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What Size Hot Water System Do I Really Need?

Picking the right hot water size is less about the number of people in your home, and more about whether your system can handle the busiest part of the day (usually the morning shower rush). Two homes with the same number of people can need completely different sizes if one has two bathrooms and overlapping showers.

Sydney’s climate is fairly mild, so most sizing issues come from busy mornings and simultaneous use, not winter temperatures.

Household sizeElectric storage (tank)Gas storage (tank)Continuous flow (L/min)
1–2 people80–160L90–135L16–20
3–4 people160–250L135–170L20–26
5+ people250–400L170–250L26+

1–2 people (80–160L electric, 90–135L gas, 16–20 L/min continuous)

Rheem 125L electric https://australianhotwater.com.au/product/rheem-125-litre-electric-hot-water-system-price-installed/ 

AquaMax 125L electric https://australianhotwater.com.au/product/aquamax-125-litre-electric-hot-water-system-price-installed/

Rheem 135L external gas https://australianhotwater.com.au/product/rheem-135-litre-natural-gas-external-hot-water-system-price-installed/ 

Dux 135L gas https://australianhotwater.com.au/product/dux-135-litre-natural-gas-hot-water-system-price-installed/

Rinnai InfinityB 16L https://australianhotwater.com.au/product/rinnai-16-litre-infinityb-natural-gas-hot-water-system-price-installed/ 

Rheem 20L continuous flow https://australianhotwater.com.au/product/rheem-20-litre-natural-gas-hot-water-system-price-installed/

3–4 people (160–250L electric, 135–170L gas, 20–26 L/min continuous)

Electric storage example (250L): Rheem 250L electric https://australianhotwater.com.au/product/rheem-250-litre-electric-hot-water-system-price-installed/

Continuous flow example (26L, good if overlap happens): Rheem 26L continuous flow (natural gas) https://australianhotwater.com.au/product/rheem-26-litre-natural-gas-hot-water-system-price-installed/

5+ people

Peak demand (what’s happening at the same time)

Instead of counting people, list what can run at once during your busiest 60 to 90 minutes. Add up the likely combinations in your home. If two showers can run at once, your peak demand can quickly climb into the 20–30+ L/min range (even before you add taps). If your home regularly runs two showers at once, use the next table before you decide.

Fixture running at the same timeTypical flow rate
Standard shower8–12 L/min
High-flow / rainfall shower12–16 L/min
Kitchen tap6–8 L/min
Bathroom basin4–6 L/min

Bathroom count (the tie-breaker)

Bathroom count often decides whether you stay in the lower end or upper end of the sizing range.

Bathroom CountMorning use patternWhat to do with your sizing
1 bathroomUsually one shower at a timeBaseline sizing guide above is usually accurate
2 bathroomsOccasional overlapMove toward the upper end of the range
2 bathroomsRegular overlap (two showers at once)Treat as a higher-demand household
3 bathroomsOverlap is likelySize for overlap (higher L/min or bigger storage), and consider multi-unit solutions if needed

Choose your system type (and what sizing means for it)

System typeHow it delivers hot waterWhat “sizing” really meansWhen people get caught out
Electric storageUses a tank of pre-heated waterTank litres (and recovery time)Off-peak heating windows, slow reheating, high-flow showers
Gas storageTank, but reheats faster than electricTank litres (with quicker recovery)Heavy overlap can still drain the tank
Continuous flowHeats water as you use itLitres per minute at peakUndersizing causes temperature drops when multiple taps run
Heat pumpEfficient electric tankTank litres (recovery is steady)Undersizing for families and expecting “instant” performance

Practical sizing by system type (Sydney-friendly ranges)

These ranges assume typical usage. If you have regular overlap, move up a bracket.

Electric storage (tank)

Household / bathroom patternCommon starting size
1–2 people, 1 bathroom125–160L
3–4 people, 1 bathroom160–250L
3–4 people, 2 bathrooms (overlap possible)250L
5+ people, 2–3 bathrooms315–400L

Electric storage (tank)

Gas storage (tank)

Household / bathroom patternCommon starting size
1–2 people90–135L
3–4 people135–170L
5+ people170–250L

Gas storage (tank)

Continuous flow (instantaneous)

Bathroom / overlap patternCommon starting size
1 bathroom, one shower at a time16–20 L/min
2 bathrooms, occasional overlap20–26 L/min
Regular overlap or 3 bathrooms26+ L/min

Continuous flow (instantaneous)

Heat pump (tank)

Household / bathroom patternCommon starting size
1–2 people200–250L
3–4 people250–300L
5+ people300L+

Heat pump (tank)

Sydney examples (common real-life setups)

Home typeElectric storageGas storageContinuous flowHeat pump
Apartment, 1 bathroom, 1–2 people125–160L90–135L16–20 L/min200–250L
House, 2 bathrooms, 3–4 people (overlap sometimes)250L170L20–26 L/min250–300L
Larger home, 3 bathrooms, 5+ people (overlap common)315–400L250L26+ L/min300L+

Apartment, 1 bathroom, 1–2 people

House, 2 bathrooms, 3–4 people (overlap sometimes)

Larger home, 3 bathrooms, 5+ people (overlap common)

Use the quick sizing guide as your baseline, then use the peak demand and bathroom tables to sanity check it. If your home has regular shower overlap, size for overlap first, that’s what usually separates a system that feels reliable every morning from one that constantly disappoints.
If you’ve used the tables above and you’re still unsure, the easiest next step is to match your household size, bathroom count and shower overlap to a system type and get a recommendation that fits your home. Get in touch with us today to speak with an expert.

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