what size hot water system do I need

What Size Hot Water System Do I Need? Use Our Calculator and Read the Guide

Choosing the right hot water system for your home can feel like a daunting task. After all, there’s a lot to think about. Heating a big hot water system that’s larger than you need is a waste of electricity (which isn’t cheap). Choose a smaller system and find yourself extra guests or a growing family and you might end up running out. And no one wants a cold shower.

So, how do you determine what size hot water system you need? Our guide and calculator below will help you figure it out.

How to Choose the Right Size Hot Water System

To choose the right size hot water system, you first need to look at how you use your hot water. Consider the number of bathrooms, appliances connected to hot water, and peak demand times.

Here’s what to do:

  • Count the Number of Bathrooms and Sinks: Start by counting the number of bathrooms and hot water taps in your home. Don’t forget your laundry if your washing machine is connected to hot water. Think about the amount of hot water you use each day.
  • Estimate Your Daily Hot Water Usage: A shower uses 9-15 litres per minute. An average bath uses 140 litres of water, with larger soaking tubs using up to 265 litres. Consider your dishwasher and washing machine, too. Do these use hot water or heat cold water? Do you have your mixer taps set to lukewarm or hot? How often do you wash up each day? Think about every time you turn on the hot water tap, not just for showering.
  • Work Out When You Use Your Hot Water: Is everyone using hot water at the same time, either before or after work? Consider when you use hot water, as this can impact whether you choose a larger storage system vs an instant hot water system.

Average Water Usage for Showers

The average water usage for a person taking a shower with a water-saving showerhead in Australia is approximately 54-72 litres. Here’s how this is calculated:

  1. A water-efficient or low-flow showerhead typically uses about 9 litres per minute.
  2. The average shower duration in Australia is around 6-8 minutes.
  3. Multiplying these figures gives us a range of 54-72 litres per shower (9 litres/minute × 6-8 minutes).

It’s worth noting that this represents a significant reduction compared to older, less efficient showerheads. For example:

  • A typical showerhead uses about 15 litres per minute, which would result in 90-120 litres per 6-8 minute shower.
  • Older style showerheads can use up to 19 litres per minute, potentially using 114-152 litres per 6-8 minute shower.

The Australian government encourages the use of water-efficient showerheads, as replacing a 15 L/min shower with a 3-star shower at 9 L/min can save a family of 4 about 70 kilolitres of water and $210 each year on water bills.

Average Usage Based on Family Size

Here are some calculations based on number of household members for electric hot water systems. These recommendations take into account that each family member showers at least once per day.

The off-peak systems are larger because they heat water during off-peak hours and store it for use throughout the day, while peak rate systems can heat water on demand. For families of 4 or more, it’s generally recommended to opt for the larger size within the given range to ensure an adequate supply of hot water, especially if multiple people are likely to shower around the same time.

Number of PeoplePeak SystemOff-Peak System
280 litres160 litres
3125 litres250 litres
4125-160 litres250-315 litres
5+160 litres or larger315-400 litres

Peak vs Off Peak Systems

The main differences between peak and off-peak rate systems are:


  • Peak rates apply during high-demand periods, typically weekday evenings (e.g. 2pm-8pm in summer, 5pm-9pm in winter).
  • Off-peak rates apply during low-demand periods, usually late night to early morning (e.g. 10pm-7am).
  • Some systems also have “shoulder” periods between peak and off-peak with intermediate rates.


  • Peak rates are the most expensive, often more than double the off-peak rates.
  • Off-peak rates are the cheapest, incentivising energy use during low-demand times.


  • Peak rates aim to reduce strain on the electricity grid during high-demand periods.
  • Off-peak rates encourage shifting energy use to times when overall consumption is lower.

Meter requirements:

  • To access time-of-use tariffs with peak/off-peak rates, you typically need a smart meter or compatible digital meter.


