What Size Hot Water System Do I Need?
What Size Hot Water System Do I Need?
Size electric, heat pump, gas, continuous flow or solar hot water from the way your household actually uses it.
One household profile, five different sizing methods. Calculate your estimated daily demand, account for peak use and compare the starting size for each major hot water technology.
Size My Hot Water
Tell us about the household once. We'll calculate daily demand and then apply the appropriate sizing method for the system you choose.
This tool provides a practical starting size, not an engineering design. Exact requirements can change with shower flow rates, inlet water temperature, product recovery rate, heating controls, solar collector configuration, boost settings and manufacturer requirements.
The Quick Answer: Start With Daily Hot Water Demand
Hot water system sizing starts with the people using it. Our base household calculation uses approximately 40 litres per person per day for light use, 50 litres for average use, 65 litres for high use and 80 litres for very high use.
Daily Hot Water Demand by Household Size
This table shows the estimated litres of hot water used each day before system technology, tariff and recovery are considered.
| 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 |
How Each Type of Hot Water System Is Sized
The same four-person household can require a different nominal system size depending on how quickly the system reheats and when that heating is available.
Electric Storage
Electric storage is sized from daily litres plus the amount of time the element has available to recover.
- Continuous supply can reheat throughout the day.
- Extended controlled supply needs a moderate storage buffer.
- Restricted overnight heating needs more stored capacity.
- Element wattage affects recovery speed.
Heat Pump
Heat pump sizing combines daily demand with slower compressor recovery and the amount of storage available through peak shower periods.
- 270–315 L is a common family-size class.
- Cold climates can reduce recovery.
- Timer windows affect available reheating.
- Boost-element strategy differs between models.
Gas Storage
Gas storage generally needs less nominal tank volume because a high-output gas burner can replace consumed hot water quickly.
- Tank litres are only part of usable capacity.
- Burner recovery rate is critical.
- Peak consecutive showers matter.
- Model recovery data should confirm final selection.
Continuous Flow Gas
Continuous flow is sized by peak litres per minute rather than stored litres.
- One shower requires less capacity than two simultaneous showers.
- Colder inlet water reduces available hot-water flow.
- Bathroom count helps identify peak use.
- Large homes may need a larger or multiple-unit solution.
Solar Hot Water
Solar hot-water sizing starts with household litres and generally needs enough storage to carry solar energy between collection periods.
- Storage tends to be generous relative to daily use.
- Collector area matters as much as tank litres.
- Roof orientation and shading affect performance.
- Electric or gas boosting covers poor solar conditions.
Compare Every Type at Once
Select Compare All Types in the calculator and we'll apply the same household profile to every technology.
This makes it easier to compare a 315 L heat pump with a restricted-tariff electric tank or a 26 L/min continuous-flow system without pretending those numbers mean the same thing.
Electric Hot Water Size Changes With the Tariff
An electric tank that can recover throughout the day does not need to behave like a tank that only receives electricity overnight.
Continuous Supply
The element can generally operate whenever the thermostat calls for heat, so recovery contributes throughout the day.
Extended Controlled Supply
Heating is available for a substantial but restricted window. A moderate storage allowance is useful where peak demand is high.
Restricted Overnight
The system may need to store enough usable hot water for most of the next day's demand before the next heating period begins.
Why Bathrooms and Simultaneous Showers Matter
Daily litres tell us how much hot water the home uses. Simultaneous fixtures tell us how quickly the system may need to deliver it.
One Shower at a Time
Peak flow is relatively modest, so continuous-flow capacity can remain lower even if the household uses a reasonable amount across the full day.
Two Showers Together
Continuous-flow capacity needs to rise because both outlets require hot water simultaneously. Storage systems also experience a larger short-term draw.
Three or More Showers
Large multi-bathroom homes can exceed the practical output of some single continuous-flow units and may need a larger or engineered solution.
Common Hot Water Size Categories
These are useful product-comparison ranges rather than universal household rules.
125–160 L
Typically associated with smaller households, light demand or technologies with strong recovery.
200–250 L
A useful middle storage category for smaller families and average-demand homes.
270–315 L
A common family-size range, especially for heat pump and electric storage systems.
