For a family of four, start with 125–160 L for continuous-supply electric storage, 250–315 L for controlled-load or off-peak electric, 135–170 L for gas storage, 270–315 L for a heat pump, or 300–360 L for solar storage. A continuous-flow system does not run out of stored water because it has no tank, but its delivery can fall short if simultaneous demand exceeds its rated flow. These ranges are indicative, so confirm the final size against current manufacturer data.
Typical sizes by system type
Storage capacities differ because heating schedules and recovery rates affect how quickly hot water is replaced.
| System type | Indicative size for four people | Key consideration |
|---|---|---|
| Electric storage, continuous supply | 125–160 L | Reheats throughout the day. |
| Electric storage, controlled load or off-peak | 250–315 L | Must cover demand between heating periods. |
| Gas storage | 135–170 L | Faster recovery may support a smaller tank. |
| Heat-pump storage | 270–315 L | Check the model’s recovery rate and operating schedule. |
| Solar storage | 300–360 L | Confirm storage capacity for the selected system. |
| Continuous flow | No tank | Size by litres per minute at the required temperature rise. |
Tank capacity alone does not show how much usable hot water a system can deliver. Storage temperature, cold-water mixing and recovery rate also affect supply.
For continuous flow, calculate the likely simultaneous demand, such as two showers running together, and compare it with the unit’s rated output at the required temperature rise. Colder incoming water can reduce the available flow rate.
When to choose the upper end
Choose the upper end of the range when demand is concentrated into busy periods or the system has limited time to recover. Relevant factors include:
- several consecutive showers;
- two showers or other hot-water outlets running together;
- long showers or high-flow fixtures;
- regular bath filling;
- frequent guests; and
- appliances connected to the hot-water supply.
Shorter, staggered showers and efficient fixtures may make the lower end suitable. Peak simultaneous demand matters more than the number of bathrooms alone.
Confirming the final size
Give the installer details of:
- the proposed system type;
- whether electricity is continuous supply, controlled load or off-peak;
- the maximum number of showers and outlets used together;
- typical shower duration and bath use;
- appliances drawing from the hot-water supply; and
- regular changes in occupant numbers.
The installer should also confirm that the selected model suits the available plumbing, gas or electrical services. Installation can involve licensed plumbing, gasfitting and electrical work, so do not attempt these connections yourself. Our Sydney plumbing team can assess your peak demand and help confirm a suitable system size.