Storage tanks run from 125 to 160 litres for one or two people, 250 to 315 litres for three or four, and 315 litres or more for larger households. Continuous-flow units are rated instead by flow rate: 16 to 20 L/min for a single bathroom, 20 to 26 L/min for two, and 26 L/min or higher where showers regularly overlap. Choose continuous flow if your home has limited space or staggered water use; choose storage if several people need hot water at once.
TL;DR:
- Households of three to four people typically need a 250 to 315-liter storage tank or a 20 to 26 liters per minute continuous flow system for overlapping showers.
- Peak-hour demand calculations should guide sizing, considering specific household routines and overlapping water use rather than relying on household size alone.
- Gas storage units recover faster and often require smaller tanks than electric systems, while heat pumps and solar systems tend to be slower but more energy-efficient.
- Proper installation checks on gas capacity, pipe runs, and household habits are essential, especially for homes with long pipe distances or multiple bathrooms.
- Oversized tanks can increase energy costs due to standing losses; choosing the right size based on actual peak use prevents unnecessary expenses.
Table of Contents
- How system type changes hot water system size
- How do you calculate peak-hour hot water demand?
- Which hot water system size fits your household?
- Recovery rate, off-peak tanks, and installation limits
- What does bigger cost in energy and running costs?
- What mistakes do people make when sizing a system?
- When should you call a plumber for a sizing check?
- Why the “just get a bigger tank” advice misses the point
- Book a hot water sizing check with Priority Plus Plumbing
- Where to check hot water sizing rules and rebates
- Sources
- FAQ
How system type changes hot water system size
The numbers change depending on how the system heats and holds water, so the same household can need a very different rating depending on the technology behind it.
Storage systems heat and hold water in a tank, ready for immediate use. Sizing here is about litres held plus how fast the unit reheats, known as the recovery rate.
Continuous-flow (instantaneous) systems heat water on demand as it passes through, so there’s no tank to run out. These are rated in litres per minute, matched to how many taps or showers might run simultaneously.
Heat pump systems extract heat from the surrounding air, which makes them efficient to run but generally slower to recover than gas. That usually means sizing up a bracket compared to an equivalent gas storage unit.
Gas storage systems tend to recover fastest of the storage options, which is why gas units can often use a smaller tank than electric for the same household.
Typical ranges by type:
- Electric storage: suitable storage bands for small to large households
- Gas storage: often one size band smaller than electric for the same household, thanks to faster recovery
- Heat pump: sizing similar to electric storage, potentially larger if recovery is slower
- Continuous flow: 16–20 L/min, 20–26 L/min, or 26+ L/min based on simultaneous outlets
How do you calculate peak-hour hot water demand?
Daily average use, around 50 litres per person, is a useful starting figure but it’s not what sizes your system. What matters is the busiest part of the day, typically the morning routine when multiple hot water uses overlap.
Work through it like this:
- List every fixture likely to run in that window (showers, laundry, dishwasher, kitchen taps).
- Assign typical litres to each: a shower uses roughly 40 to 70 litres depending on flow rate and how long people linger.
- Add laundry and dishwasher contributions, generally 15 to 25 litres per cycle for the hot component.
- Total everything that could realistically overlap, not just what happens across the whole day.
- Match that total to a tank’s first-hour rating, or to a combined L/min figure if you’re comparing continuous-flow options.
Pro Tip: If your total peak-hour figure sits right on the boundary between two capacity bands, go with the larger one, or consider consulting water treatment experts to evaluate system compatibility. Running short on hot water mid-shower is a far more common complaint than paying for a touch of spare capacity.
Which hot water system size fits your household?
Matching your household to a starting point is faster than running the full calculation, provided you sanity-check the result afterwards.
- One to two people, one bathroom: 125 to 160 L storage or 16 to 20 L/min continuous flow covers most routines comfortably.
- Three to four people, one or two bathrooms: 250 to 315 L storage, or 20 to 26 L/min continuous flow if two bathrooms are in regular simultaneous use.
- Five or more people, or two-plus bathrooms with overlapping showers: 315 L+ storage, or 26 L/min or higher continuous flow.
- Households with long baths, hot-wash laundry cycles, or teenagers who shower back to back: move up one band from whatever the headcount alone suggests.
These starting points work for most homes, but they’re not a substitute for the peak-hour method above if your routine is unusual, such as shift workers showering at odd hours or a home gym with its own shower.
Recovery rate, off-peak tanks, and installation limits
Recovery rate is how quickly a storage system reheats water after use, and it directly affects the first-hour rating, the total hot water a tank can deliver in an hour of continuous draw. A gas unit with fast recovery can supply the same usable hot water from a smaller tank than an electric unit with slower recovery.
Off-peak electric systems reheat overnight only and thus usually require a larger storage tank than continuously heated systems for the same household.
