Instant Hot Water Sizing Calculator Australia
Work out the right electric instant or gas continuous flow hot water system for your home. Tell us which taps and showers run at once and what power and gas you have, and get the size you need and the units that fit.
- Electric instant and gas continuous flow
- Sized for your winter mains water
- Checks single phase vs 3-phase power
Hot water units that fit your home
Each unit below covers your hot water in winter.
How to size an instant hot water system
An instant hot water system is sized on the hot water it has to make at the busiest moment, not on the number of people in the house. Add up the showers and taps that run at the same time, then work out how much heat it takes to lift your cold winter mains water to shower temperature. For one standard 9 litre a minute shower in a Sydney winter, that is about 17 kW of electric power, or a gas unit rated 20 litres a minute.
The two things people miss are the season and the switchboard. Mains water in winter can be 6 to 10 degrees colder than in summer, so a unit that feels fine in January can run lukewarm in July. And the bigger electric units need 3-phase power. This calculator checks both, then matches units from our range that pass.
Quick guide: electric power needed by city
Power needed in winter at 40°C, using standard 9 litre a minute showerheads.
| City | One shower | Two showers | Kitchen sink |
|---|---|---|---|
| Brisbane | 15.1 kW | 30.1 kW | 12.1 kW |
| Perth | 16.3 kW | 32.7 kW | 13.0 kW |
| Sydney | 17.0 kW | 33.9 kW | 13.4 kW |
| Adelaide | 18.2 kW | 36.4 kW | 14.2 kW |
| Melbourne | 18.8 kW | 37.7 kW | 14.7 kW |
| Canberra | 20.7 kW | 41.4 kW | 15.9 kW |
What each size really gives you in winter
Litres a minute of 40°C water. A standard shower uses about 9, a water-saving one about 7.
| Unit | Brisbane | Sydney | Melbourne | Canberra |
|---|---|---|---|---|
| 9.5 kW electric, single phase | 5.7 | 5.0 | 4.5 | 4.1 |
| 19.4 kW electric, 3-phase | 11.6 | 10.3 | 9.3 | 8.4 |
| 28 kW electric, 3-phase | 16.7 | 14.9 | 13.4 | 12.2 |
| 20 L/min gas | 20.8 | 18.5 | 16.7 | 15.2 |
| 26 L/min gas | 27.1 | 24.1 | 21.7 | 19.7 |
Electric or gas?
Electric instant units are compact, need no flue and suit point of use jobs like a basin, a kitchen sink or an ensuite. A single phase home can run a small unit up to about 9.5 kW. For a whole house on electric you need 3-phase power and usually the 19.4 or 28 kW size.
Gas continuous flow wins when two showers run at once. A 20 litre a minute unit handles two showers in Brisbane, Perth and Sydney. In Melbourne, Adelaide and colder areas, choose 26 litres a minute. If you are on LPG bottles, check the unit is approved for LPG before you buy.
Three tips that save money
- Fit water-saving showerheads. Dropping from 9 to 7 litres a minute cuts the power needed by about a fifth, and often lets you buy one size smaller.
- Stagger showers. If two people rarely shower at once, size for one shower plus a basin, not two showers.
- Keep bathrooms at 50°C. Australian rules limit hot water to bathrooms to 50°C for new installs. Units with a 60 or 70°C setting need a tempering valve on the bathroom line.
Not sure which way to go? Send us your postcode and what runs at once and we will check it for you, or browse all instant hot water systems.
Instant hot water size: common questions
What size instant hot water system do I need for one shower?
For one standard 9 litre a minute shower at 40°C in winter, you need about 15 kW in Brisbane, 17 kW in Sydney and 19 kW in Melbourne, so a 19.4 kW electric unit covers most of Australia. Canberra and Tasmania need about 21 kW, so step up to 28 kW there. A gas unit rated 20 litres a minute covers one shower anywhere.
Can I run an instant electric hot water system on single phase power?
Yes, but only small units. The largest single phase models are around 9.5 kW, which gives about 5 litres a minute at 40°C in a Sydney winter. That suits a basin, a kitchen sink or a water-saving shower in a warm climate. A whole-house electric system needs 3-phase power.
How many showers can a 28 kW instant hot water system run?
In winter a 28 kW unit gives about 13 to 17 litres a minute at 40°C, depending on where you live. That is one standard shower plus a basin, or two water-saving 7 litre a minute showers in warmer areas like Brisbane. Two standard showers at once need gas or two units.
Is a 20 or 26 litre gas continuous flow unit better?
A 20 litre unit suits one or two bathrooms and runs two showers at once in Brisbane, Perth and Sydney. In Melbourne, Adelaide and colder areas, two showers at once need a 26 litre unit, because the colder mains water takes more heating.
Why does the calculator ask for my postcode?
Because the temperature of the incoming mains water changes the power needed. Winter mains water is about 16°C in Brisbane but around 10°C in Melbourne and 7°C in Canberra, so the same shower needs about a third more power in Canberra.
How do I know if I have 3-phase power?
Look at your switchboard. A main switch with three linked levers, or a meter labelled 3-phase, usually means you have it. If you are not sure, ask a licensed electrician before choosing a whole-house electric unit.
Want us to check it before you order?
Tell us your postcode, how many bathrooms and what power and gas you have, and we will confirm the right unit, free. Australian owned, based in Perth, delivering nationwide.