What Size Hot Water System Do I Need?

Storage tank hot water systems — including electric, heat pump, and solar — hold a fixed volume of heated water ready for use. Choosing the correct tank size ensures your household has enough hot water during peak demand without paying to keep excess water heated unnecessarily.

For 1 to 2 people, a tank between 125 and 180 litres is typically sufficient. This covers daily showers, hand washing, and light kitchen use without maintaining a large volume of hot water. For 3 to 4 people, which is the most common Gold Coast household size, a 250 to 315 litre tank is the recommended range. This provides enough capacity for morning and evening peak usage, including multiple showers, dishwashing, and laundry. For households of 5 or more people, a 315 to 400 litre tank ensures adequate supply even during high-demand periods.

These figures assume standard usage patterns — roughly 50 litres of hot water per person per day. If your household takes longer showers, uses a bathtub regularly, or runs hot water appliances simultaneously, you should size toward the upper end of the range. Conversely, if your household is water-conscious with short showers and efficient appliances, you can size toward the lower end.

Continuous flow (also called instantaneous or tankless) hot water systems heat water on demand as it flows through the unit. Instead of measuring capacity in litres, continuous flow systems are rated by their flow rate in litres per minute (L/min) — the volume of hot water they can deliver simultaneously.

For homes with 1 to 2 bathrooms, a 16 to 20 L/min unit provides enough flow to run one shower and a tap simultaneously. This is suitable for couples or small families who rarely need hot water at multiple points at the same time. For 2 to 3 bathrooms, a 24 to 26 L/min unit handles two simultaneous showers or one shower plus a dishwasher and kitchen tap. This is the most popular size for Gold Coast family homes. For homes with 3 or more bathrooms or high simultaneous demand, a 32 L/min unit ensures no one runs out of hot water even during peak morning usage.

When sizing continuous flow, think about the maximum number of hot water outlets you would realistically use at the same time. Each shower uses approximately 8 to 10 L/min, a kitchen tap uses 4 to 6 L/min, and a dishwasher uses 2 to 4 L/min. Add up the flow rates of the fixtures you might run simultaneously, and choose a unit that meets or slightly exceeds that total.

Several factors beyond the number of people in your home influence the ideal hot water system size.

The number of bathrooms matters more than you might expect. A 3-person household in a home with three bathrooms may need a larger system than a 4-person household with one bathroom, simply because the potential for simultaneous usage is higher. Think about realistic peak demand scenarios — a weekday morning when everyone is getting ready at once is usually the critical period.

Baths versus showers make a significant difference. A standard shower uses 60 to 80 litres of hot water. A full bath uses 150 to 200 litres — nearly three times as much. If anyone in your household takes regular baths, size up by at least one bracket.

Peak usage patterns also matter. Some households spread their hot water usage throughout the day, while others concentrate it into a one-hour morning rush. If your usage is concentrated, you need either a larger storage tank or a higher-flow continuous system to cope. The Gold Coast climate helps here — warmer inlet water temperatures (18 to 24°C depending on season) mean your system does not need to work as hard to reach the target temperature, effectively stretching your capacity.

Finally, if you are installing a heat pump on a controlled load tariff (like Tariff 31), be aware that the system may only have power for part of the day. In this case, the tank needs to be large enough to store all the hot water you need during off-power hours. We typically recommend sizing up one bracket for Tariff 31 heat pump installations.

It is tempting to choose the biggest system available as insurance against ever running out of hot water. However, oversizing your hot water system is a genuine waste of energy and money.

Storage tanks lose heat through their walls constantly — this is called standby heat loss. A larger tank has a greater surface area, which means higher standby losses even if you are not using any hot water. A 400-litre tank that is only half-used still heats and maintains 400 litres of water at temperature. The energy used to keep that unused water hot is pure waste. On the Gold Coast, standby losses are lower than in cooler climates thanks to higher ambient temperatures, but they still add up over the course of a year.

An oversized system also costs more to purchase and install upfront. You pay for capacity you do not use, then pay again through higher running costs to maintain it. The sweet spot is a system sized correctly for your actual usage with a modest buffer — typically one size bracket above your minimum requirement if you occasionally have guests or anticipate your household growing.

Our sizing calculator takes all of these factors into account and provides a personalised recommendation. If you are unsure, a professional assessment of your household's usage patterns will ensure you get the right size the first time.

Different hot water technologies have distinct sizing considerations that are worth understanding before you make a decision.

Heat pump systems generally need a slightly larger tank than a standard electric system for the same household. This is because heat pumps take longer to reheat a full tank of water — typically 3 to 5 hours compared to 1 to 2 hours for a high-powered electric element. The larger tank acts as a thermal battery, storing enough heated water to last through periods when the heat pump is not running or is on a controlled load tariff. A 4-person household that might use a 250L electric tank should consider a 270 to 315L heat pump.

Solar hot water sizing depends heavily on collector area and orientation. A system with a larger collector area can heat water faster and maintain higher temperatures, potentially allowing a slightly smaller tank. Conversely, a smaller collector paired with a larger tank gives the system more thermal storage to carry over from sunny days to cloudy ones. On the Gold Coast, a typical 2-panel solar system with a 315L tank covers most family households comfortably.

Continuous flow gas systems are the simplest to size because there is no tank — you simply choose the flow rate that matches your peak simultaneous demand. However, if you are switching from a storage system to continuous flow, be aware that you may need a gas supply upgrade if your current gas line cannot deliver sufficient volume to the new unit.

Regardless of system type, we always perform a proper sizing assessment before recommending a specific model. Getting the size right is one of the most important decisions in your hot water upgrade.