What size hot water system do I need?
Answer four questions about the household and see the right size for every type of system: electric off-peak and any-time, gas storage, gas continuous flow, heat pump and solar. Show the customer, then quote it.
Size a hot water system
No sign-up. Four questions, every type of system compared, and nothing is uploaded.
Open the calculatorOpens in a new tab. Works on a phone, tablet or computer.
What it does
- Every type side by side: electric storage on off-peak and any-time tariffs, gas storage, gas continuous flow, heat pump and solar with a booster.
- From the household, not guesswork: people in the home, showers running at once, how much hot water they use, and the climate.
- Common sizes: rounded up to tank sizes and continuous flow ratings you can buy, with the next size up for heavier use or a growing family.
- Show the customer: clear enough to go through on site, and you can copy the sizes into a quote or message, or print them.
Typical hot water system sizes
| People | Off-peak electric | Gas storage | Continuous flow | Heat pump |
|---|---|---|---|---|
| 1–2 | 125 L | 90 L | 16 L/min | 160 L |
| 3–4 | 250 L | 135 L | 26 L/min | 180–250 L |
| 5 | 315 L | 170 L | 32 L/min | 315 L |
| 6 | 400 L | 260 L | 32 L/min | 400 L |
Average use in a mild climate. Continuous flow assumes one shower at a time for 1–2 people, two for 3–4 and three for 5–6.
How to use it
- Count the people who live there, and the showers that can run at the same time.
- Pick how much hot water they use and where the home is.
- Compare the systems with the customer and pick the type that suits their tariff, gas supply and budget.
- Check the manufacturer’s chart for the model, then quote it.
Common questions
What size hot water system does a family of four need?
As a rough guide for average use in a mild climate: about 250 L for off-peak electric, 125 L for electric on an any-time tariff, 135 L for gas storage, 26 L/min for gas continuous flow with two showers at once, 250 L for a heat pump and about 300 L for solar. The calculator adjusts for use and climate.
Why is an off-peak tank so much bigger?
Off-peak electric only heats overnight, so the tank has to hold the whole day’s hot water. Gas storage and any-time electric reheat during the day, so a smaller tank covers the morning rush.
How do I size a continuous flow unit?
By the hot water running at once, not the number of people. One shower needs about a 16 L/min unit, two showers at once 26 L/min, and three 32 L/min. Colder water needs more heating, so cold climates may need a size up.
What temperature should hot water be?
Storage systems must keep water at 60 °C or hotter to control Legionella, and new installations limit hot water to bathrooms to 50 °C (AS/NZS 3500.4), usually with a tempering valve.
How accurate is it?
It gives typical starting sizes from the household’s daily hot water use, rounded up to common tank sizes and gas ratings. Always check the manufacturer’s sizing chart for the model, and the household’s real habits.
Is anything uploaded?
No. It all runs in your browser, works offline once open, and remembers your last answers on your device.
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