Quick Answer

For most Melbourne households, the rule of thumb is 50 litres of storage capacity per person for electric and gas storage systems, or a continuous flow unit rated at 16–20 L/min for 1–3 people and 20–26 L/min for 4+ people. Getting the size right matters: too small means cold showers, too large means wasted energy heating unused water.

Storage Tank Size by Household

Storage systems hold pre-heated water ready to use. The key figure is how much hot water your household uses at peak demand — typically the morning shower rush. These guidelines apply to gas and electric storage systems in Melbourne homes:

Household Size Recommended Tank Size Notes
1–2 people 125–160 litres Solo or couple, one bathroom
3–4 people 160–250 litres Family with one main bathroom
4–5 people 250–315 litres 2+ bathrooms or high usage
5–6+ people 315–400 litres Large family, frequent simultaneous showers
Heat pump systems 200–315 litres Slower recovery, so slightly larger recommended
Pro tip: Heat pump systems recover more slowly than gas, so size up by one step. A family needing 250 L gas should choose a 270–315 L heat pump to avoid running out during the morning rush.

Continuous Flow System: Flow Rate by Household

Continuous flow (instantaneous) systems are rated in litres per minute (L/min) rather than tank size. The key calculation: add up all hot water outlets that could run simultaneously at peak time.

Household / Usage Recommended Flow Rate Example Scenario
1–2 people, 1 bathroom 16 L/min One shower + kitchen tap
2–3 people, 1–2 bathrooms 20 L/min One shower + dishwasher
3–5 people, 2+ bathrooms 24–26 L/min Two showers simultaneously
Large families, 3+ bathrooms 26–32 L/min Two showers + kitchen

A standard shower uses approximately 9 L/min (water-efficient showerhead) to 12 L/min (older showerheads). A bathroom tap uses 3–6 L/min. Add up your simultaneous worst-case and choose a unit with at least 10–15% headroom above that figure.

What Else Affects the Right Size?

Cold Water Inlet Temperature

Melbourne’s winter mains water temperature drops to 8–12°C in June and July, compared to 18–22°C in summer. A continuous flow system must work harder in winter to reach 50–60°C, which effectively reduces its usable flow rate by 15–20% in cold weather. Size for winter peak demand, not summer.

Off-Peak Electric Timing

Off-peak electric storage systems heat overnight and are not permitted to reheat during peak tariff hours. This means the tank must hold enough water for the entire next day in a single heat cycle. For a 4-person household, that typically means a 250–315 L tank, even if steady-state daily use would be lower.

Heat Pump Recovery Rate

Most residential heat pump hot water systems heat at 1–3 kW compared to 3.6–6 kW for an electric element. This means recovery from cold takes longer. Rheem’s 270 L heat pump recovers at about 185 L per 3.5 hours, while a gas storage system of the same size recovers in under an hour.

Safety warning: Never reduce the thermostat below 60°C on a storage tank to “save energy.” Water stored below 60°C risks Legionella bacteria growth. 60°C at the tank, tempered to 50°C at the tap outlet, is the Australian standard (AS 3500.4).

Sizing Tips and Gotchas

  1. Always size for peak demand, not average use. A couple that hosts family regularly needs more capacity than their everyday usage suggests.
  2. Check physical clearances first. A 315 L tank is approximately 1.7 m tall and 600 mm wide. Measure the installation space before ordering.
  3. Shared hot water in dual-occupancy. Two units sharing one system often undersize — each dwelling should be calculated independently and the totals added.
  4. Tankless under-sink boosters. In large homes with long pipe runs, a small 10–15 L point-of-use electric unit at a distant bathroom can supplement the main system and eliminate long waits.
  5. Gas pressure matters for continuous flow. Units need 200–350 kPa minimum inlet pressure. Low-pressure streets in older parts of Frankston or Mornington can cause ignition issues with undersized units.
  6. Ask your plumber to confirm recovery time. Especially with heat pumps, the gap between “tank capacity” and “usable hot water in the morning” can surprise families switching from gas storage.
  7. Solar systems need a quality electric or gas boost. On cloudy Melbourne winter days, solar collectors produce minimal heat. The boost element or gas burner capacity should be sized for full household demand, not just top-up.
  8. VEU rebates are size-specific. Some Victorian Energy Upgrades heat pump rebates specify minimum tank sizes (often 200 L+). Check the ESC website before choosing a smaller unit.

FAQ

Is a 125 L tank big enough for two people?

For most couples with moderate usage — one shower each in the morning and normal kitchen use — a 125 L gas or 160 L electric storage system is adequate. If one or both take long showers or you regularly run a bath, step up to 160–200 L. Electric off-peak systems should be sized more generously since they cannot reheat during the day.

What happens if I buy a hot water system that is too small?

The tank runs cold before everyone has showered in the morning. With electric off-peak, you may wait until the next overnight cycle for it to reheat — which means no hot water until the following morning. With gas, recovery is faster (30–60 minutes) but still disruptive. Consistently running a tank empty also stresses the element and shortens its lifespan.

Does a bigger tank always cost more to run?

A larger tank loses slightly more heat through its walls (standby loss), so running costs are marginally higher even if you don’t use all the water. However, the difference between a 160 L and 250 L well-insulated tank is usually only $10–$30 per year. The cost of running out of hot water — in frustration and potential damage from cold-water reheats — is generally higher than the standby loss penalty.

My household varies — sometimes 2 people, sometimes 6. What size do I choose?

Size for your highest-occupancy situation and accept slightly higher standby losses during low-occupancy periods. Alternatively, a gas continuous flow unit removes this problem entirely — it produces hot water on demand regardless of household size, with no tank to run dry or waste energy maintaining.

Do I need a licensed plumber to install or replace a hot water system in Victoria?

Yes. In Victoria, all hot water system installation — including like-for-like replacements — must be performed by a licensed plumber. Gas systems additionally require a licensed gasfitter. Check your installer’s VBA registration at vba.vic.gov.au. Unlicensed work voids manufacturer warranties and may affect your home insurance.

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