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What size hot water system do I need?

Getting the right sized hot water system for your home is critical. The perfect size will mean rarely running out of hot water, never wasting money heating water you don't use, as well as potentially saving on the purchase price of your new system. Many factors need to be considered to ensure the water heater you choose is ideal for your home, family and lifestyle.

How to choose the right size hot water system for your home

How to find the right size hot water system for your home?

On average each person in your home will use around 50 litres of water per day, however, this can vary from person to person dependent on how often you wash your clothes in hot water, use the dishwasher or the length of your showers.

Points to consider include:

How many people living in your home?

A family of seven is obviously going to need a bigger hot water system than a couple. However, a standard storage hot water system will last around 10 years so make sure you plan for the future as well. Are you planning on having children (or more children), renting out rooms to students, or sub-letting part of your home? Ensure the system you choose will be able to cater for the additional demands of the future.

How many bathrooms?

Modern houses or apartments often have at least two bathrooms, plus the kitchen sink, dishwasher, laundry tub, and washing machine competing for hot water. If your home has more bathrooms or an extra kitchen your system will need to be larger to maintain hot water levels for the whole house.

When does your home use hot water?

During the evening is often when the demand for hot water peaks. If everyone in your home showers in the evening, plus you run the dishwasher and throw a load of washing on when you finish work you'll need a large system to avoid running out. A household where some are morning showerers and some evening, the clothes washing is done during the day and the dishwasher timer is set to run at 2 am to take advantage of off-peak tariffs will require a much smaller system.

Peak or off-peak power?

If your chosen power source is electricity and you'd like to keep the costs of running a hot water heater down by connecting the system to an off-peak tariff (often between 10 pm and 7 am) you'll need to install a larger system as it will need to supply hot water to your home for the whole day and will only reheat at night (during the off-peak times).

This applies to solar hot water systems as well as they only heat water during the day (without the booster function).

Once you've gathered this information and considered all these points we recommend speaking with your plumber or hot water specialist to get professional advice specific to your requirements.

HANDY TIP - for some ideas or help working out the perfect sized system take a look at Rinnai's Hot Water Selector

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Hot water size guide

Please note these recommendations should be used as a guide only, speak to your local hot water specialist for specific advice for your home and hot water needs.

Electric hot water size guide

Electric storage hot water connected to an off-peak tariff (eg 10 pm to 7 am)

Household size    Electric storage hot water system size   
1 - 3 people 250 litre
3 - 5 people 315 litre
5 - 8 people 300 litre

Electric storage hot water connected to an economy shoulder tariff

Household size    Electric storage hot water system size    
1 - 3 people 125 litre
3 - 5 people 160 litre
4 - 6 people 250 litre
6 - 8 people 315 litre

Electric storage hot water connected to peak supply tariff

Household size    Electric storage hot water system size   
1 person 25 litre
1 - 2 people 50 litre
2 - 3 people 80 litre
3 - 5 people 125 litre

Ref - https://www.ergon.com.au/retail/residential/tariffs-and-prices/economy-tariffs/hot-water-tank-sizes

Gas hot water size guide

Gas storage hot water system using either natural gas or LPG

Household size    Gas storage hot water heater size   
1 - 3 people 90 litre
2 - 4 people 130 litre
3 - 5 people 170 litre
4 - 6 people 200 litre

Gas instant hot water system using either natural gas or LPG. Rather than the number of residents, instant hot water systems sizing works on the number of outlets which may be used simultaneously. For example, if you have two bathrooms and the kitchen which could need water at the same time in the evening you would need to purchase a 24L unit to ensure there is enough hot water generated to cater for all taps.

Number of outlets     Gas instant hot water   
1 16 litre
2 20 litre
2 - 3 24 litre
3 or more 32 litre

Solar hot water size guide

Solar hot water systems only heat water during the day. Most systems are fitted with either an electric or gas booster for high demand times or overcast days.

Number of people     Solar hot water system size   
1 - 2 180 litre
3 - 4 300 litre
5 - 6 440 litre

Heat pump hot water size guide

Heat pump water heaters are very energy efficient, even more so if connected to an economy tariff (off-peak 1 or off-peak 2). 

Small - less than 200L    Medium - 200 - 300L    Large - 300L+   
Apartment or small house    Average sized house  Large house
1-2 bedrooms & 1 bathroom    3 bedrooms & 1 bathroom    4 or more beds & 2 or more bath   
Maximum 2 people  Maximum 3 people Minimum 4 people
Not normally suited to off-peak tariffs    May not be suited to off-peak. Some can run on Restricted hours tariff Restricted hours and off-peak tariff possible

Ref - http://www.energyrating.gov.au/sites/new.energyrating/files/documents/HeatPumpWaterHeaterGuide_toWeb_0.pdf

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