What Size Hot-Water System Do I Need?

If you’re trying to work out what size hot water system to buy, you’re probably hoping for a quick answer like “2 people = this size” or “4 people = that size”. But it’s not that simple. Headcount gives you a good starting point, but what really matters is how your home uses hot water at its busiest times.

Two people taking long, back-to-back showers can easily use more hot water than a family of four who take shorter showers and space out their usage instead of running everything at once.

That’s why sizing is not just about numbers but also about your routine and a bunch of other things we’ll walk you through in this guide. It’ll help you choose a system that actually works for your home without running out or being oversized.

TL;DR

System type matters first because storage systems are measured in litres, while continuous flow systems are measured in litres per minute, so you must know which type you’re using.
Daily usage is a baseline, as a person uses around 50L/day, so you multiply it by household size to estimate tank size, but the final size still changes based on other factors.
Peak-time usage matters the most, as busy hours can push demand over 200 L, so storage systems need a 20–30% buffer, and continuous flow systems must handle combined flow at once.
The number of bathrooms affects sizing, as more bathrooms increase the chance of multiple showers running at the same time, which increases the required tank size or flow rate.
Appliances add a hidden load, as washing machines (50–70L hot cycles) and dishwashers (15–20L bursts) increase demand during peak times and can push you to size up.
Long showers and high-flow heads (15–25 L/min) use much more water than efficient WELS-rated heads (6–7 L/min), significantly affecting system size.
Energy source affects tank size:
- Electric = slower recovery → larger tanks needed
- Gas = fast recovery → smaller tanks possible
Future-proofing is a must because hot water systems last 8–20 years, so it’s usually smarter to size slightly larger now to allow for future family growth or renovations.

How Hot Water System Sizing Differs (By System Type)

Storage Systems

Storage systems heat water and keep it in a tank, so these systems are sized in litres (L) and come in a wide range of sizes – from small 25L units for single-point use (like a single tap) to large 400L+ systems designed for full family homes. The number tells you how much hot water is available in storage at one time before the system needs to recover.

Continuous Flow Systems (Instantaneous Systems)

In continuous flow systems, since there is no tank and the system heats water as it passes through the unit, they can’t be sized based on a litre capacity; instead, they are sized by flow rate in litres per minute (L/min).

Continuous flow systems usually range from 12 L/min to 32 L/min+. This number shows how much hot water the system can deliver at one time, which determines how many showers or taps can run together without losing temperature or pressure.

Now that you understand how sizing works differently for each system, you can apply the right method to the system you’re installing in your home and work out the correct size from there.

The Main Factors That Determine What Size Hot Water System You Need

Number of People In The Household

The number of people living in your home is the starting point for understanding your hot water use, as more people means more showers, dishwashing, laundry, etc., which all add up quickly.

In Australia, one person uses about 50 litres of hot water per day. Based on this per-person usage and the number of people in your house, you can calculate your family usage and the size of your storage tank.

For instance, if you have a family of four, 4 people × 50 L each = 200 litres of hot water usage.

Now, since your family uses around 200 litres a day, it might seem logical to go for a 200L tank, but please don’t. Because, for instance, in real life, someone might take a longer shower, or there might be an extra load of laundry to do, which can increase your family’s usage from the calculated value and empty the tank earlier. That’s why you always add a buffer of around 50–100 litres, so you don’t run out of hot water.

For a continuous flow system, the number of people doesn’t directly decide the size because it heats water as you use it, not based on how much is used overall in a day. But it still helps you understand the overall system size. For instance, more people mean a higher chance of multiple showers or taps running at the same time, which pushes you towards a higher flow rate system.

Peak-Time Hot Water Usage

Peak time is the busiest hour in your day – usually the morning before school or work, or the evening after sports and daily routines – when most hot water is being used at the same time.

In a storage system, the tank needs to hold enough hot water to cover everything used during this peak window.

To calculate this peak usage, look only at your busiest 60 minutes and add them up.

A shower uses about 60 litres (around 7 minutes at 9 L/min)
A dishwasher uses about 15-20 litres per cycle
A washing machine hot cycle uses roughly 50–70 litres

Now imagine most of this happening in the same hour.

