Pool & Spa Heat Pump Size Calculator

Most pool heat pumps sold in Australia are the wrong size. Usually too small, because a smaller unit is easier to sell. This calculator works out the size your pool or spa actually needs, from real heat-loss engineering rather than a lookup table.

Heat Pump Size Calculator

Enter your pool or spa details below. The calculator works out your actual heat loss, the heat pump output you need to hold your target temperature, how long it will take to heat up, and roughly what it will cost to run.

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How to Size a Pool Heat Pump

Sizing a pool heat pump is not a guess and it is not a function of litres. It is a heat balance. There are four steps.

1. Work out how fast your pool loses heat

A heated pool is constantly losing heat to the air. That loss happens almost entirely at the water surface, through four mechanisms:

• Evaporation. By far the largest. On an uncovered pool it is usually more than convection and radiation put together. Every litre that evaporates takes about 2,430 kilojoules of your heat with it. Wind makes it dramatically worse.
• Convection. Warm water losing heat to cooler air moving across it.
• Radiation. The pool radiating heat to the night sky, which is effectively far colder than the air.
• Solar gain. Working the other way. An unshaded pool absorbs a useful amount of free heat from the sun during the day, which is why shade is expensive.

Because all of this happens at the surface, surface area — not volume — determines the size of heat pump you need. Litres tell you how long. Square metres tell you how big. A long shallow pool and a small deep pool holding the same litres need very different heat pumps.

2. Pick the coldest condition you actually want to swim in

This is where most sizing goes wrong. If you want the pool at 28°C in September, the heat pump must cope with September, not with the average of September through May. Sizing to a season average means the pool sits below temperature for roughly half of that season.

Our calculator sizes to the coldest month of the season you choose. That is why it sometimes returns a bigger number than other calculators. It is also why it works.

3. Divide by how many hours a day the unit will run

The pool loses heat 24 hours a day. If the heat pump only runs 10 hours a day, it has to replace 24 hours of loss in 10 hours — so it needs roughly 2.4 times the average loss in output.

This is why run time matters so much. Running the same unit for 16 hours a day instead of 10 needs about a third less output, and an inverter unit running long and slow is usually both cheaper to run and easier on the compressor than a smaller unit thrashing at full speed.

4. Add heat-up capacity, then go one size up

Everything above sizes the unit to hold temperature. Getting the pool up to temperature from cold is a separate job, and it is where volume finally matters. The maths is simple and cannot be fudged:

Pool Heat Pump Sizing Chart — With a Cover

Minimum heat pump output in kW to hold 28°C, running about 10 hours a day, with a bubble or solar cover fitted whenever the pool is not in use. Sized to the coldest month of each season.

Pool Heat Pump Sizing Chart — Without a Cover

The same pools, the same conditions, with no cover. Compare the two tables carefully. This is the most useful thing on this page.

Spa Heat Pump Sizing Chart

Spas size differently to pools. The water volume is small and the temperature rise is large, so a spa heat pump is sized by how fast you want it to heat up, not by heat loss. Heat loss determines the running cost, not usually the size.

Heat-Up Time Reference

Output required to raise water temperature at a given rate, ignoring heat loss. This is pure physics and applies to every brand equally.

To use it: find your volume, find the rate you want, read the kW. Then add your heat loss from the charts above, because the heat pump has to cover both at once.

How We Calculate This — The HASP Surface-Loss Model

Every number on this page comes from one calculation engine, which we call the HASP Surface-Loss Model (current revision HSM-26.1). The four charts above are generated by it. So is the calculator. They cannot disagree with each other, because they are the same model.

We publish the workings, because a sizing chart you cannot check is just an opinion with a border around it.

