What Size Hot Water System Do I Need?

HOT WATER OUTLET • HOT WATER HELP CENTER

Calculate a practical starting size from household demand, peak shower use and the type of hot water system you want to buy. Then jump straight into the HWO range that matches the result.

Electric Storage Heat Pumps Gas Storage Instant Gas

Sizing is a starting point only. Final selection can change with recovery rate, element or burner output, inlet water temperature, heating windows, shower flow rates and the exact manufacturer model.

HOT WATER SIZE CALCULATOR

Find My Starting Size

About 60 sec
1–8 people
2. How heavily do you use hot water?Per person / day
Peak demand
Adds concentrated demand
Choose technology
Affects storage buffer
Your household profile4 people • average use
200 L/dayestimated demand
Electric storage starting point
250L class
A practical starting point for an average four-person household on an extended controlled-load supply. Confirm the exact model's element rating, recovery and tariff suitability before ordering.
Shop Matching Range See How Sizing Works

Guide only — not an engineering or installation design. A licensed installer should confirm final product suitability, services and installation requirements.

THE QUICK ANSWER

Start With Daily Hot Water Demand

Hot water sizing begins with how much hot water the household actually uses. A useful starting range is about 40 litres per person per day for light use, 50 litres for average use, 65 litres for high use and 80 litres for very heavy use. That is daily demand — not automatically the tank size.

HOUSEHOLD DEMAND

Estimated Daily Hot Water by Household Size

Use this table as a first-pass estimate before system recovery, tariff, simultaneous showers and technology are considered.

40LLight use / person / dayShort showers and conservative hot-water use.
50LAverage use / person / dayA sensible starting point for normal household demand.
65LHigh use / person / dayLonger showers, baths or repeated back-to-back use.
80LVery high use / person / dayUseful as a stress test for unusually heavy demand.
People Light Average High Very High
1 40 L/day 50 L/day 65 L/day 80 L/day
2 80 L/day 100 L/day 130 L/day 160 L/day
3 120 L/day 150 L/day 195 L/day 240 L/day
4 160 L/day 200 L/day 260 L/day 320 L/day
5 200 L/day 250 L/day 325 L/day 400 L/day
6 240 L/day 300 L/day 390 L/day 480 L/day
7 280 L/day 350 L/day 455 L/day 560 L/day
8 320 L/day 400 L/day 520 L/day 640 L/day
Daily litres and tank litres are not the same thing.

Electric storage, heat pumps, gas storage and instant gas all recover differently. Two households with the same daily litres can need very different nominal product sizes.

SIZE BY TECHNOLOGY

How Each Hot Water System Is Sized

The same household profile needs a different sizing method depending on how the system stores and replaces hot water.

Electric Storage

Tank + recovery

Electric storage is sized around daily demand, tank capacity, element output and how many hours the element is allowed to heat.

  • Continuous supply can recover during the day.
  • Controlled-load systems need more stored reserve.
  • Element kW changes recovery speed.
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Heat Pump

Tank + compressor

Heat pumps combine storage with compressor recovery, so daily demand, climate, timer windows and boost strategy all matter.

  • 270–315L is a common family-size class.
  • Cold conditions can reduce recovery.
  • Operating schedules can change available reheating.
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Gas Storage

Tank + fast burner

Gas storage can use a smaller tank than some electric systems because a high-output burner can replace consumed hot water quickly.

  • Burner recovery is critical.
  • Peak consecutive showers matter.
  • Model recovery data should confirm the final size.
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Instant Gas

Litres per minute

Continuous-flow gas is sized by peak hot-water flow rather than stored litres. Simultaneous showers are one of the biggest factors.

  • One shower can suit a smaller flow class.
  • Two simultaneous showers often push into 24–26L/min.
  • Cold inlet water reduces available hot-water output.
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ELECTRIC STORAGE

Electric Hot Water Size Changes With the Heating Window

An electric cylinder that can reheat whenever the thermostat calls for power can use recovery throughout the day. A tank that only heats during a restricted off-peak window needs more stored reserve.

Continuous Supply

Heating can generally operate whenever required. This allows a smaller stored buffer than a comparable restricted overnight system, provided the element and circuit suit the demand.

Extended Controlled Load

Heating is available for a substantial but restricted period. A moderate storage allowance helps cover peak demand between heating windows.

Restricted Overnight

The cylinder may need to store most of the next day's hot-water requirement before the next heating period, so capacity often needs to rise.