  • Peak/off-peak systems benefit households that can shift major energy usage to off-peak times.
  • They may not be ideal for those who use most energy during peak periods.

Appliance considerations:

  • Some hot water systems may be on dedicated off-peak circuits.


  • Users on time-of-use tariffs need to be more conscious of when they use energy to maximise savings.

To take advantage of peak/off-peak rates, you need to be on a time-of-use tariff, have a compatible meter, and be able to adjust your energy usage patterns to favor off-peak times.

Try Our Hot Water System Size Calculator

The team at FullFlow Plumbing and Drainage have created a calculator to help you determine what size hot water system you need. The recommendation is based on 50 litres of water being used per shower (ie – a 5 minute shower). If you have a teenager in your home who loves to spend 20 minutes washing their hair, consider a larger size than recommended.

Australian Hot Water System Size Calculator

Types of Hot Water Systems Available

Choosing the right hot water unit for your needs depends on various factors, including your household size, energy preferences, and the amount of water you use daily. Below are the main types of hot water systems, each with its unique features and benefits.

Electric Hot Water Systems

Electric hot water systems are widely used for their ease of installation and availability. These systems primarily come in two types: storage and instantaneous.

Storage electric systems heat water and store it in an insulated tank, making hot water readily available. These tanks typically range from 25 litres to 400 litres, with larger sizes suited for bigger households.

In contrast, instantaneous hot water systems (also known as tankless systems) heat water on demand, offering an energy-efficient alternative because there is no heat loss from stored water.

These systems are great for homes with limited space. However, they might not be able to manage well if multiple outlets require hot water simultaneously.

Gas Hot Water Systems

Gas hot water systems use natural gas or LPG to heat water, often proving more cost-effective than electric options in areas with access to gas. These systems are available as both storage tanks and instantaneous units.

Storage gas systems retain water in a tank, commonly sized between 135 litres and 170 litres, providing consistent hot water supply.

Instantaneous gas systems, also known as continuous flow systems, heat water only when needed. This saves on storing water, but you do need to run water through at a decent pressure to keep the pilot light going.

They are suitable for large families and households with higher hot water demands, as they can provide hot water continuously without running out.

Solar Hot Water Systems

Solar hot water systems leverage solar energy to heat water, reducing reliance on electricity or gas and cutting down on electricity bills. These systems incorporate solar collectors installed on the roof and a storage tank. The system heats the water using solar energy during the day, and excess water can be stored for later use.

There are two types: active systems, which use pumps to circulate water, and passive systems, relying on gravity and natural convection. For cloudy days or high-usage times, a booster—usually electric or gas—is included to ensure a consistent hot water supply.

When choosing your solar hot water system size, important factors include the number of people in your home, local climate, and available roof space for collectors. Proper sizing of both the storage tank and the solar collector area is essential.

Heat Pump Hot Water Systems

Heat pump water heaters function by transferring heat from the air to heat the water, making them highly energy-efficient. These systems work similarly to a refrigerator but in reverse and can be up to three times more efficient than conventional electric systems.

They have a storage tank where heated water is kept. Heat pump systems are ideal for moderate to warm climates and a popular choice for Brisbane and Gold Coast homes.

However, the initial installation cost may be higher, though the energy savings often compensate for the upfront expenses over time.

Things to Consider When Buying a New Hot Water System

Choosing the right hot water system involves balancing upfront costs, ongoing savings, and potential government incentives. Each factor impacts your long-term budget and energy efficiency.

Temperature Rise

Temperature rise refers to the increase in temperature needed to heat incoming cold water to the desired hot water temperature. It’s an important factor in choosing the right size in colder climates and not as important for South East Queensland homes, given our milder climate. A larger temperature rise requires more energy and potentially a larger system capacity to meet your hot water needs efficiently.

