What Size Heat Pump Hot Water System Do I Need?

The right tank size depends primarily on how many people use hot water daily and their usage habits. Here is the standard sizing guide used by Australian installers:

These are guidelines. Actual usage depends on shower length, whether you have a bathtub, dishwasher habits, and laundry practices. A household of 3 with short showers may use less than a couple who take long baths.

The golden rule: when in doubt, size up. A slightly oversized tank costs a little more upfront but avoids the frustration and inefficiency of running out of hot water. The running cost difference between a 270L and 315L system is negligible.

Australia is divided into climate zones that affect heat pump performance. In colder zones, heat pumps work harder and take longer to heat water, which means you may need a larger tank.

Zones 1-3 (tropical/subtropical): Darwin, Cairns, Townsville, northern WA. No adjustment needed. Heat pumps perform at peak efficiency year-round with warm ambient temperatures. You might even get away with a slightly smaller tank.

Zones 4-5 (temperate): Sydney, Brisbane, Perth, Adelaide, most coastal areas. Standard sizing applies. This is the "design condition" most manufacturers use for their recommendations.

Zone 6 (cool temperate): Melbourne, Canberra, Hobart, Blue Mountains, parts of regional NSW and VIC. Size up one step from the household guide above. For example, a 3-person household in Melbourne should consider 315L instead of 270L. Winter mornings with ambient temperatures of 2-8C slow recovery times by 30-50%.

Zone 7 (alpine/cold): Ballarat, Orange, alpine areas of VIC/NSW/TAS. Size up one step and choose a CO2 model. Cold ambient temperatures (frequently below 5C in winter) significantly slow heat pump recovery. A larger tank and a cold-climate-rated system (like Sanden or Reclaim) ensures you have adequate hot water even on the coldest days.

Climate zone maps are available from the Bureau of Meteorology and the NatHERS website. Your installer should automatically factor in your climate zone when recommending a system size.

The most critical sizing factor is your peak demand period, which is typically the morning rush hour when everyone showers, or the evening when showers, dishwashing, and laundry coincide.

A standard shower uses approximately 35-50 litres of hot water (at 40C mixed temperature) over 5-7 minutes with a standard showerhead. Water-efficient showerheads (WELS 3-star) reduce this to 25-35L.

Calculate your peak demand scenario:

• 2 showers back-to-back = 70-100L
• 3 showers back-to-back = 105-150L
• 4 showers + dishwasher = 160-220L
• 4 showers + dishwasher + one load of washing = 200-260L

Your tank must hold enough hot water (at 60-65C storage temperature) to satisfy this peak demand without running cold. Remember that stored water at 65C is mixed with cold water at the tap, so 1 litre of stored hot water provides roughly 1.5 litres of usable warm water at 40C.

A 315L tank storing water at 65C provides approximately 470L of usable hot water at 40C. That is enough for 4-5 consecutive showers, a dishwasher cycle, and a load of washing before the tank runs cold.

Recovery time is how long it takes your heat pump to reheat the tank after it has been depleted. This is a critical difference from gas systems (which recover in 1-2 hours) and matters for your daily routine.

Typical recovery times for a full reheat (from cold to 60C):

• 150-170L tank: 2-3 hours (heat pump only)
• 250-270L tank: 3-4 hours (heat pump only)
• 315L tank: 4-5 hours (heat pump only)
• 400L tank: 5-6 hours (heat pump only)

These times assume moderate ambient conditions (15-20C). In summer, recovery is faster. In winter, it can be 30-50% slower.

Most heat pump systems also have an electric element boost (typically 1.5-2.4kW) that can supplement the heat pump during high-demand periods. With boost engaged, recovery time drops to 1.5-3 hours for most tanks. However, using the element significantly increases energy consumption, so it should be reserved for peak demand situations rather than daily use.

Pro tip: If you have solar panels, set the heat pump to run during the middle of the day (10am-3pm) when solar production is highest. The tank then stores solar-heated water for the evening and morning peaks, essentially using your tank as a thermal battery.

There are several situations where choosing a larger tank than the standard recommendation makes sense:

• Solar PV system: A larger tank lets you store more solar-heated water during the day for use in the evening. Think of it as a free thermal battery. If you have 5kW+ solar, a 315L or 400L tank maximises self-consumption.
• Cold climate: As discussed above, zones 6-7 should size up to compensate for slower recovery times in winter.
• Teenagers in the house: Shower habits in households with teenagers can consume significantly more hot water than adult-only households. Plan for the worst case.
• Regular guests or Airbnb: If you frequently have overnight guests or use part of your home for short-term rental, size for the maximum occupancy, not just your usual household.
• Bathtub use: A standard bath uses 120-180L of hot water (mixed temperature). If anyone takes regular baths, you need to account for this in your peak demand calculation.
• Future-proofing: Planning to add a bathroom or expecting family growth? Size for where you will be in 5 years, not where you are today. The marginal cost of a larger tank is small compared to the cost of replacing an undersized system.

The cost difference between a 270L and 315L system is typically $200-$400. Between a 315L and 400L system, it is $300-$500. These are small premiums compared to the inconvenience and efficiency loss of an undersized system.

Pairing a heat pump with rooftop solar is one of the most effective ways to minimise hot water costs. Here is how to optimise the combination:

How much solar do you need? A heat pump draws 0.6-1.5kW during operation. A modest 3kW solar system will typically have enough surplus power during the day to run the heat pump entirely on solar. Larger systems (5kW+) can easily power the heat pump with energy to spare.

Timer settings: Most heat pump hot water systems have a built-in timer or can be connected to an external timer. Set the heat pump to operate during peak solar hours (typically 10am-3pm). The system will heat the full tank during the day and store it for evening/morning use.

Tank as thermal battery: A well-insulated 315L tank at 65C holds approximately 18kWh of thermal energy. This is more than most battery storage systems and costs a fraction of the price. By heating this tank on solar during the day, you are effectively storing solar energy for free.

Diverters and smart controllers: Some homeowners install solar diverters (like Catch Power or Solar Analytics) that automatically route excess solar production to the heat pump. This ensures the heat pump only runs when there is free solar energy available, maximising the financial benefit.

With solar pairing optimised, annual hot water running costs can drop to $50-$100 per year, consisting mainly of the small amount of electricity used on cloudy days or during winter when solar production is lower than heat pump demand.