For example, in a typical family morning:

3 people showering = around 180 litres
Plus a dishwasher cycle = ~20 litres

That already puts you at around 200+ litres in one peak hour, without even adding anything extra like taking longer showers. So, for extra usage, step up to a safe working range by choosing a tank size 20–30% larger than the calculated peak demand.

If you are sizing a continuous flow system, you need to find the litres per minute (L/min) that will be enough to supply all running fixtures during peak time.

Now, for the same example, let’s calculate the litres per minute (L/min) required:

3 Showers: 27 L/min
Dishwasher (20 litres for a 10-minute cycle): 2 L/min

So total simultaneous demand becomes 29 L/min.

But just like with a storage system, you should always add around a 2–5 L/min margin to your calculated peak load to cover unexpected usage.

Number of Bathrooms/Showers

In Australia, around 80% of people shower daily, so showers are the biggest and most regular use of hot water in most homes.

In a storage system, the number of bathrooms affects how quickly the tank will be empty because more bathrooms mean more chances of multiple showers running at the same time (higher simultaneous use), which creates a faster drain on the tank, so you need a bigger tank.

For calculation, you can use this formula:

(Number of Showers) x (9 L/min average shower flow rate) x (average shower time)

For example, if you have a 2-bathroom home and two people take a 10-minute shower each, the calculation looks like this:

2 showers × 9 L/min × 10 minutes = 180 litres

That means in just 10 minutes, you can use around 180 litres of hot water.

If you only have a 160L tank, it will run out before both showers are finished. That’s why you always add a buffer of 50–100 litres, so the system can comfortably handle the showers and any other hot water use happening at the same time without running short.

With continuous flow (instantaneous) systems, sizing is based on adding up the flow rates of every bathroom outlet that might run at once.

1-Bathroom Home: Your maximum simultaneous demand is usually just one shower (9 L/min) plus tap use (5 L/min), which totals 14 L/min. You should choose a 16L/min unit as a minimum, or a 20L/min unit for better stability.
2-Bathroom Home: You calculate for two people showering at the same time (9 L + 9 L = 18 L/min). Then you add a small buffer for short tap use, bringing the total to around 22–24 L/min. This is why a 26L/min unit is the standard recommendation for 2-bathroom homes.
3+ Bathroom Home: You calculate for three showers running together (9 L × 3 = 27 L/min). With other usage in the home, demand can go higher, so you will need a 32L/min unit or two smaller systems to maintain a steady temperature and pressure.

Appliances That Use Hot Water

When people think about hot water system sizing, they mostly focus on showers. But other appliances, mainly the washing machine and dishwasher, pull a large amount of hot water in a short time, so you must take them into account, too.

Some washing machines, especially older ones, are hot-fill machines. That means they pull hot water directly from your hot water system and mix it for the wash cycle. A single warm wash can use around 50 to 70 litres of hot water, depending on the cycle and load size.

So if you run two loads in the morning with each using ~60 L, that’s already around 120 litres gone before anyone has even stepped into the shower.

Dishwashers also use water in short bursts during cycles. On average, a cycle uses about 15–20 litres of hot water, depending on efficiency and settings.

For a storage tank, this means going up one size. For example, if a family of four would normally choose a 250 L tank, moving to a 315 L tank gives you enough extra capacity to cover laundry use without compromising your shower supply.

For a continuous flow system, add roughly 4–6 L/min to your total demand to cover appliance use during peak times, so the system can handle both without affecting temperature or pressure.

Shower Length and Water-Efficiency Fittings

Another important factor in hot water sizing is how long people actually stay in the shower.

In a storage hot water system, every minute under the shower pulls water directly from a fixed supply in the tank. So, longer showers mean faster tank drain. This is why storage systems need a 20% to 30% extra capacity to make sure the last person isn’t left in the cold.

With a continuous flow system, shower length isn’t a limiting factor because the system keeps heating water as long as you’re using it.

The type of showerhead fittings is also important. Across Australia, showerheads are rated using WELS (Water Efficiency Labelling and Standards), which tells you how much water they use per minute. This rating changes everything in system sizing.