Upfront Costs vs. Long-Term Savings

Hot water systems vary in initial costs depending on the type and size. Electric hot water systems are often cheaper to install but may have higher running costs. On the other hand, gas systems usually have moderate installation costs and better long-term savings due to lower energy usage. Investing in solar power or heat pump systems can be costly initially, yet they offer significant savings on energy bills over time.

Off-peak tariffs can reduce electricity costs for electric storage systems, making them more cost-effective in the long run. Consider how much hot water your household uses to determine the most economical option.

Maintenance and Longevity

The longevity and maintenance needs of your hot water system affect ongoing costs. Electric storage systems require regular checks and have a lifespan of around 10-15 years.

Solar and heat pump systems often come with higher maintenance costs but can last 15-20 years. Regular upkeep ensures energy efficiency and prevents the inconvenience of running out of hot water. Consider both the cost and frequency of maintenance when selecting your system to avoid unexpected expenses.

Government Incentives and Rebates

The Australian government offers incentives and rebates that can reduce the overall cost of installing energy-efficient systems. Programs like the Small-scale Renewable Energy Scheme (SRES) provide financial benefits for choosing solar power or heat pump systems.

Rebates for switching to energy-efficient systems can lower upfront costs significantly. Check eligibility criteria and application processes to take full advantage of these incentives. Exploring these options can make a costly investment in green technology more affordable and beneficial for your household.

Lifestyle Changes

When selecting a hot water system, consider your lifestyle changes over the next 10 years. A growing family or adding new occupants can increase hot water consumption.

Opt for systems that provide flexibility. Continuous flow units can be advantageous since they don’t rely on a tank and can handle varying demand. If you anticipate lifestyle changes, ensure your system can scale up or down without requiring complete replacement. This can save time, money, and hassle in the long term.

Technological Advancements

Emerging technologies can significantly enhance energy efficiency and sustainability.

Modern hot water systems might feature solar power integration, smart controls, and energy-efficient heating elements.

Investing in a system compatible with future enhancements can reduce energy costs and environmental impact.

For example, hybrid systems that use both electricity and solar power can adapt to technological advancements and energy tariffs. Installing a system that can integrate with home automation and smart devices will also make your heating more efficient and convenient. Speak to your plumber about options that offer these features.

Frequently Asked Questions

How does household size affect the hot water system size I need?

Household size directly impacts the size of the hot water system needed. Generally, larger households require larger systems to meet increased hot water demand.

What’s the difference in sizing for electric, gas, and solar hot water systems?

Electric systems typically need larger storage tanks than gas systems. Solar systems require proper sizing of both storage tanks and solar collectors.

How do peak usage times impact the size of hot water system required?

Peak usage times influence system size as larger systems are needed to meet high demand periods. Off-peak systems require larger storage to provide hot water throughout the day.

Should I choose a tank or continuous flow system for my home?

Storage tanks provide a reserve of hot water but can run out, while continuous flow systems heat water on demand. Choose based on household size and usage patterns.

What role does climate play in determining the right hot water system?

Colder climates generally require larger systems due to increased hot water usage and greater heat loss. Warmer climates may allow for smaller systems.

How do energy efficiency and running costs factor into sizing decisions?

More energy-efficient systems may have higher upfront costs but lower running costs. Consider both initial and long-term expenses when sizing.

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

For a family of four, typical recommendations are 125-160L for electric storage systems and 135-160L for gas storage systems. Instantaneous systems should be sized based on the number of simultaneous hot water outlets used.

How do shower head types and flow rates impact system size requirements?

Water-efficient shower heads reduce hot water consumption, potentially allowing for a smaller system. High-flow shower heads increase demand, requiring larger systems.

How does the temperature rise needed affect hot water system sizing?

A larger temperature rise requires more energy, potentially necessitating a larger system. Factor in your cold water temperature and desired hot water temperature.

How do I determine if I need a larger tank or a higher flow rate for my needs?

Choose a larger tank for high simultaneous demand or if hot water is needed over extended periods. Opt for a higher flow rate if quick hot water recovery is more important.