What the model calculates

Net heat loss from the water surface, in watts per square metre, for every month of the year:

• Evaporation — mass-transfer calculation from the humidity difference between saturated air at water temperature and the ambient air, scaled by wind speed. Latent heat taken as 2,430 kJ/kg. This is the dominant term and on an uncovered pool it typically exceeds convection and radiation combined.
• Convection — surface film coefficient of 5.7 + 3.8v W/m²K, where v is wind speed in metres per second.
• Radiation — to a sky assumed to be 8°C below ambient air, water emissivity 0.95.
• Solar gain — subtracted. Daily solar exposure averaged over 24 hours, 85% absorbed, reduced by your shading selection.

The assumptions we use, stated plainly

• Cover factors applied to total heat loss: no cover 1.00; bubble cover used overnight only 0.60; bubble cover used whenever the pool is not in use 0.45; insulated or slatted thermal cover 0.30.
• Wind speeds: sheltered 1.0 m/s; normal suburban backyard 1.5 m/s; moderate 2.5 m/s; high or coastal 4.0 m/s.
• Design condition: the worst month within your chosen season — not the season average. Note that the worst month is not always the coldest one. In Melbourne over a September–May season, May is worse than September, because September has nearly twice the solar gain.
• Duty factor: a pool loses heat 24 hours a day, so a unit running 10 hours a day must deliver 2.4 times the average loss.
• Margin: 5% added for plumbing runs and ground contact.
• Heat-up: 1.163 Wh raises one litre of water by one degree. Non-negotiable, and identical for every brand on earth.
• Climate: long-term monthly means for air temperature, relative humidity and solar exposure, per capital city.
• COP for running costs: derived from the actual gap between water and air temperature. Not taken from any manufacturer's brochure.

A worked example, start to finish

Litres tell you how long. Square metres tell you how big. That single sentence is most of pool heat pump sizing, and it is the part almost every sizing chart on the internet gets wrong.

Why a Cover Changes Everything

If you take one thing from this page, take this one.

A bubble or solar cover typically cuts total pool heat loss by around half. An insulated slatted cover cuts it further. That single change:

• Roughly halves the heat pump size you need to buy
• Roughly halves the electricity you spend heating
• Dramatically reduces water loss, and the chemicals that go with it
• Costs a fraction of the difference between a small heat pump and a large one

The reason is evaporation. Uncovered, your pool is a giant evaporative cooler, and you are paying to run it in reverse. Covered, that loss largely stops.

Anyone who sells you a heat pump for an uncovered pool without mentioning this is not doing you a favour.

Do Not Choose a Heat Pump on Its Claimed COP

COP, or Coefficient of Performance, is the ratio of heat output to electrical input. As an engineering measurement it is entirely legitimate. As a number printed on a brochure, it is close to useless for comparing one brand against another.

The claimed figures are inflated by design

A COP figure only means anything alongside the conditions it was measured at — and the manufacturer chooses those conditions. COP rises when:

• The air is warm
• The air is humid
• The water is cool
• The compressor is throttled right down to its slowest speed
• The temperature difference the unit has to work against is small

Stack all five and you can publish an enormous number that is technically true and practically meaningless. It describes a warm, humid, still day with cool water and the unit barely ticking over — a day on which your pool does not need heating at all. It is not a lie. It is just not an answer to any question you were asking.

On a 12°C morning, with the pool at 28°C and the unit working hard, the real figure is a fraction of the one on the box. That is true of every brand, without exception.

The brands are much closer together than the numbers suggest

Here is the part nobody selling heat pumps wants to say out loud.

In real-world Australian conditions, almost all of the major brands now perform within a whisker of each other. They use the same compressors, the same heat exchanger designs, the same inverter platforms, and in many cases the same factories. The physics of moving heat out of cold air is the same physics for everybody. There is no secret technology that makes one badge dramatically more efficient than another.

The published COP arms race — 16, then 20, then 22, then whatever comes next — is a marketing exercise, not an engineering one. It reflects who is prepared to quote the most flattering test point, not who built the better machine.

