some granny flats have indoor electric storage systems in cupboards or laundries. Others have outdoor tanks, gas units or heat pumps.

Indoor systems need checks for:

• Safe drainage

• Overflow tray requirements

• Access

• Cupboard dimensions

• Electrical connection

• Valve access

• Compliance with current requirements

Outdoor systems need checks for:

• Base or slab

• Weather exposure

• Drainage

• Clearance

• Access

• Noise placement

• Boundary distances

• Wall mounting if gas

• Airflow if heat pump

The replacement system should suit the existing location unless there is a strong reason to move it.

Should you replace like-for-like?

For granny flats, like-for-like replacement is often the simplest option.

This may mean replacing:

• Electric storage with electric storage

• 80L with 80L

• 125L with 125L

• 160L with 160L

• Instant gas with instant gas

• Natural gas with natural gas

• LPG with LPG

• Same location where practical

If the old system worked well, a similar replacement may be practical. If it regularly ran out or no longer suits the occupant, compare a larger size before ordering.

When should you upgrade the system size?

You may want to upgrade if:

• The current system runs out of hot water

• The granny flat is now occupied full-time

• A second person has moved in

• The property is becoming a rental

• A laundry has been added

• Shower use has increased

• The old system was only sized for occasional use

• The occupant wants more comfort

For many granny flats, moving from 80L to 125L or from 125L to 160L can make a noticeable difference, provided the space and electrical setup suit.

Supply only vs supplied and installed

Installed Today offers supply-only ordering across major Australian metro areas for customers who already have their own licensed installer arranged. Supplied-and-installed options are available on selected products in eligible Brisbane and Gold Coast service areas where shown.

For granny flats, always check what is included before ordering. Product pricing may not include installation, valves, electrical work, gas work, pipework changes, drainage changes, trenching, long service runs, old unit removal, disposal or compliance upgrades unless those items are clearly selected or included.

This is especially important where the granny flat is detached from the main house or has separate services.

Hot water systems for granny flats in Brisbane and Gold Coast

For eligible Brisbane and Gold Coast service areas, Installed Today offers supplied-and-installed options on selected hot water products where available.

Common granny flat options include:

• 50L electric storage systems for very low use

• 80L electric storage systems for compact setups

• 125L electric storage systems for everyday use

• 160L electric storage systems for more comfort

• 16 L/min instant gas systems where suitable

• Heat pump systems only where location and demand suit

Before ordering, check the product page, delivery options, installation availability and any add-ons required for your property.

Common mistake: choosing a system that is too small

A granny flat may be small, but the occupant still needs enough hot water for normal daily living.

A very small system may run out quickly if:

• The occupant takes long showers

• Two people live there

• The kitchen uses hot water regularly

• There is a laundry

• Guests stay over

• The granny flat is rented full-time

If the granny flat is a proper independent dwelling, size it like a small home, not just a spare room.

Common mistake: oversizing the system

Oversizing can also be a mistake. A large family-sized system may cost more, take up more space and store more hot water than a granny flat needs.

A 250L or 315L system is usually unnecessary for a one-person or two-person granny flat unless there is a specific reason for the extra capacity.

For most granny flats, 80L, 125L or 160L will be the main size range to compare.

Common mistake: connecting to the main house without checking demand

Connecting a granny flat to the main house hot water system can seem simple, but it may not work well if the pipe run is long or the main system is undersized.

Problems can include:

• Long wait times for hot water

• Wasted water

• Heat loss through pipework

• Main house system running out

• Pressure and flow issues

• Harder maintenance later

A separate system may be more practical, especially for full-time occupancy or rental use.

Common mistake: choosing gas without checking supply

A gas system can be compact and practical, but the granny flat must have suitable gas supply and a compliant external location.

Before ordering gas, confirm:

• Natural gas or LPG

• Correct model type

• Gas pipe sizing

• External location

• Clearances

• Water pressure

• Electrical connection

• Licensed gas fitter requirements

If the granny flat does not already have gas, the cost of adding it may outweigh the benefit compared with electric storage.

Common mistake: choosing heat pump without enough space

A heat pump needs outdoor airflow, drainage and clearance. Some granny flats have tight side spaces or small courtyards that may not suit a heat pump.

Before choosing heat pump hot water, check:

• Outdoor space

• Airflow

• Noise placement

• Distance from bedrooms and neighbours

• Condensate drainage

• Base or slab

• Delivery access

• Electrical suitability

A heat pump can be a strong option, but only where the location supports it.

Final recommendation

For most granny flats, start by comparing 80L, 125L and 160L electric storage hot water systems. An 80L system may suit very low-use setups, a 125L system is a practical everyday choice for many 1–2 person granny flats, and a 160L system gives more comfort for full-time use, rentals, longer showers or laundry demand.

If the granny flat has suitable natural gas or LPG supply, compare a 16 L/min continuous flow gas hot water system. If energy efficiency is the main goal and the location has suitable outdoor space, airflow and drainage, compare a heat pump hot water system.

The best hot water system for a granny flat is the one that matches the actual use of the dwelling. A granny flat used as a spare room may only need a small system. A granny flat used as a full-time home should be sized as a compact independent household.

To compare suitable options, browse the Installed Today hot water systems range, including electric hot water systems, instant gas hot water systems and heat pump hot water systems.

Long-tail FAQs

What size hot water system do I need for a granny flat?

Most granny flats suit an 80L, 125L or 160L electric storage hot water system. An 80L unit may suit low-use single occupants, 125L suits many everyday granny flats, and 160L gives more comfort for two people or rental use.

Is a 125L hot water system enough for a granny flat?

A 125L hot water system is often enough for a 1–2 person granny flat with one bathroom and moderate daily hot water use. If the granny flat has longer showers, laundry use or two full-time occupants, a 160L system may be worth comparing.

Is an 80L hot water system enough for a granny flat?

An 80L system may be enough for one person with light hot water use. It may be too small for two people, long showers, a rental granny flat or a dwelling with regular laundry hot water use.

Can a granny flat have its own hot water system?

Yes. Many granny flats work best with their own dedicated hot water system, especially where they have a separate bathroom, kitchen or laundry. A separate system can avoid long pipe runs from the main house and reduce pressure on the main home’s hot water supply.

Is instant gas hot water good for a granny flat?

Instant gas hot water can be good for a granny flat with suitable natural gas or LPG supply and a compliant external location. A 16 L/min system may suit many one-bathroom granny flats, but the gas setup must be checked by a licensed gas fitter.

Can I use a heat pump hot water system for a granny flat?

A heat pump can suit some granny flats where there is enough outdoor space, airflow, drainage and budget. It may not be necessary for very small or occasional-use granny flats, and it may not suit tight courtyards or enclosed spaces.

Should a granny flat share hot water with the main house?

A granny flat can share hot water with the main house in some setups, but it may cause long wait times, heat loss and extra demand on the main system. A separate hot water system is often more practical for full-time occupancy or rental use.

What is the cheapest hot water option for a granny flat?

A small electric storage system is often the lowest upfront option for many granny flats, especially where the property already has suitable electrical supply. The cheapest option still needs to be correctly sized and installed safely.