Hot Water System Calculator Australia — Capacity & System Sizing
Household details
Recommended capacity
Usage estimate
Storage system sizing
Continuous flow (instantaneous)
✓ Recommended system for your household
This hot water calculator estimates the appropriate storage capacity (litres) and continuous flow rate (L/min) for Australian households based on occupant numbers, usage patterns, and climate zone. The sizing model uses industry-standard figures from the Australian Institute of Refrigeration, Air Conditioning and Heating (AIRAH) and AS/NZS 3500.4 (Hot Water Services): approximately 50 L per person per day at 60°C is the baseline, adjusted upward for higher usage and colder climates. Off-peak electric storage tanks are typically sized at 1.5× the daily demand to account for the overnight-only heating window. Solar and heat pump systems are sized differently — consult a licensed plumber for those.
Frequently Asked Questions
What size hot water system do I need for a family of 4 in Australia?
For a family of four with average usage, the typical recommendation is: 315–360 L electric off-peak storage; 160–200 L gas storage; or a continuous flow gas unit rated at 20–24 L/min. Cold climates require slightly larger systems. Off-peak electric tanks are sized larger because they only heat overnight. Gas storage units reheat quickly and can be smaller.
What is the difference between off-peak and continuous electric hot water?
Off-peak (controlled load) electric hot water heaters are connected to a separately metered circuit that is only energised during designated off-peak periods — usually overnight (10 pm to 7 am) at a lower electricity tariff. This requires a larger tank to store enough hot water for the whole day. Continuous supply systems heat whenever the thermostat calls for it and can be smaller, but use more expensive peak-rate electricity.
Is a heat pump hot water system worth it in Australia?
Heat pump hot water systems are one of the most energy-efficient options for Australian homes, using 60–75% less electricity than a conventional electric element by extracting heat from ambient air. They are eligible for STCs (Small-scale Technology Certificates) under the Australian Renewable Energy Target, providing an upfront rebate. They perform best in temperate and subtropical climates where ambient temperatures stay above 10°C for most of the year.
What temperature should hot water be set to in Australia?
AS/NZS 3500.4 requires hot water to be stored and delivered at a minimum of 60°C to prevent Legionella bacteria growth. However, water above 50°C can cause scalding, particularly for young children and elderly people. A tempering valve (thermostatic mixing valve) must be installed on new systems to deliver water at no more than 50°C to bathrooms and 60°C to kitchens and laundries, per the NCC.
What flow rate do I need for a continuous flow (instantaneous) hot water system?
A single shower uses approximately 7–9 L/min of hot water (at mixed temperature). For one simultaneous shower, a 16 L/min unit is typically sufficient. For two simultaneous showers, 20–24 L/min is recommended. Large households with three or more simultaneous outlets should consider 26–32 L/min units or a gas storage system instead. Always factor in the incoming cold water temperature for your climate zone.