400 L+
More common for high-use households, large families or installations relying heavily on restricted heating periods.
16–20 L/min
Common smaller continuous-flow category where only one significant hot-water outlet normally operates at once.
24–32 L/min+
Higher-capacity continuous-flow category for multiple bathrooms and simultaneous hot-water use.
What Can Push You Up or Down a Size?
Household size gives us the starting point. How the home uses hot water decides how much buffer or recovery is required.
Long Showers
Longer shower duration increases both daily household consumption and the amount of stored hot water used during peak periods.
High-Flow Showers
Higher litres-per-minute shower heads increase instantaneous demand and can change both tank and continuous-flow sizing.
Regular Baths
Filling a bath creates a large concentrated hot-water draw and can justify additional storage capacity.
Back-to-Back Showers
Several showers in succession place more pressure on stored capacity before the system has time to recover.
Heating Window
Electric tanks and heat pumps with restricted operating periods need enough stored hot water to bridge the time between reheating cycles.
Climate
Colder inlet water increases required temperature rise and can reduce the effective output of heat pumps and continuous-flow systems.
How to Choose the Right Hot Water Size
Calculate Daily Litres
Start with occupants multiplied by the appropriate 40, 50, 65 or 80 litre usage level.
Check Peak Demand
Bathrooms, simultaneous showers and baths tell us how quickly the household may need that hot water.
Apply Technology
Electric, heat pump, gas storage, continuous flow and solar all turn the same household demand into different system sizes.
Compare Actual Products
Confirm tank capacity, recovery data, manufacturer requirements and installation conditions before purchasing.
Hot Water System Sizing FAQs
What size hot water system does a family of four need?
Four people may use approximately 160 litres per day with light use, 200 litres with average use, 260 litres with high use or 320 litres with very high use. The actual system size then depends on the technology, recovery rate, heating tariff and peak demand.
What size hot water system do two people need?
Two people may use around 80 litres per day with light use, 100 litres with average use, 130 litres with high use or 160 litres with very high use. Tank size can still vary significantly by technology and recovery rate.
What size hot water system do five people need?
Five people may use approximately 200 litres per day with light use, 250 litres with average use, 325 litres with high use or 400 litres with very high use.
Is a 250 litre hot water system enough for four people?
It can be. Four average users represent about 200 litres of estimated daily demand, but whether a 250 litre system is suitable depends on recovery performance, tariff, peak shower demand and technology.
Is a 315 litre hot water system enough for a family?
A 315 litre system is a common family-size category. It can suit many households, although very high-use families or restricted-heating installations may require more storage or stronger recovery.
What size heat pump hot water system do I need?
Heat pump sizing starts with daily household litres and then adds allowance for compressor recovery, climate and peak usage. Around 270 to 315 litres is a common family-size category, while smaller or high-demand households may require a different capacity.
What size electric hot water system do I need?
Electric storage sizing depends on daily demand and the available heating window. A tank on restricted overnight electricity generally needs more stored capacity than one connected to continuous electricity.
What size continuous flow hot water system do I need?
Continuous flow is sized in litres per minute. One shower may suit a smaller unit, while two or more simultaneous showers generally require 24 to 32 litres per minute or more depending on inlet water temperature and required temperature rise.
What size solar hot water system do I need?
Solar hot water sizing starts with household daily demand and usually allows generous storage so collected solar energy can be carried between heating periods. Tank size, collector area, roof orientation, climate and boost system all need to be considered together.
Should I replace my old hot water system with the same size?
Not automatically. Household size, usage patterns, tariffs and available technology may have changed since the existing system was installed. Recalculate current demand first.
Can a hot water system be too small?
Yes. An undersized storage system can run out during consecutive showers or before its next heating period, while an undersized continuous-flow unit may not maintain adequate flow when several outlets operate together.
Can a hot water system be too big?
Yes. Excess storage can increase purchase cost and standing heat loss without providing a meaningful household benefit. The objective is adequate usable capacity with a sensible recovery buffer.
Found Your Size? Compare the Systems That Fit.
Compare hot water systems by technology, capacity, recovery performance and installation requirements — not simply by the size of the old tank.
