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What Size Hot Water Heater Do I Need?

There are many factors that you will need to consider when determining the right size hot water heating system you will need. These include:

  • Household Size
  • Number of Outlets Simultaneously Used (Tankless only)
  • Maximum Water Consumption (Storage only)
  • Type of Hot Water System (Gas, Electric or Solar)

Note: This will only give you an estimate size based on Australian households and water usages. If you want professional advice about your requirements and which hot water system is best for you, talk to the experts. Sun City Hot Water & Plumbing are hot water specialists based with years of experience. We provide quality advice, installations, replacements and repairs on hot water systems Sunshine Coast and Brisbane wide. Call 1300 728 122 for free advice on which size hot water heater is best for you.

Household Size

The first thing you will need to consider is your household size. In other words, how many people will be using the hot water heater. You will also need to consider any appliances that require hot water. For example, if you have a dishwasher you will need to add another person to your household size to compensate for the extra water usage.

Number of Outlets Simultaneously Used

This step only applies if you have a tankless or instantaneous hot water system.

As you may well know, tankless hot water systems do not require a storage tank. This is because tankless systems heat water as it is needed and thus, can supply hot water for an unlimited amount of time with no waiting period. However, because the water is heated on demand, using multiple hot water outlets simultaneously will reduce the flow rate.

This means that you will need to identify how many hot water outlets will be used at one time to determine the maximum flow rate that your system will need to support.

To determine the maximum number of outlets that will be used simultaneously, start by considering how many bathrooms you have. If you count each bathroom as one outlet all you need to do is add up the rest of the outlets in the house that may be used at the same time. This could include the kitchen sink, a dishwasher, a washing machine and any other hot water consuming appliance in your house.

Maximum Water Consumption

This step only applies if you have a storage type hot water system.

In order to determine the size of hot water system required for your household, you will need to calculate the maximum amount of water used in your household on a daily basis.

Below is a table with the average water usage of daily activities that require hot water.

UseAverage Litres Of Hot Water Used (Litres)
Washing dishes by hand or food preparation15
Washing Machine26

Note: estimated amounts are rounded to the nearest litre. Figures adapted from energy.gov worksheet https://energy.gov/energysaver/sizing-new-water-heater

You can use this table to help calculate the maximum daily water consumption of your household. To calculate the use per day of each activity multiply the litres required to complete the activity by the total number of times that activity is performed each day.

Litres Used x Use Per Day = Total Water Required

Add the total water required for each activity to determine the amount of hot water required for your household per day.

Note: The maximum water consumption of your household is not necessarily equal to the size of storage tank you need for your hot water system. For example, a household may have a maximum daily water usage of 100 litres. But, if 25 litres of that is used in the morning with 75 litres being consumed at night, the tank would only need to store 75 litres at a time.

Type of Hot Water System

To work out the size of the system, you will need to consider what type of hot water system you want. For an in depth guide see our [link: Guide to choosing a hot water system]. Below is a comparison of the average size needed for each type of hot water system.

Electric Hot Water System (Storage):

Household SizeTank Size Required (Litres)
1 – 3160
2 – 4250
3 – 6315
5 – 8400

Gas Hot Water System (Storage):

Household SizeTank Size Required (Litres)
1 – 390
2 – 4130
3 – 5170
4 – 6200
5 – 9260

Gas Hot Water System (Tankless):

Number of Outlets Used SimultaneouslyAverage Flow Rate (L/min)
1 – 220
2 – 324
3 – 532

Solar Hot Water System:

Household SizeMaximum Amount of Daily Hot Water (Litres)Required Tank Size (Litres)Solar Collector Size Required (m^2)
1 – 21201802
3 – 42003004
5 – 63004406

Note: All figures are estimated amounts based on average use. Required water is rounded to the nearest litre. You will need to add an extra person to your household if you have a hot water dishwasher installed. Figures adapted from sustainability.vic.gov tables https://www.sustainability.vic.gov.au/resources/documents/Choosingahotwatersystem.pdf

There are a lot of factors that contribute to deciding which is the best size hot water system for you. Choosing the right sized hot water system can save you money and save you from an unexpected cold shower.

The best way to ensure that you buy the right size hot water system is to ask an expert. Sun City Hot Water & Plumbing are your hot water specialists. Call 1300 728 122 for no obligation advice on which size hot water system is the best for you.


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