When it comes to hot water systems, size actually matters. So whether you’re upgrading or installing a new system, you must pick the best-sized unit for your daily needs.

This article will tackle some of the most important factors when determining the right size for your hot water system. By keeping them in mind, you’ll optimise your comfort and ensure you have an energy-efficient and cost-effective unit.

Water Usage

Understanding water usage is crucial in selecting the appropriate heater size. If you have a high demand for hot water due to a large household or frequent use, a bigger unit may be necessary to ensure an adequate supply. But if your water usage is relatively low, opting for a minor system can help you avoid unnecessary expenses.

Tips to Determine Your Water Usage

  • Monitor the Water Meter. Keep track of your household’s water meter readings over a specific period, such as a week or a month. Record the starting and ending readings to calculate the total water consumption during that period.
  • Time Hot Water Usage. Use a stopwatch or a timer to measure the duration of hot water usage for activities like bathing, dishwashing, and cleaning. Multiply the average time by the number of times these activities occur daily to estimate daily hot water usage.
  • Use Water Flow Rate Devices. Install flow rate devices on faucets and showerheads to measure the gallons per minute (GPM) flow rate. This information can help assess simultaneous hot water demand during peak times.
  • Assess Appliance Usage. Identify hot water-consuming appliances like washing machines and dishwashers. Check their water usage specifications or use a flow rate device to measure their GPM. Multiply the flow rate by the frequency of use to estimate daily hot water consumption.
  • Consider Seasonal Variations. Account for seasonal variations in water usage. Hot water demand might increase in colder months due to longer showers.
  • Review Water Bills. Analyse past water bills to get an idea of your household’s average daily water consumption. Compare previous bills during different seasons to understand any variations in usage.

Household Size

Your household size and the number of occupants directly influence your hot water demand. The larger the family, the more hot water is usually required. When considering the right size for your hot water system, you should consider the number of people living with you.

When considering the hot water demand, take into account various activities that contribute to the consumption. These activities include showering, dishwashing, laundry, and other hot water usage in your household.

Identifying peak usage times is also vital in estimating how much hot water you need during specific periods. For example, mornings or evenings might be the peak showering times, requiring a higher flow rate to meet the demand.

What Size Water Heater Do I Need for a Family of 4?

A family of four in Australia typically requires a water heating system that can provide around 125 to 360 litres daily. Bear in mind that this size will depend on certain factors, including the type of heater used and the frequency of water consumption.

What Size Hot Water System Do I Need for 3 People?

The average Australian uses around 76 litres of hot water daily, so we can safely assume that a 3-person household consumes up to 228 litres per day. Again, this will fluctuate based on the factors mentioned previously.

Electric Hot Water System Size Guide (Storage)

Household Size Off-Peak Tariff (Tank Size) Continuous Tariff (Tank Size)
1 - 2 people 125 litres 50 litres
2 - 3 people 160 litres 80 litres
3 - 4 people 250 litres 125 litres
4 - 6 people 315 litres 160 litres
5 - 9 people 400 litres 250 litres

Gas Hot Water System Size Guide (Storage)

Household Size Tank Size
1 - 2 people 90 litres
2 - 3 people 130 litres
3 - 4 people 170 litres
4 - 6 people 200 litres

Gas Instant Hot Water System Size Guide

Number of Bathrooms System Size
1 16 litres
1 - 2 20 litres
2 - 3 26 litres
3 or more 32 litres

Hot Water Size Calculator

With varying household sizes and energy tariff options, it can be challenging to determine the ideal system capacity. Our Hot Water Heater Size Calculator simplifies this process for you. By inputting your household size and preferred tariff type, you’ll receive a recommended tank size tailored to your needs.

Type of Hot Water System

When determining the size of a hot water system, it’s also essential to consider the type of hot water system and energy source. Your choice can have a significant impact on the size and capacity requirements of your unit.

Common Tank-Based Systems Available in Australia

  • Gas Systems. These heaters are powered by liquid petroleum gas and are known for quick heat recovery and efficiency. They’re a popular choice for households with high hot water demand. When sizing a gas hot water system, it’s important to consider the gas supply available and the usage patterns of hot water in your household.
  • Electric Systems. Electric water heaters are standard in many homes and generally easier to install and maintain than other options. However, they tend to have higher running costs and may have longer recovery times. Sizing an electric system involves considering the electricity supply available and the number of appliances requiring hot water.
  • Solar Systems. Solar hot water systems utilise sun energy to heat water, making them the most environmentally friendly option. When determining the right size for a solar heater, you need to consider the amount of sunlight available in your area and the size of the collector and storage tank needed to meet your hot water demands.
  • Heat Pump Systems. Heat pump hot water systems extract heat from the air or ground to heat water. They are highly energy-efficient but may have higher upfront costs. To determine the right size for your heat pump system, check the temperature of the air or ground and the hot water demand in your household.

