Guide · last reviewed 10 September 2026
What size heat pump hot water system do I need?
The short version: work on about 50 litres of hot water per person per day with roughly 1.5 days of storage, which lands a typical four-person home at around 300 litres. The twist with heat pumps is that bigger is not better — the smallest tank that genuinely covers your household is usually the cheapest to run.
The rule of thumb, and where it comes from
Victoria's official solar hot water buyers guide states that each person uses around 50 litres of hot water a day, and that a system should hold about 1.5 days of capacity. Multiply those together and a four-person household comes out at 300 litres — which matches the guide's own worked example, a three-bedroom house with a 300-litre tank.
That is a planning figure, not a measurement of your home. Two long showers a day and a bath at the weekend is a different house from four quick showers before 8am. Treat the table below as a starting point you then adjust with an installer.
| Household | Typical tank | Worth knowing |
|---|---|---|
| 1–2 people | 125–150 L | Compact units suit units and couples; check the unit is rated for your climate zone. |
| 3 people | around 200–250 L | The usual sweet spot for a small family with staggered showers. |
| 4 people | around 250–300 L | Solar Victoria's buyers guide puts a three-bedroom house at a 300-litre tank. |
| 5–6 people | 300–400 L | Or a smaller tank plus a bigger compressor — ask your installer to model recovery, not just volume. |
Why smaller is usually cheaper to run
Every litre you keep hot leaks heat to the air around the clock, whether you use it or not. Solar Victoria's guidance is blunt about it: the smallest tank size will nearly always give the lowest energy use for heat pumps — with the equally important caveat not to go smaller than your household actually needs.
Heat pumps can also afford to be sized tighter than the tanks they replace, because they can reheat efficiently at any time of day rather than only overnight. That is the opposite of the old logic and it is where most people go wrong — see below.
The most common mistake: copying your old tank
Plenty of Australian homes have an oversized electric storage tank for a reason that no longer applies. On a controlled-load (off-peak) tariff, the element could only run overnight, so the tank had to be big enough to carry the household through the entire next day. A heat pump has no such constraint — it can top up in the afternoon.
So matching your old tank litre-for-litre often over-buys: a larger unit costs more upfront and then spends a decade keeping water hot that nobody uses. Ask your installer to size from your household, not from the badge on the tank being removed.
Size the house, not just the people in it
The buyers guide makes a point that is easy to miss: size for the dwelling, because people sell houses and move on, but the hot water system stays. A couple in a four-bedroom house who fit the smallest unit available may save a little now and hand the problem to whoever lives there next — including themselves, if a child or a housemate arrives inside the system's ten-plus year life.
Four things that shift the answer
- Simultaneous use. Volume is only half the story — how fast the unit reheats (its recovery rate) decides whether back-to-back showers work. If your household bunches its hot water into one hour, say so when you get quotes.
- Running it on solar. If you plan to time the compressor to your rooftop solar window, the tank has to carry the evening and morning demand on what it made in the middle of the day. That is an argument for not going to the very smallest unit.
- Cold climates. In genuinely cold areas — Ballarat, Bendigo, Canberra, Tasmania — a heat pump works harder and recovers more slowly on the coldest mornings. Check the model is rated for your climate zone rather than assuming.
- Where it will physically go. An outdoor compressor needs airflow and a sensible spot away from bedroom windows. Sometimes the space you have, not the litres you want, decides the model.
Does tank size change your rebate?
Not directly, and it is the wrong thing to optimise for. Federal STCs are calculated per approved model and climate zone, and state incentives have their own eligibility rules — so the incentive varies between models rather than rising neatly with litres. Pick the size your household needs first, then compare the rebate-adjusted price of the eligible models at that size. Our costs guide covers what moves the installed price.
Frequently asked
What size heat pump hot water system do I need for a family of 4?
Around 250–300 litres for most four-person households. The Victorian Government's solar hot water buyers guide works on roughly 50 litres of hot water per person per day with about 1.5 days of storage capacity, which puts a four-person home at 300 litres — and it gives a three-bedroom house at a 300-litre tank as its own worked example. If your four people all shower in the same hour, size to the top of that range.
How many litres of hot water does one person use a day?
About 50 litres a day per person is the planning figure used in Australian government guidance, though your own household can sit well either side of it. Long showers, baths and a dishwasher on a hot-water connection push it up; short showers and efficient fixtures pull it down.
Is a bigger tank better for a heat pump?
No — and this is where heat pumps differ from what you may be used to. Solar Victoria's guide states the smallest tank size will nearly always give the lowest energy use for a heat pump, because a bigger tank means more water kept hot and more standing heat loss. The caveat matters just as much: do not go smaller than your household actually needs, or you will run out.
Should I just match the size of my old electric hot water system?
Usually not, and copying it is the most common sizing mistake. Old off-peak electric storage tanks were often deliberately oversized because they could only heat overnight on a controlled-load tariff. A heat pump can reheat during the day, so it does not need to carry the same buffer — matching the old tank litre-for-litre often means paying for and heating storage you never use.
Should I size for the people in the house now, or the house itself?
The house. The Victorian buyers guide makes the point plainly — people sell houses and move on, but the hot water system stays. A system sized only for a couple can make a three or four bedroom home harder to live in, and harder to sell, for the ten-plus years the unit is installed.
Get the size confirmed on site
A local installer can check your demand, your space and your climate zone — then quote the eligible models at that size.
General information drawing on published Australian government guidance as at 10 September 2026 — not a quote, a design or advice. Sizing figures are planning rules of thumb; your own usage, water pressure, climate zone and available space change the answer. Always have an installer confirm the size on site.