Before you settle on a size, a professional will usually check:
- Gas line capacity, since larger burners need adequate supply pipe
- Pipe run length between the plant room and bathrooms, which affects wait times and losses
- Roof space and orientation if solar is on the table
- Ambient temperature around the unit, which affects heat pump efficiency
What does bigger cost in energy and running costs?
A larger storage tank holds more heat, which sounds good until you factor in standing losses, the slow heat leakage that happens even when nobody’s using hot water. Continuous-flow systems sidestep this entirely because they only heat water as it’s drawn.
Heat pumps and solar systems generally cost less to run day to day, but they come with their own sizing quirks: heat pumps need adequate airflow and can be slower to recover, while solar systems need a backup element or booster for cloudy stretches.
Before buying, check the energy rating label on any unit you’re considering, and look into whether rebates apply in your area for heat pump or solar upgrades. These can shift the economics enough to justify a different size or technology than you first assumed.
- Standing losses rise with tank size on storage units
- Continuous flow avoids storage losses but draws more power or gas per minute of use
- Rebates can offset the higher upfront cost of heat pump or solar systems
What mistakes do people make when sizing a system?
The most common error is replacing like for like without checking whether current household demand has actually changed since the old unit went in. A family that’s grown, or added a second bathroom, often needs a size up even if the old tank “worked fine.”
- Don’t assume the old system’s size was ever correctly matched to begin with
- Upsize if simultaneous use is frequent, or if the household is likely to grow
- Consider twin continuous-flow units for homes with heavy overlap and limited roof or floor space
- Favour continuous flow in space-limited homes with staggered use; favour storage where several people need hot water at the same time
When should you call a plumber for a sizing check?
Getting the size right on paper is one thing. Confirming it holds up against your actual gas supply, pipe runs, and bathroom layout is another job entirely, and it’s where Priority Plus Plumbing’s hot water expertise earns its keep after more than 25 years in the trade.
A proper sizing visit typically covers a busiest-hour walkthrough of your household, a first-hour rating check against your current or proposed unit, a gas-line capacity test where relevant, and an inspection of pipe runs for heat loss or lag.
Consider booking a sizing visit if you have frequent overlapping showers, questions about your gas line capacity, or long pipe runs to a remote bathroom.
Why the “just get a bigger tank” advice misses the point
Most sizing guides push readers toward buying capacity as insurance, and it’s easy to see why: a bigger tank feels like a safe hedge against running cold. But that advice ignores standing losses, ignores gas-line and pipe-run constraints, and ignores the fact that two households with identical headcounts can have wildly different peak-hour demand depending on shower habits alone.
The peak-hour calculation matters more than any headcount table, because it’s the only method that accounts for how your specific household actually behaves in the busiest 60 to 90 minutes of the day. A retired couple with staggered routines might do fine on a smaller continuous-flow unit than a family of four with synchronised morning showers, even if the raw numbers on paper suggest otherwise.
What the reader should prioritise first isn’t brand, price, or even fuel type. It’s an honest audit of when hot water actually gets used in your home, then matching the system type, gas capacity, and tank size to that reality rather than to a generic rule of thumb.
— Peter Honey
Book a hot water sizing check with Priority Plus Plumbing
Professional plumbing services can offer practical alternatives to guessing your way through a hot water system upgrade, including onsite assessments of gas capacity, pipe runs, and peak-hour demand.
Experienced plumbers provide site assessments, quotes, and system recommendations matched to how your household uses hot water, not just by household size. If your current system is already failing or you’re facing a burst pipe or total hot water loss, emergency plumbing support is available around the clock.
Ready to get the right size installed the first time? Book a hot water system replacement with Priority Plus Plumbing and get an upfront quote before any work begins.
Where to check hot water sizing rules and rebates
- Sizing tables and buying advice: Choice buying guide
- Off-peak sizing rules: Solar Victoria buyers guide
- Energy rating labels: energyrating.gov.au
- Rebate eligibility: Australian Government energy rebates
Sources
FAQ
Is a 250L hot water system enough?
A 250 L storage tank generally suits three to four people with moderate simultaneous use, but frequent overlapping showers or long baths may push you toward 315 L or a continuous-flow unit rated 20 to 26 L/min.
How many showers can you get from a 300 litre tank?
A tank sized for three to four people typically provides hot water for several average showers, depending on shower duration and flow rate.
How big should my hot water system be?
Match your system to your household’s busiest 60 to 90 minute demand rather than headcount alone: 125 to 160 L for one to two people, 250 to 315 L for three to four, and 315 L+ for larger households, or the equivalent L/min bands for continuous flow.
What is the difference between a 2.4 and 3.6 kW hot water system?
The kilowatt figure refers to the heating element’s power, which affects recovery speed, not storage capacity; a higher-kilowatt element reheats water faster than a lower-kilowatt element in the same size tank.
Should I upsize when replacing an old hot water system?
Not automatically, but you should recheck your household’s current peak-hour demand rather than matching the old unit exactly, since family size, bathroom count, and habits often change over a system’s lifespan.