What size hot-water system do I need?

If you’re replacing your system, our calculator gives you a starting point for a heat-pump tank and the questions to take to an installer. You can use it whoever supplies your electricity.

Published household guides are a starting point. We’ll also show the checks your installer needs to make.

Before you begin, note how many people regularly use hot water and when the busiest showers happen. Add whether anyone often has longer showers or baths, whether regular visitors stay, when the tank can reheat, whether your home gets frosts or sub-zero winter temperatures, and whether the current system still works. The calculator asks for each of these.

A storage tank’s capacity is measured in litres, or L. An instantaneous system heats water as you use it and is commonly rated in litres per minute, or L/min. These describe different jobs. When you change system type, have the installer size the replacement for your household rather than simply matching the old unit’s number.

The litres a household uses across a day are a third quantity again:

For scale, heating 250 litres of water by 20°C stores about 5.8 kWh of heat, ignoring heat losses. That is the heat in the water, not the electricity used to put it there: a conventional electric element and a heat pump need different amounts of electricity to add the same heat.

Our calculator retains the current hub’s published capacity examples. Matching examples cover two to six people regularly using hot water. For one person, or seven or more, it asks for an individual assessment rather than giving a range.

The two columns answer different questions. The benchmark estimates water used across a day; the range comes from published capacity examples for households of that size. Neither one shows when the showers happen.

A worked example

Four people regularly use hot water. The calculator gives a starting range of 270 to 302 L, and the daily-use benchmark is about 200 litres.

Now suppose all four shower one after another before school and work, one often has a longer shower, and the tank can only reheat in a limited tariff window. The starting range stays the same, but the calculator adds checks for the installer: whether the model can cover that whole busy period and reheat before the next use, the usual shower length and showerhead flow, and whether the model suits the circuit and can reheat in the actual available window. Those answers help the installer decide where in the range, or outside it, your home sits.

Several showers in a row draw on the same store of hot water. Showers spread across the day may leave more time for reheating. Recovery time is how long the system takes to heat water again. Ask the installer how the proposed model will handle your busiest period and what hot water will be available afterwards.

Your Home uses about 50 litres of hot water per person per day as a general planning guide. That is water use across the day, not a rule for choosing tank capacity. Your own shower habits and other hot-water use can change it.

What to write down

• When each person showers on a typical weekday, and whether the showers happen close together.
• Usual shower length, and the showerhead flow if you know it.
• How often anyone has a bath, and the other hot-water use around it.
• Regular visitors, and when they shower.
• When the tank can reheat: during the day, in a limited tariff window, or across the day and night.

If the tank will heat only during the day, check that daytime heating can leave enough stored hot water for the next busy period, including on a cloudy winter day. If you are not sure when it heats now, check your hot-water heating hours or tariff before relying on the available reheating time.

For cold-weather performance, the ACT Government’s heat-pump buyer guide recommends checking that the model can operate on the heat pump alone down to −5°C. Ask about winter reheating and booster use for the exact model and your address. A booster is an additional electric heating element used by some systems. Colder inland homes and milder coastal homes can need different assessments.

An operating-temperature limit alone does not prove enough hot-water recovery. Ask the installer to explain how the exact tank and heat pump will recover after your busiest period on a cold morning, and when the booster would run. The Australian Government notes that not all heat pumps are designed for cold locations. For a home on the coast, have the installer assess the local conditions and proposed placement too.

If you want to heat during a solar or cheaper electricity period, tell the installer. The available heating window still needs to cover your hot-water use and the system’s required hygiene cycles. Keep approved temperature settings and safety controls in place.

A timer knows the time, not how much solar is available, so a scheduled daytime run can draw grid electricity when generation is low. A separate controlled-load circuit can also have different supply times and rates from the rest of the house. Hot water that is too cool can allow Legionella bacteria to grow, so a schedule must never stop the system completing the heating it needs for safe operation.

• Your calculator result and the busiest shower time in your home.
• The old system’s model and tank size, if it has a tank, plus whether it has failed.
• Details of baths, regular visitors and any planned change in household size.
• Your hot-water tariff or heating schedule, if you know it.
• Whether your home gets frosts or sub-zero winter temperatures.
• Your postcode, and whether the old system is gas or electric, so support can be checked.

Ask: “Why does this size suit our household, and how will it reheat after our busiest period in winter?” If your system has failed, contact an installer about replacement timing and ask whether temporary hot water is available.

If two installers suggest different sizes, ask each one to explain the busy-period, reheating and winter assumptions behind the number. Compare those answers alongside the complete installed price for each option.

Electrify brings together upgrade discounts and local installers. Check the rebates and ActewAGL offers available for your home before committing to a quote. Our own discounts have appliance, location and electricity-account requirements.

What you pay depends on your address, the system you are replacing and the complete installation. In the ACT, an eligible concession rebate can arrive after you pay for the job. In NSW, an eligible installer discount can go into the quote. Ask for the discount and installed price in dollars, and assess running-cost savings separately.

Check what you could save on a new hot-water system (opens in a new tab)

Related advice: Is your hot-water tank already a battery? · Heat-pump hot-water rebates in the ACT and NSW: what will I actually pay? · What should I electrify first?