Temperature Rise

To determine the correctly sized hot water system for your needs, you must also understand the concept of temperature rise. Temperature rise refers to the increase in temperature needed by the heater to ensure the hot water produced has the desired temperature.

The greater the required temperature rise is, the larger your hot water system must be. Thus, you need to consider it when picking a new heater for your home.

How to Calculate Temperature Rise

To determine the temperature rise, you must consider two key factors: the incoming cold water temperature and the desired hot water temperature. The formula is simple:

Temperature Rise = Desired Hot Water Temperature – Incoming Cold Water Temperature

So let’s say your desired hot water temperature is 48°C. If the incoming cold water temperature is 15°C, then the temperature rise would be 33°C.


The climate of your region not only affects the temperature of your tap water; it can also influence the size of the hot water system you need. If you live in a colder area, you might need a larger heater to ensure you enjoy an adequate hot water supply. But if you’re located somewhere warm and humid, you can settle with a smaller unit as there’s less need for hot water.

Tips to Factor in Climate When Determining Your Heater’s Size

  • Check the Incoming Water Temperature. Take into account the average incoming water temperature in your area. Hot water systems must work harder to reach the desired temperature in colder regions where the water is colder. This means you may require a more extensive capacity system to meet your hot water needs during colder months.
  • Consider Heating Efficiency. Warmer climates allow for greater heating efficiency, as the water heater will have to raise the temperature less to reach the desired hot water level. A minor capacity system may suffice to provide an adequate hot water supply in such areas.
  • Remember Seasonal Variations. Consider seasonal variations in your region. Hot water usage may increase during winter or colder months due to longer hot showers. Factor in these variations when sizing your hot water system to ensure it can meet the peak demand.
  • Get Solar Water Heaters. Solar water heaters work best in sunny climates with ample sunlight. A Cambridge Energy Alliance study even found that the solar heating tank alone can provide enough hot water for your needs during warmer months.
  • Choose Heat Pump Water Heaters. Warmer climates are more favourable for heat pump water heaters as they rely on extracting heat from the surrounding air or ground. In colder climates, the efficiency of heat pump systems may decrease, necessitating a different unit.


The price of a hot water system can significantly impact its size. Smaller-sized systems may fetch cheaper but also have reduced capacity and features. Meanwhile, higher-priced models often offer larger tanks and more perks.

Opting for a more expensive, more extensive heating system may provide better performance and meet the hot water demands of a larger household. Thus, balancing your budget with the system’s capacity is essential to ensure cost-effectiveness.

Tips to Save Money When Determining Your Heater’s Size

  • Assess Your Budget. Determine your budget for the hot water system upfront. Be clear about how much you will spend on the purchase, installation, and potential ongoing maintenance.
  • Compare Prices. Research and compare prices of different hot water systems with varying capacities. Look for models that fit your budget while meeting your hot water requirements.
  • Energy Efficiency. Consider the long-term savings on energy bills by opting for an energy-efficient system. A higher initial investment in a more efficient system may result in lower operating costs.
  • Consider Off-Peak Electricity Rates. Off-peak times in Australia are often between 10 pm and 7 am. Heating your water during these hours will help you save up on energy bills. You can save enough hot water for the day by choosing a larger heater.
  • Size Vs Efficiency. Strike a balance between the hot water system’s size and energy efficiency. A larger system may offer better performance but at a higher upfront cost. A smaller system may be more affordable but might not adequately meet your needs.
  • Consider Financing Options. Explore financing options, incentives, or rebates offered by manufacturers or government programs that can help reduce the initial financial burden of a larger and more efficient hot water system.

Energy Efficiency

Having an energy-efficient hot water system offers two benefits. One, it can help you reduce your utility bills. Two, it allows you to get smaller units since they can deliver the same amount of hot water larger, less-efficient systems do.

Additionally, a study by the National Renewable Energy Laboratory noted that centralising your water heater can increase energy efficiency by 35%. So if you’re environmentally conscious or cost-savvy, getting an energy-saving heater is a must.

