Heat Pump Hot Water Sizing Guide: What Size System Does Your Home Need?

For most homes, tank size should be based on how much hot water the household actually needs — not simply the number of people living there today.

Solar Victoria recommends thinking about the dwelling, because people move in and out but the hot-water system remains with the property. Its guide gives a general benchmark of around 50 litres of hot water use per person per day, with storage systems traditionally allowing approximately 1.5 days of capacity.

However, heat pumps change the sizing conversation. Unlike a conventional electric storage heater, a heat pump can efficiently reheat the tank, so you don't necessarily need a huge tank to cover every possible peak.

That's the balance between comfort, energy use and purchase price.

A heat pump uses electricity to move heat from the surrounding air into the water rather than relying entirely on an electrical resistance element. That means the system's performance depends on more than the tank's litres.

You need to consider:

Solar Victoria explains that heat pumps can be operated using timers, including during periods of solar generation, and that they can often work efficiently at different times of day.

This is one of the most important things to understand before buying. A product might be advertised as 250L Heat Pump, but that tells you only part of the story. You should also ask:

Solar Victoria's current 2026–27 guidance recommends:

It also recommends at least 75L of stored hot water per person as a general sizing consideration.

So don't turn the table above into: "Four people always need 300L." Instead use it as a starting point for the assessment.

Let's make the choice easier.

This range can be relevant for:

• 5+ occupants
• Multiple bathrooms
• Frequent simultaneous showers
• Large baths
• High hot-water consumption
• Extended peak-demand periods
• Large households with limited recovery opportunities

It might sound logical: "If 250L is enough, 300L must be better." Not necessarily.

An oversized heat pump can:

• Cost more upfront
• Take longer to install
• Require more space
• Hold more water than the household needs
• Potentially cycle unnecessarily
• Increase standby losses
• Provide little additional value if the household rarely uses the extra capacity

Going too small can be worse. A household may regularly run out of hot water if:

• Everyone showers around the same time
• The home has several bathrooms
• The household takes long showers
• There are frequent guests
• The tank is too small for peak demand
• Recovery is too slow

Solar Victoria identifies running out of hot water as one of the key risks of undersizing.

Instead of asking only: "How many people live here?" ask these seven questions.

1. How many people live in the home? Start here.
2. How many bathrooms? Two homes with four occupants can have very different peak demand if one has one bathroom and the other has three.
3. When does everyone use hot water? Morning-heavy demand is different from evenly distributed demand.
4. How long are your showers? Five-minute showers and 15-minute showers create very different requirements.
5. Are there baths? Baths can create large single-event demand.
6. Do the dishwasher and washing machine use hot water? Many modern appliances heat their own water internally, which can reduce the household's hot-water storage requirement.
7. Do you have solar PV? If yes, your heat pump can potentially be scheduled to operate during the day and use excess solar generation.

This can change how you operate the system. A heat pump doesn't have to run only when someone is taking a shower. You can often use a timer or control system to heat water during the day.

This effectively turns the hot-water tank into a form of thermal energy storage. Solar Victoria specifically identifies daytime operation using excess solar PV as a useful strategy.

It may be tempting to think: "I have lots of solar, so I should buy the biggest tank." Not necessarily.

Your solar system still has to generate enough excess electricity to operate the heat pump. A large tank can also require more energy to heat from cold.

So the correct calculation is:

— not simply solar system size.

Heat pump performance can change with ambient conditions. In Melbourne, winter brings:

• Colder mornings
• Shorter daylight hours
• Lower solar generation
• Lower outdoor temperatures

This matters if your strategy is: "I'll only run the heat pump during solar hours."

Here's a practical way to think about it.

Again, these are decision ranges, not universal product prescriptions.

This is probably the most common sizing question. Suppose a Melbourne family has:

• Four occupants
• Three bedrooms
• Two bathrooms
• Normal shower usage
• Dishwasher heats its own water
• Washing machine rarely uses hot water
• Rooftop solar

A sensible starting conversation might be 225–300L rather than automatically selecting the largest tank. Then the installer should check:

• Product recovery rate
• Operating temperature range
• Heat-pump output
• Expected hot-water usage
• Solar generation
• Installation location

For two people, 150–225L is a useful starting range. But consider your actual habits. A couple who:

• Takes short showers
• Uses efficient showerheads
• Rarely uses baths
• Has appliances that heat their own water

— may not need as much storage as another couple with heavy hot-water use.

For a larger family, don't try to save money by choosing a small tank without checking recovery. A starting point of 300L+ is more appropriate, with the final size determined by actual demand and product specifications.

Imagine two systems:

If the household's peak demand is predictable, System B could potentially provide adequate service without storing as much water.

This is why comparing only litre capacity can lead to the wrong conclusion. Ask for:

• Usable/storage capacity
• Heat-pump output
• Recovery performance
• Operating range
• Efficiency data
• Boost element details

Heat pumps work by extracting heat from ambient air. Therefore, the minimum operating temperature matters. Solar Victoria's current 2026–27 requirements state that a hot-water heat pump's specified minimum operating temperature should be lower than the minimum recorded temperature for the location, using Bureau of Meteorology data.