Older or high-flow showerheads: 15-25 L/min
Modern water-efficient (WELS-rated) showerheads: 6-7 L/min

So, let’s suppose that a person takes a 7-minute shower.

With older high-flow showerheads: 15–25 L/min × 7 minutes = 105–175 litres of hot water.
With efficient WELS 3-star showerheads: 6–7 L/min × 7 minutes = 42–49 litres total. That is more than half the demand just from changing the showerhead.

For a continuous flow system, if you run an old high-flow shower, you need a system with a much higher flow rate, usually 32 L/min or more, just to keep the pressure from dropping. But if you have efficient heads, a standard 26 L/min unit can easily handle the shower and other usage.

So, basically, a water-saving and modern showerhead allows you to buy a smaller hot water system because you aren’t rushing the water out of the pipes.

Energy Source And Recovery Rate

‘Recovery rate’ means how quickly your system heats cold water back up to usable hot water. This recovery time is affected by the energy source (electric or gas) you choose, which in turn affects the sizing. Here is how:

Electric Storage (Slow Recovery): These heat water slowly because they use electric elements with limited heating power. Also, they are often connected to off-peak electricity tariffs like Tariff 33, which supply cheaper power for 16 hours a day (not continuously). Because of this, the system can’t always reheat water when you are using it, especially during peak times when cheaper power is off. That’s why you need a larger tank than your calculated daily use, so it has enough stored hot water to last until it gets the next heating cycle.
Gas Storage (Fast recovery): Gas heats water much faster because it uses a high-powered burner, so the system starts reheating almost immediately while you are still using hot water. Because of this quick recovery, a smaller tank (compared to an electric one) can still comfortably support your family, as it keeps heating new water while the stored water is being used.
Continuous Flow (No storage recovery): Since there is no stored water, recovery doesn’t apply here, as water is heated instantly as it is used.

As a rule of thumb, the slower the recovery, the bigger the tank you need. The faster the recovery, the smaller the tank can be.

Future-Proofing: Size for the Next Version of Your Home

Most hot water units last around 8 to 20 years, depending on their type, so they are a long-term investment. That’s why sizing should not only match who lives in the house right now but should also reflect how your household is likely to change over time.

If you size a system perfectly for a couple today but plan to have children, bring in a relative, or simply expect more people in the home later, that perfect size can quickly start feeling too small during busy periods.

If you are planning a renovation that adds a second bathroom or a large freestanding soaking tub, your hot water needs will jump significantly.

That’s why we recommend going one size up from the start, rather than trying to upgrade later. In most cases, increasing capacity upfront is far more cost-effective and less hassle than replacing an undersized system a few years later when it can’t keep up anymore.

The Australian Hot Water Sizing Guide

To make choosing the right size easier, we’ve created a simple table that breaks down common household sizes and the standard hot water system sizes they typically need, so you can quickly see what suits your home.

Household Size	Electric Storage (Off-Peak)	Gas Storage (High Recovery)	Continuous Flow (Instant)
1–2 People	125L – 160L	90L – 135L	16L – 20L per min
3–4 People	250L – 315L	135L – 170L	24L – 26L per min
5+ People	400L+	170L – 200L+	32L per min

Final Thoughts

Before you size a hot water system, know that different systems are sized differently. Storage systems are measured in litres, and continuous flow systems are measured in litres per minute, so you need to know what system you plan to install and then use the correct sizing method.

Once that’s clear, the starting point is always the same: how many people live in the house. That gives you a basic idea of daily hot water use. But to get the right size, you then look at how that water is actually used, especially during peak times. Things like how long people shower, whether multiple bathrooms are used at the same time, and how efficient your fixtures are can all push demand higher than the daily average.

After you’ve worked out the size, always add a buffer and think a little ahead. People’s routines change over time, families grow, and homes get busier. So choose a slightly larger system now to face fewer problems later and enjoy more consistent hot water.

And if all of this feels confusing, our team at 1touch Plumbing can help. We look at your home, your usage, and your setup and then help you choose the right size hot water system so you can have reliable hot water when you need it, without worrying about running out or oversizing it.