Why there's no single answer
Four people who shower in the same hour ask far more of a system than four people who spread their showers across the day. Same house, same headcount, different size.
The type changes the question too. A storage tank, a heat pump and a solar water heater all keep heated water in a tank, so their size is about volume. A continuous flow unit has no tank, so the question becomes how many hot taps it has to supply at once.
What to count and measure
- Your busiest hour. How many showers run back to back, morning or night.
- Outlets running together. Two showers at once, or a bath filling while someone showers, is a bigger ask than one at a time.
- Big users. A deep bath or a large shower head uses more hot water in one go.
- The label on your current unit. It shows the model and size. If it has kept up so far, that's a useful starting point.
- The space. Measure where the unit sits now and anywhere else a new one could go, including the height.
- Your energy setup. Do you have mains gas? Is the electric hot water on its own tariff? The plumber will ask.
Running out sooner than you used to?
If the hot water used to last and now it doesn't, and nobody new has moved in, the system may have a fault rather than being too small. A failing element, thermostat or burner can look exactly like a size problem. Get it checked before you pay for a bigger unit. Our no hot water guide covers the first checks.
What changes the answer for each type
Heat pump. It heats the water in its tank using an air source heat pump. Ask how long the model takes to reheat after the tank is used up, because that decides how many showers in a row it can keep up with.
Solar. Some solar water heaters have an electric booster: Consumer Affairs Victoria refers to an electric-boosted solar water heater. Ask when the booster runs, because that affects how much hot water you have on a dull day.
Continuous flow. Ask whether the unit will keep up with your two busiest outlets running together, and whether your gas pipe and meter suit it.
Gas in Victoria. From 1 March 2027, a worn-out gas system in an existing Victorian home must be replaced with an efficient electric one (check current rules). If you have a tankless gas unit now, an electric replacement such as a heat pump means finding room for a tank.
Rules that touch size
STC limits. STCs, the federal discount, only apply to air source heat pumps under 425 L and solar water heaters under 700 L, and only for models on the Clean Energy Regulator's register. If you're looking at a very large system, check it sits under those limits before you count on the discount. More on STCs.
Storage temperature. Size doesn't change how hot stored water has to be. The building regulator says Legionella growth is reduced significantly in water stored at 60°C and above, so a bigger tank isn't a reason to run it cooler.
Space. For Victorian rentals, one exemption from the new hot water standard is "insufficient space to install a heat pump system or rooftop solar panels". That tells you space is a real limit, so measure before you fall for a model.
Questions to ask the plumber
- How did you work out this size for our household?
- Will it keep up when two showers run at the same time?
- How long does it take to recover once the hot water is used up?
- Does the size or model change which discounts apply?
- Where will it go, and is there room to service it later?
What we would do
Start from what you have. If your current system has kept up, the same size is a sensible baseline, and the bigger question is which type to get. If it hasn't kept up, count your busiest hour rather than an average day, and ask the plumber to show how they reached their number. Sizing is worth a proper conversation with a plumber who specialises in hot water before you sign, because you live with the answer every morning.