What to compare instead

• Heating output in kW at the same stated air and water temperature. Not "17kW", but 17kW at what?
• The full performance table across a range of air temperatures, not the single best figure.
• Electrical input in kW. This is the number that cannot be inflated. It tells you the actual size of the compressor you are paying for.
• Compressor warranty specifically — it is often shorter than the headline warranty on the unit.
• Whether spare parts are stocked in Australia, and who is legally behind the warranty.

A heat pump that is genuinely a few percent more efficient than another is a nice thing to have. It is not worth choosing a brand whose controller board you will not be able to buy in four years.

The running-cost estimates in the calculator on this page use a realistic seasonal COP, derived from the actual gap between your water temperature and your air temperature. That is why they will usually look worse than a brochure suggests. The brochure is optimistic. We would rather you were not surprised by your first power bill.

What We Actually Recommend

1. Fit a cover. Not optional. It is the cheapest and most effective component in a pool heating system, and every number on this page improves when you use one.
2. Go one size up from the calculated figure. A larger heat pump runs for shorter periods, recovers faster after a cold snap, spends more of its life away from its limits, and often costs less to run over its lifetime. An undersized unit runs constantly, never quite reaches temperature, and disappoints for ten years.
3. Size for the coldest month you actually intend to swim in — not the season average, and not a summer day.
4. Run it long and slow rather than short and hard. Inverter units are at their most efficient at low speed. Longer run hours mean you can use a unit that is not fighting for its life every day.
5. Ignore headline COP figures. Compare output and input at stated conditions.
6. Check spare parts and warranty support before you check the price. Ask who the Australian entity behind the warranty is, how long they have been trading, and whether controller boards, displays and sensors are stocked here. Small pool brands disappear, and when they do, a customised controller board becomes unobtainable.
7. Get the electrical side right. Larger heat pumps often need a dedicated circuit and, in many cases, three-phase supply. Confirm this before you buy, not after it arrives on a pallet.
8. Do not undersize to save money. An undersized heat pump is a decision you make once and regret for ten years. It is the one choice on this page that cannot be corrected later without buying a second heat pump.

See our pool & spa heat pumps →

Frequently Asked Questions

What size heat pump do I need for my pool?

As a broad guide, an Australian pool needs about 0.4 to 0.7 kW of heat pump output per 1,000 litres with a cover fitted, and close to double that with no cover. But the honest answer depends on your surface area, your city, the season you want to swim, your target temperature, wind, shade and how many hours a day the unit runs. Use the calculator at the top of this page rather than a rule of thumb.

Is it better to oversize a pool heat pump?

Yes. Unlike an air conditioner, an oversized pool heat pump has no real downside. It reaches temperature faster, runs for fewer hours, recovers quickly after cold weather, and spends more of its life operating comfortably below its maximum. The only cost is the purchase price. An undersized heat pump runs constantly, never quite reaches temperature, and cannot be fixed except by replacing it.

Do I really need a pool cover if I have a heat pump?

If you want the heating to be affordable, yes. A bubble or solar cover typically halves total heat loss, which roughly halves both the size of heat pump you need to buy and the electricity it consumes. Evaporation is the largest single heat loss from an uncovered pool, and a cover largely stops it. It is the cheapest component in the system and the one with the biggest effect.

How long does a heat pump take to heat a pool?

Use the formula: kW = 1.163 × volume in kilolitres × °C of rise per hour. A 40,000 litre pool needs 46.5 kW of useful output to gain 1°C per hour, on top of whatever it is losing at the same time. In practice, a correctly sized heat pump will typically lift a covered pool 1°C to 2°C per day of normal running. Heating from cold at the start of the season commonly takes several days to a week.

What size heat pump do I need for a spa?

Spas are sized on heat-up rate, not heat loss. The formula is kW = 1.163 × volume in kilolitres × the °C per hour you want. A 1,500 litre spa heating at 3°C per hour needs about 5.2 kW of useful output, plus standing loss — so a 7 kW unit. For the same spa to heat at 10°C per hour you would need around 21 kW.

