What Size Hot Water System Do I Need?

Solar Hot Water System On Roof

When it comes to hot water systems, size matters. That’s because the system you choose needs to be able to meet the demands of your household. If it’s too small, you’ll be left taking cold showers. And if it’s too large, you’ll be wasting energy and money.

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How Do You Know What Size System To Choose?

The first step is to determine the peak demand of your household. This is the maximum amount of hot water that will be used at any one time. To do this, simply add up the flow rates of all the appliances that will be using hot water simultaneously. Once you have your peak demand, you can choose a hot water system that meets your needs.

The following are the four different types of hot water systems and a recommendation on what size you will require:

Sizing A Hot Water System


The size of the electric system you will require will vary depending on your water usage, the number of people in your household and the tariff you are on. These recommendations should be treated as a guide only – if you have higher water usage than normal, you may need a larger sized electric hot water system.

Size guide:

Off-peak tariff

1-3 residents: 250l
3-5 residents: 315l
5-8 residents: 400l

Economy shoulder tariff

1-3 residents: 125l
3-5 residents: 160l
4-6 residents: 250l
6-8 residents: 315l

Peak supply tariff

1 resident: 25l
1-2 residents: 50l
2-3 residents: 80l
3-5 residents: 125l


A gas storage hot water system is a type of system that uses either natural gas or liquid petroleum gas (LPG). The tank is typically located above ground and the water is heated by a gas-fired burner.

Size guide:

1-3 residents: 90l
2-4 residents: 130l
3-5 residents: 170l
4-6 residents: 200l


Solar hot water systems are a great way to save energy and money. They work by using the sun’s energy to heat water, which is then stored in a tank for later use. One of the main advantages of solar hot water systems is that they can significantly reduce your energy bills. In fact, they often pay for themselves within just a few years. However, it’s important to keep in mind that solar hot water systems only heat water during the day.

Size guide:

1-2 residents: 180l
3-4 residents: 300l
5-6 residents: 440l

Heat pump

Heat pump water heaters are becoming increasingly popular due to their high level of energy efficiency. By using electricity to heat water, they can slash water heating costs by up to 60 per cent when compared to traditional electric resistance water heaters. Even more savings are possible if the heat pump water heater is connected to an economy tariff (off-peak 1 or off-peak 2).

Size guide:

1-2 residents: 200l or less
2-3 residents: 200-300l
3-4 residents: 300l or more

For more information or assistance choosing the right hot size hot water system for your requirements, contact the team at David Lewis Plumbing on (02) 6621 8088 or email admin@davidlewisplumbing.com.au.