What size hot water system? Peak hour, not people
Every sizing chart starts with "how many people". That is the wrong question, and it is why a household of three can run out while a household of five never does. What decides it is your busiest twenty minutes.
There is a conversation that happens in most Australian laundries and it goes like this: the old system has died, the plumber asks how many people live here, someone says four, and a tank the same size as the old one goes in. Sometimes that works. When it does not, the household spends the next decade rationing showers.
The reason is that the number of people in a house tells you how much hot water gets used in a day. What decides whether you run out is how much gets used at once.
Why "how many people" is the wrong question
Take two households. In the first, four people all shower between 7:00 and 7:30 on a weekday, and the dishwasher and washing machine are set to run in the morning too. In the second, five people shower across the whole evening, one at a time, and the appliances run overnight.
The second household uses more hot water. The first one is the one that runs out. Same daily total, completely different demand at the moment it matters, and only one of those two numbers is on the sizing chart.
So the question to work from is: how many hot water outlets run in the same twenty to thirty minutes, on your worst morning? Not your average morning — the one where everyone is trying to leave at the same time.
Nominal volume is not the hot water you get
A tank's rating is its nominal capacity, and the amount of genuinely usable hot water is always less. Three things eat into it:
- Stratification. Hot water sits on top of cold, and as you draw it off, cold mains water enters at the bottom. The boundary between them is not a clean line, so the last part of the draw is already cooling.
- Tempering. Water is stored hot enough to keep bacteria such as Legionella under control, then blended down with cold before it reaches bathroom outlets. That is a plumbing requirement, not an option — see the maintenance pillar for what the tempering valve does and how it fails.
- Standby loss. A tank sitting all day loses heat through its jacket and its pipework, and in an uninsulated outdoor location it loses more.
This is why "it's a 250, that's plenty" is not an answer on its own. It is half of one.
For a storage tank, volume and recovery are one number
The other half is recovery rate: how fast the system reheats once you have drawn the tank down. A smaller tank with a fast recovery can outperform a larger one with a slow one, if your demand comes in two bursts rather than one.
Recovery is where the heat source shows up in the sizing:
| Heat source | How it behaves on sizing | What to ask for |
|---|---|---|
| Electric resistance | Slow recovery. On a controlled-load tariff it only heats in set hours, so the tank must hold a whole day. | Volume, and whether the tariff is controlled load or continuous |
| Gas storage | Fast recovery — the usual reason a gas tank can be smaller than the electric one it replaced. | Volume and recovery rate together |
| Heat pump | Deliberately slow. It extracts heat gently, so the tank is sized to carry you through, not to reheat mid-morning. | Volume, and performance on a cold morning |
| Solar thermal | Depends on the day. Sized with the booster, never without it. | Tank volume and what the booster is |
| Continuous flow | No volume at all. Sizing is a flow rate. | Litres per minute at a realistic winter temperature rise |
The controlled-load trap
This one catches people out when they change tariff rather than when they change systems. A tank on a controlled-load (off-peak) tariff heats only during the hours the network allows. That means it has to store the entire day's hot water, so it is sized noticeably larger than the same household would need on a continuously powered circuit.
Move an existing tank onto controlled load without resizing and you get a system that was fine last month and now runs cold every evening. Nothing has broken. It is simply being asked to do a different job.
Where 250 litres actually sits
250 litres is one of the standard nominal sizes for a whole-house storage tank in Australia, along with 315 litres above it — they are the sizes you will be quoted most often for a main electric storage system, particularly on controlled load.
What we are not going to do is tell you that 250 litres suits a family of four. That is exactly the shortcut this page exists to argue with. Whether 250 is right for you depends on:
- how many showers overlap on your worst morning, and how long they run;
- whether the tank is on controlled load or heats on demand all day;
- whether the bathroom outlets are tempered (they will be) and what that does to the delivered volume;
- whether anything else — a spa bath, a large laundry load, a second bathroom — lands in the same window.
Two households can be given the same tank and only one of them will be happy with it.
Continuous flow: litres per minute, at a real temperature rise
A continuous flow unit never runs out, which is the headline. What it does instead is cap the rate. It can only lift water by so many degrees at so many litres a minute, and those two figures trade against each other.
The important part: the flow rate on the brochure is quoted at a particular temperature rise, and winter mains water in a cold part of the country is a great deal colder than the assumption behind that figure. The same unit that comfortably ran two showers in February can struggle with one in July.
So the number to insist on is litres per minute at the temperature rise your climate actually demands in winter, not the headline rating. There is more on how this plays out day to day in instantaneous versus storage.
The physical limits that override all of this
Sizing is the part everyone discusses. What frequently decides the answer is the part nobody mentions until the day of installation:
- Footprint and access. A bigger tank is taller and wider, and it has to get to where it is going. A tank in a roof space or an internal cupboard has a ceiling above it and a doorway behind it.
- The slab or stand. Larger tanks are heavier when full, and what is under the old one was sized for the old one.
- Clearances. Heat pumps need air around them and somewhere for the noise to go; gas units need siting rules met. A unit that fits the space may still not fit the clearances.
- The existing connection. Moving from one heat source to another can change what the system needs at the meter or the switchboard, and that changes the job.
What to ask before you agree to a size
Four questions, and the answers to them together are the sizing:
- How many outlets did you size this for running at once, and for how long? If the answer is a number of people rather than a number of simultaneous outlets, ask again.
- Is it on controlled load or continuous supply? The same household needs a different tank for each.
- What is the recovery rate (storage) or the flow rate at a winter temperature rise (continuous flow)?
- What will need to change to fit it — the stand, the clearances, the circuit, the pipework?
How far can you take this yourself?
You can do all of the thinking: count the overlapping outlets, note what tariff you are on, measure the space the current system occupies and check what is above and around it. That is genuinely useful and it is yours to do.
What you cannot do is install it, connect it or alter the pipework. Hot water installation is licensed work in every state and territory, and the licensing body is a state one — the plumbing regulator for your state or territory is who sets and checks it. If you are unsure where the line falls on any plumbing job, what you can legally do yourself sets it out.
Frequently asked questions
How do I work out what size hot water system I need?
Count the hot water outlets that run in the same twenty to thirty minutes on your busiest morning, not the number of people in the house. Then pair that with the recovery rate for a storage tank, or the flow rate at a realistic winter temperature rise for a continuous flow unit.
Is a 250 litre hot water system big enough for a family?
It depends on when the hot water is used rather than how many people use it. A household where showers overlap in one narrow window needs more stored volume than a larger household that spreads them out. It also depends on whether the tank is on a controlled-load tariff, because an off-peak tank has to store a whole day's use.
Why does my tank hold 250 litres but run out sooner than that?
Nominal capacity is not usable hot water. Cold mains water enters the bottom of the tank as you draw hot water off the top, so the last part of the draw is already cooling, and water is stored hot then tempered down with cold before it reaches bathroom outlets. Standby heat loss takes a little more.
Can I just fit the same size as the old system?
Often, but check two things first. If you are changing heat source, recovery rates differ enough that the equivalent size can change. And if the tariff is changing to or from controlled load, the same household needs a different volume, because an off-peak tank must carry the full day.
Sources
What this page explains is based on these official sources. If you check one and it does not match what you read here, send it to us and the article gets corrected.
- Australian Government, energy.gov.au. Hot water systems — types, installation and energy performance
- Australian Building Codes Board. National Construction Code — Volume Three (Plumbing Code of Australia)