Storage hot-water systems need appropriate temperature management. Solar Victoria states that the entire volume of stored water should reach 60°C for at least 30 minutes at least once a week to help prevent Legionella growth.

There isn't one universal daily electricity figure. Actual electricity consumption depends on:

Solar Victoria notes that heat pumps can reduce energy use substantially compared with standard electric storage systems; its current consumer guidance cites potential reductions of almost 80% for some heat pump systems compared with standard electric storage water heaters.

A common mistake is sizing the heat pump entirely around the solar system. For example: 10kW solar = automatically 300L heat pump.

Instead:

Then:

And:

A correctly sized system can help reduce the energy required for hot water. But don't calculate savings simply as: old hot-water bill × 80%.

The actual result depends on the old system, tariff, usage, climate, heat-pump performance and how the system is operated.

The table is a planning guide, not a substitute for product-specific sizing.

A heat pump can be an attractive replacement pathway for a home moving away from gas. But the sizing calculation should still be based on actual hot-water demand.

The operating characteristics are different. A qualified installer should consider:

• Existing usage
• Tank size
• Household demand
• Pipework location
• Electrical requirements
• Heat-pump recovery

This is another common opportunity. A standard electric storage heater uses a resistive element. A heat pump instead transfers heat from the air into the water. Solar Victoria says modern heat pumps can use substantially less energy than standard electric storage systems.

But before replacing the system, check:

• Existing tank capacity
• Household consumption
• Electrical circuit
• Switchboard location
• Drainage
• Noise considerations
• Manufacturer clearance requirements

A heat pump isn't just a tank. Your installer needs to consider where the system will be located. Check:

• Outdoor space
• Airflow
• Clearances
• Noise
• Weather exposure
• Pipe runs
• Condensate/drainage
• Electrical supply
• Service access

Victorian homeowners may currently be eligible for a hot-water rebate of up to $1,000, or up to $1,400 for eligible locally made products, subject to program eligibility and product requirements.

The household income eligibility threshold changed on 1 July 2026 to $150,000 for the relevant Solar Victoria solar PV and hot-water rebate programs.

The Victorian hot-water rebate has specific eligibility rules. Solar Victoria states that its hot-water rebate applies to installations replacing an existing hot-water system that is at least three years old and is not available for new-build homes under that rebate.

Don't compare quotes only by price. Ask each installer for:

Solar Victoria itself recommends asking installers about system sizing, bathrooms, occupants, warranties, STCs, timers, maintenance and other installation details.

The right size isn't: the biggest tank you can afford. And it isn't: the smallest tank that technically works.

It's the system that provides:

• Enough hot water — without regularly running out
• Enough recovery — to meet the next peak period
• Appropriate efficiency — without unnecessary cycling or standby losses
• Appropriate climate performance — for Melbourne/Victoria
• Appropriate integration — with solar, tariffs and household energy use

Samsun Energy's current product portfolio includes iStore, Emerald, Ecogenica, Reclaim Energy, Rinnai and Rheem heat-pump brands. The important part is that the brand should come after the sizing assessment.

A good installer shouldn't simply say: "Four people? Buy 300L." Nor: "You have solar? Buy the biggest tank." Nor: "This model has a rebate, so it's the best option."

That is the level of assessment a major hot-water purchase deserves.

Consider a hypothetical Melbourne household:

• 4 occupants
• 2 bathrooms
• Moderate shower use
• Dishwasher heats its own water
• Rooftop solar
• Most hot-water demand in morning and evening

Solar Victoria's general sizing guidance would put a three-bedroom, four-person home around 300L, but its heat-pump-specific guidance says a smaller tank may work where recovery and manufacturer recommendations support it.

So the correct comparison might be:

The question isn't simply which holds more water. You should compare:

• Recovery rate
• Electricity consumption
• Winter performance
• Solar operating window
• Upfront price
• Available space
• Warranty
• Expected household demand

Larger Heat Pump

Smaller Heat Pump

A tank-size chart cannot tell you the right product for your home. It doesn't account for:

• Your shower duration
• Water pressure
• Number of bathrooms
• Simultaneous demand
• Recovery rate
• Local climate
• Solar generation
• Tariff
• Installation location
• Product operating range

1. What tank size are you recommending and why?
2. How many litres of hot water does our household actually need?
3. What is the heat-pump recovery rate?
4. What happens during Melbourne winter conditions?
5. What is the minimum operating temperature?
6. Can it be scheduled to use excess solar?
7. What is the expected electricity consumption?
8. What rebate and STCs apply to this product?
9. Are electrical and plumbing upgrades included?
10. What happens if the system fails or needs servicing?

For most Melbourne homes, choosing a heat pump hot water system comes down to getting the balance right. As a starting point:

But these are starting ranges, not automatic recommendations. Solar Victoria's guidance makes the same broader point: sizing is a trade-off between cost and performance, and heat pumps can sometimes use smaller tanks because they can efficiently reheat water.

The final choice should consider:

• Solar Victoria — Solar Homes Program
• Solar Victoria — Hot Water Rebate
• Energy Safe Victoria — Electrical Safety Requirements
• Essential Services Commission — Victorian Energy Upgrades Program
• Bureau of Meteorology — Melbourne Climate Data