Tips to Find the Best Energy-Efficient Water Heater

  • Look for Energy Star Rating. Check for water heaters with the Energy Star rating, which indicates compliance with strict energy efficiency standards set by the Australian government. Higher-star-rated models are more energy-efficient and can lead to reduced electricity or gas consumption.
  • Consider Solar Water Heaters. Solar water heaters harness the sun’s energy to heat water, making them highly energy-efficient and environmentally friendly. Solar water heaters can be a cost-effective and sustainable option in Australia’s sunny climate.
  • Check for High Energy Factor (EF). Evaluate the Energy Factor (EF) of different water heaters. A higher EF indicates better energy efficiency and lower operating costs. Look for models with higher EF values to ensure optimal efficiency.
  • Insulation and Tank Material. For tank-based water heaters, consider models with better insulation and high-quality tank materials. Well-insulated tanks and durable materials can reduce standby heat loss and improve efficiency.
  • Explore Heat Pump Water Heaters. Heat pump water heaters are energy-efficient alternatives that work by extracting heat from the surrounding air. They can be an excellent option for Australian climates and significantly reduce energy consumption.

Building Type

The type of building plays a crucial role in determining the size of your hot water system. Larger facilities like multi-story homes or commercial properties require higher hot water capacities to meet the demand of multiple occupants. In contrast, an apartment or smaller buildings may only need a modest heater due to their lower hot water requirement.

Tips to Factor in Building Type When Determining Your Heater’s Size

  • Assess Occupancy. Evaluate the number of occupants in the building. Larger buildings with more occupants have higher hot water demand, requiring a wider capacity system.
  • Number of Fixtures. Consider the number of fixtures in the building, including bathrooms, kitchens, and laundry rooms. More fixtures mean a higher hot water requirement, necessitating a larger system.
  • Peak Usage Times. Consider peak hot water usage times in the building. Buildings with simultaneous hot water usage, such as during mornings or evenings, require a system that can handle higher flow rates.
  • Space Availability. Assess the available space for installing the hot water system. Larger buildings may have more room for accommodating larger tank-based systems or multiple tankless units. In comparison, smaller establishments may require more space-efficient solutions.
  • Consult Professionals. Seek advice from a plumbing expert or heating specialist who can analyse the building’s hot water needs and recommend the appropriate system size based on the specific building type and usage patterns.

Plumbing Infrastructure

Having older or inefficient plumbing can slow down, if not clog, the water flow rate in your home, necessitating a larger heater to meet your hot water demands. By fixing or replacing your plumbing infrastructure, you’ll improve water flow and increase your options when looking for new water heating systems.

Tips When Fixing Your Home Plumbing

  • Check for Leaks. Regularly inspect all pipes, connections, and fittings for any signs of leaks. Leaks can waste water and decrease the efficiency of your water heating system. Fix any leaks promptly to prevent further damage and conserve water.
  • Insulate the Pipes. Insulate hot water pipes to reduce heat loss and ensure hot water reaches your fixtures faster. This will save energy and improve the overall performance of your water heating system.
  • Flush Out Sediment. Periodically flush the water heater tank to remove sediment buildup. Sediment can affect the efficiency of the heating element and reduce the amount of available hot water. Flushing the tank will help maintain optimal performance.
  • Check Pressure Valves. Inspect the temperature and pressure relief valves on your water heater. These safety features are essential for preventing excessive pressure buildup and potential tank ruptures. Test and replace the tap if necessary.
  • Address Water Temperature Issues. If you’re experiencing inconsistent water temperatures, check the thermostat settings on your water heater. Adjust them to ensure your water heater provides water at the desired temperature.

Future Expansion

Considering future changes is essential when determining the size of your hot water system. As your household grows in size, the demand for hot water increases. You might need a larger water heater to meet this growing need.

Moreover, suppose you plan to expand your home or add new fixtures in your bathroom, sink, and kitchen. To keep enjoying hot water, you need to make sure your heater is big enough to accommodate these changes.

Installing a hot water system with sufficient capacity from the outset ensures it can accommodate additional family members or occupants without sacrificing comfort or performance. This proactive approach saves you from replacing or upgrading the system later, avoiding extra costs and inconvenience.


Determining the right size for your hot water system is crucial to meet your household’s hot water demands efficiently. By considering the factors listed above, you can make a well-informed decision and find the perfect unit for your house.