Sizing hot water is two questions, not one
Ask "what size hot water system do I need" and you get a number in litres. That is only the right answer for a storage system, where the whole job is holding enough hot water to get the household through a day and recover overnight.
A continuous flow unit holds nothing. It heats water as it passes through, so the question is not how many litres a day but how many litres a minute it has to deliver at once. Two showers and a kitchen tap is a flow problem. A 250L tank and a 26 L/min instantaneous unit can serve the same house and neither number tells you anything about the other.
The calculator above answers both, so you can compare a tank and an instant on the same job without doing the sum twice.
Two temperatures that are not sizing choices
Whatever system goes in, two figures come from AS/NZS 3500.4 and Australian public health requirements rather than from the sizing:
- Stored water is kept at 60°C or above. This is about Legionella, which grows in warm water and is knocked down by hot. Turning a tank down to save energy is not a trade-off a plumber gets to make.
- Water delivered to a sanitary fixture used for personal hygiene in a home is tempered to 50°C maximum. That is a scald limit — a small child can take a full-thickness burn from 60°C water in about a second. It is done with a tempering valve or a thermostatic mixing valve on the outlet, not by turning the tank down.
Store hot, deliver warm. Those two rules pull in opposite directions on purpose, and the valve in between is what makes both true at once. Hot water work is licensed work — the install, the valve and the certificate of compliance are a licensed plumber's job.
A worked example
Four people, average usage, a Sydney house:
- Baseline: 50 litres per person per day at 60°C
- 4 × 50 = 200 L, times the temperate climate factor of 1.10 = 220 L a day
- Next standard tank up from 220 L is a 250 L unit
- Same house on continuous flow: two showers at 9 L/min each = 18 L/min, so a 20 or 26 L/min unit depending on winter inlet temperature
Storage size by household, average usage
| People | Warm (QLD, NT) | Temperate (Sydney, Perth) | Cool (Melbourne, Tas) |
|---|---|---|---|
| 1 | 80 L | 80 L | 80 L |
| 2 | 125 L | 125 L | 125 L |
| 3 | 160 L | 170 L | 250 L |
| 4 | 250 L | 250 L | 250 L |
| 5 | 250 L | 315 L | 315 L |
| 6 | 315 L | 400 L | 400 L |
These come straight from the sum above, using the same standard tank sizes the calculator picks from. They are a starting point for a quote, not a substitute for asking the customer how they actually live.
Off-peak electric needs a bigger tank
An off-peak electric storage system only heats overnight, so the tank has to carry the whole day's hot water plus a margin — there is no top-up at 4pm. As a rule of thumb, step up a size or two from the figure above. A four-person house on continuous tariff might sit fine on 250 L; the same house on controlled load 1 usually wants 315 L or 400 L.
Picking the technology
- Gas storage — recovers fast, so it forgives a slightly undersized tank. Needs flue clearances and a gas supply that can carry it.
- Gas continuous flow — endless hot water, sized on flow. Ratings are quoted at a temperature rise, and winter inlet water is colder, so a 26 L/min unit does not deliver 26 L/min in July in Melbourne.
- Electric storage — cheapest to buy, dearest to run on peak tariff. Size up for off-peak.
- Heat pump — very efficient, but recovery is slow, it is noisy enough to matter next to a bedroom window, and cold ambient air hurts output. Do not undersize one.
- Solar with boost — sized on collector area and tank together. The tank is usually larger than the equivalent gas or electric unit because it is banking a sunny day.
What people forget on site
- Dead legs. A long uninsulated run to an ensuite wastes litres and makes the customer think the system is undersized when it is not.
- Peak, not average. Four people showering in forty minutes is the sizing case. A daily total that looks comfortable can still run out at 7:15am.
- Relief valve discharge and seismic restraint. Neither changes the size, and both will change the install.
- Replacements are not like-for-like. The old unit might have been the wrong size for years, and the customer may have simply lived with it.
Quoting a hot water changeover
The unit is the obvious line. The ones that eat the margin are the tempering valve, the isolation and relief plumbing, the electrical or gas connection, the base or bracket, disposal of the old unit, and the time spent draining a 250 L tank that has been in place for fifteen years. Price them as their own lines and the customer can see what they are paying for.
Sized the system. Now get the job priced and paid.
Yamate turns the unit, the valve and the hours into a quote the customer accepts on their phone, then an invoice, then a payment — with GST handled and the follow-ups chased for you.
Try Yamate free → Free to try · no card · sign in with a link, no password to rememberHot water system sizing FAQ
What size hot water system do I need for 4 people?
About 220 litres a day in a temperate climate, which points to a 250 litre storage tank. On continuous flow, two showers at once needs roughly 18 litres a minute, so a 20 or 26 L/min unit. Off-peak electric should step up to 315 litres.
Why does hot water have to be stored at 60 degrees?
To control Legionella bacteria, which multiply in warm water and are killed at hot storage temperatures. It is a health requirement, not an efficiency setting, so a tank is never turned down below it to save running cost.
Why is the water at the tap only 50 degrees then?
Because 60°C water can give a small child a serious burn in about a second. Water delivered to a bathroom fixture in a home is tempered down to a 50°C maximum by a tempering or thermostatic mixing valve on the outlet, while the tank itself stays hot.
How many litres of hot water does one person use a day?
Around 50 litres a day at 60°C for a normal household. Showers only with efficient fittings drops it to about 35. Baths, long showers or a spa push it to 75 or more.
Do I need a bigger tank for off-peak electric?
Yes. An off-peak system only heats overnight, so the tank carries the entire day with no top-up. Step up one or two sizes from the figure the calculator gives.
How do I size a continuous flow hot water unit?
On simultaneous flow, not daily volume. Add up the outlets that could run at the same time — a 9 L/min shower plus another 9 L/min shower is 18 L/min. Then check the unit rating at the temperature rise you actually need in winter, because the headline figure assumes warmer inlet water.
Can I install a hot water system myself?
No. Hot water installation in Australia is licensed plumbing work, and gas or electrical connection is licensed work too. This calculator is a sizing aid for the trade, not a DIY guide.
Other calculators you'll want
General information for Australian trades, and an estimate only — not tax, financial or legal advice. It is not engineering, building or design advice either: a calculator works out a quantity from the figures you enter, and anything that needs sign-off still needs your engineer, certifier or building surveyor. Sizing figures are typical Australian baselines published in hot-water guidance and vary with fittings, habits, inlet temperature and system type. The 60°C storage minimum and 50°C tempered delivery maximum are safety requirements — confirm the current wording against AS/NZS 3500.4 and your state's regulator. Hot water installation is licensed plumbing work; this calculator is an estimating aid, not an installation instruction or a specification.