Element Output

A 3.6kW element recovers faster than a lower-output element, but the product and electrical circuit must be designed for the selected rating.

PEAK USE MATTERS

What Can Push You Up or Down a Size?

Long ShowersLonger shower duration increases daily litres and the amount of stored water used before recovery can catch up.
Two Showers TogetherSimultaneous demand matters most for continuous flow, but it also creates a stronger short-term draw on storage systems.
Regular BathsBath filling is a large concentrated hot-water draw and can justify additional storage capacity.
Restricted HeatingShorter electric or heat-pump heating windows reduce available recovery and increase the storage buffer required.
Cold Inlet WaterColder mains water increases the temperature rise needed and can reduce effective heat-pump or instant-gas output.
High RecoveryStrong element, compressor or burner recovery can reduce how much stored reserve is needed for the same household demand.
COMMON PRODUCT CLASSES

Common Hot Water Size Categories

These are product-comparison ranges, not fixed household rules. Always check recovery, heating schedule and the exact model.

125–160LCompact to medium storageOften compared for smaller households, lighter demand or systems with strong recovery.
200–250LMid-size storageA useful middle range for smaller families and average-demand homes.
270–315LFamily storageA common family-size range across heat pump and electric storage products.
400L+High-demand storageMore common for large households, heavy use or restricted heating periods.
16L/minSmaller instant gasGenerally considered where one significant hot-water outlet operates at a time.
20L/minMid-range instant gasExtra flow for stronger single-outlet demand and selected smaller multi-outlet situations.
24–26L/minFamily instant gasCommonly compared for homes where two bathrooms or simultaneous hot-water use are possible.
32L/min+High peak flowLarge homes or unusually high simultaneous demand can require larger or multiple-unit solutions.
BUYING PROCESS

How to Choose the Right Hot Water Size

1Calculate Daily Litres

Start with occupants multiplied by an appropriate 40, 50, 65 or 80 litre usage level.

2Check Peak Demand

Bathrooms, simultaneous showers and bath use show how quickly the home may need hot water.

3Apply Technology

Electric, heat pump, gas storage and instant gas convert the same demand into different product sizes.

4Compare Actual Models

Confirm capacity, recovery, dimensions, services and manufacturer requirements before purchasing.

HOT WATER SIZING FAQ

Hot Water System Size Questions

Common questions about family size, electric storage, heat pumps, continuous flow and replacing an existing tank.

What size hot water system does a family of four need?
Four people may use about 160L/day with light use, 200L/day with average use, 260L/day with high use or 320L/day with very high use. The actual product size then depends on technology, recovery rate, heating window and peak demand.
What size hot water system do two people need?
Two people may use around 80L/day with light use, 100L/day with average use, 130L/day with high use or 160L/day with very high use. A fast-recovery system can often use less storage than a restricted-heating system.
Is a 250L hot water system enough for four people?
It can be. Four average users represent roughly 200L/day of estimated demand, but whether a 250L system is suitable depends on the heating technology, recovery performance, tariff and peak shower use.
Is a 315L hot water system enough for a family?
315L is a common family-size storage class and can suit many households. High-use families, restricted heating periods or unusually heavy peak demand may justify more storage or faster recovery.
What size heat pump hot water system do I need?
Heat-pump sizing starts with household daily litres and then allows for compressor recovery, climate, operating windows and peak use. Around 270–315L is a common family-size class, but smaller and larger households can need different capacities.
What size electric hot water system do I need?
Electric storage sizing depends on daily demand plus the hours available for reheating. A tank on restricted overnight electricity generally needs more stored capacity than one with continuous power and equivalent element recovery.
What size continuous-flow gas hot water system do I need?
Continuous-flow systems are sized in litres per minute rather than tank litres. One shower can suit a smaller unit, while two simultaneous showers often move the starting comparison into the 24–26L/min class depending on inlet-water temperature and required temperature rise.
Should I replace my old hot water system with the same size?
Not automatically. Household size, shower habits, tariffs and available technologies may have changed since the old system was selected. Recalculate current demand and then compare the existing services, dimensions and connections.
Can a hot water system be too big?
Yes. Excess storage can increase purchase cost and standing heat loss without adding a useful household benefit. The goal is enough usable hot water plus an appropriate recovery buffer.
READY TO SHOP?

Found Your Starting Size? Compare the Systems That Fit.

Use the calculator result as the starting point, then compare capacity, recovery, power or gas requirements, dimensions, stock and delivery on the actual HWO products.

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