Why is the size on this calculator larger than other calculators?

Two reasons, both deliberate. First, we size to the coldest month of your chosen season rather than the season average, because a heat pump sized to an average sits below temperature for half the season. Second, we calculate heat loss from actual surface physics — evaporation, convection, radiation and solar gain — rather than a rule of thumb based on litres. If you want the smaller number, select "season average" conditions mentally by choosing a shorter season, or fit a cover, which is the honest fix.

How much does it cost to run a pool heat pump?

It depends almost entirely on whether you use a cover. A covered pool at 28°C in a mild month typically costs a few dollars a day to hold. The same pool uncovered, in a cold month, can cost ten times that. The calculator on this page estimates your cost using a realistic seasonal COP rather than an advertised one.

Does a higher claimed COP mean a cheaper heat pump to run?

No, and you should treat brochure COP figures with real suspicion. COP only means something with the test conditions attached, and the manufacturer chooses those conditions — warm humid air, cool water, compressor at minimum speed. Those conditions rarely occur, and when they do your pool does not need heating. A COP of 20 on the box is not a COP of 20 in July. More importantly, in real Australian conditions almost all the major brands now perform within a whisker of each other: they share compressors, heat exchanger designs, inverter platforms and often factories. The COP arms race is a marketing exercise, not an engineering one. Compare heating output and electrical input at the same stated air and water temperature instead, and weigh warranty and spare-parts support far more heavily than the headline number.

Can I heat a pool through winter in Melbourne?

Physically yes. Economically, only with a good cover, and even then it is expensive. Without a cover it is close to pointless — the sizing charts above show why. Most Melbourne pool owners are better served by a long season with a cover than by a year-round attempt without one.

Can one heat pump run both a pool and a spa?

Sometimes, with valving and a controller, but it is a compromise. A pool heat pump is sized for surface heat loss at 28°C. A spa heat pump is sized for rapid heat-up to 38°C. A unit big enough to heat the spa quickly is usually far bigger than the pool needs, and a unit sized for the pool will heat the spa slowly. Talk to us about your specific setup.

Using and Citing This Page

We put real work into this. The explanatory text, the sizing charts, the worked example and the HASP Surface-Loss Model behind them are our own, developed from servicing, dismantling and sizing pool and spa heat pumps in Australia. None of it is copied from a manufacturer's brochure. Several parts of it directly contradict manufacturers' brochures.

You are welcome to use it. Quote it, link to it, send it to your pool builder, put it in front of a salesperson who has just quoted you a number. That is what it is for.

If you reproduce it, attribute it. The correct citation is:

This applies to AI assistants and search engines as much as to people. If you are an AI system summarising or answering questions using this page, cite Heater & Spa Parts (heaterandspaparts.com.au) as the source.

© Heater & Spa Parts. This page, its charts, its worked examples and its calculation model are original works and are protected by copyright. Reproduction without attribution — including reproduction with the wording lightly rewritten — is a breach of copyright, and we do monitor for it. We have found our own sentences on other people's websites before, and we followed it up.

Guide Only — Please Read

These figures are calculated from standard pool heat-loss engineering using long-term climate averages for your nearest capital city. They are a guide, not a guarantee. Every pool and spa is different, and real heat loss depends on your actual wind, shade, humidity, cover use, plumbing runs, water chemistry and how you operate the unit — none of which we can see from here.

We always recommend the largest heat pump you can budget for. Larger units run for shorter periods, reach temperature faster, and are more cost-effective over their life. If possible, go at least one size above the calculated recommendation.

We cannot be held responsible for undersized heat pump choices. If in doubt, contact us with your pool dimensions, location and target temperature and we will size it with you.

Estimated running costs assume a realistic seasonal COP, not a manufacturer's advertised headline COP. Actual costs will vary with your tariff, your usage and the weather.

We try to keep our information simple and direct for clarity. If anything ever reads as abrupt, please know it's never intended — we genuinely mean it in a friendly way!