How Long Does a Heat Pump Take to Heat a Pool?
Straight numbers for Australian pool owners who have just switched the thing on and are wondering why the water is still cold.
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A correctly sized pool heat pump takes 24 to 36 hours of run time to lift a typical Australian backyard pool from about 18 degrees to a swimmable 28 degrees. Spread over a normal filter cycle of ten to twelve hours a day, that is two to three days for the first heat up of the season. After that, holding the temperature takes a few hours a day, not days.
The number that matters is run hours, not calendar days. Everything below is about how many run hours you need and what makes that figure blow out.
The short answer by pool size
A correctly sized heat pump lifts a pool roughly 2 to 3 hours per degree, almost regardless of pool size. That surprises people. It happens because a bigger pool gets matched to a bigger heat pump, so the two scale together and the hours per degree stay in the same band.
Heat up time by pool volume
| Pool volume | Matched heat pump | Run time per degree | Run time, 18 to 28 degrees |
|---|---|---|---|
| 20,000L plunge or small pool | 13kW | About 2.2 hours | 20 to 24 hours |
| 30,000L small family pool | 16kW to 17kW | About 2.5 hours | 25 to 30 hours |
| 40,000L standard family pool | 20kW | About 2.9 hours | 29 to 35 hours |
| 50,000L large family pool | 24kW | About 3 hours | 30 to 36 hours |
| 65,000L large or lap pool | 35kW | About 2.7 hours | 27 to 33 hours |
These figures assume a covered pool in mild conditions with the heat pump running near its rated output. Leave the pool uncovered in a cold snap and you can double them, or never quite get there at all. More on that below.
Not sure which unit matches your pool? Start with the sizing guide, What Size Pool Heat Pump Do I Need?, then come back to the timing.
The maths behind the table
Lifting 1,000 litres of water by 1 degree takes 1.16 kWh of heat energy. That is fixed physics and no brand gets around it. So a 40,000 litre pool needs about 46 kWh of heat for every single degree.
Feed that 46 kWh in with a 20kW heat pump and the pure maths says 2.3 hours. Real pools lose heat to the air and the ground while you are heating them, so add roughly 25 percent and you land at about 2.9 hours per degree. That is where the table comes from.
To run it for your own pool: multiply your volume in thousands of litres by 1.16, divide by the heat output in kW, then add a quarter. A 25,000 litre pool with a SensaHeat ES 13kW works out at 29 divided by 13, which is 2.2 hours, plus a quarter, so about 2.8 hours per degree.
Why the first heat up of the season is the slow one
The first heat up is slow because you are moving the entire body of water a long way, from winter cold to swimming warm, and the pool is fighting you the whole time. Ten degrees of lift on 40,000 litres is roughly 460 kWh of heat. That is a genuine amount of energy. No heat pump does it in an afternoon.
Once the pool is at temperature the job changes completely. You are no longer heating water, you are replacing what leaked out overnight, which is usually one to two degrees. That is three to six hours of run time a day in shoulder season, and often less in the middle of summer.
So the pattern for most owners is a slow two or three day start in about September, then an easy season after it. Switching the heat pump off for a fortnight and starting again means paying the slow start twice, which is why most people leave it set and let it tick over.
What actually changes the number
Air temperature, the biggest factor by a mile
Air temperature changes heat pump output more than anything else on this page. A heat pump pulls warmth out of the air, so cold air means less heat available and a lower output. Rated kW figures are measured at around 26 degrees air, 26 degrees water and 80 percent humidity, which is a warm summer day.
Run the same unit in 15 degree air and output can fall by 30 to 40 percent. A 20kW unit behaves like a 13kW unit, and your 3 hours per degree becomes closer to 4.5. That is the whole reason early spring heating feels so slow while January heating feels instant. Inverter models such as the Guardian Platinum 20kW inverter and the Fairland Aqua X20 16kW hold up better in cool air than fixed speed units, because they can push the compressor harder when conditions get tough.
Whether the pool is covered
A pool cover roughly halves the heat you lose, which roughly halves the time you spend replacing it. An uncovered pool loses most of its heat through evaporation off the surface, and it does it hardest overnight when the air is coolest and driest.
Heating an uncovered pool in early spring is close to pointless. You put ten hours of heat in during the day and the pool hands most of it back to the sky by 6am. If you buy one accessory this season, buy the cover before you buy extra kilowatts.
How long the filter pump runs
A pool heat pump only heats while water is flowing through it, so filter pump run time is a hard cap on heating time. Set the filtration to four hours a day and four hours is all the heating you get, no matter what the heat pump is capable of.
For a first heat up, extend the filter cycle to ten or twelve hours, or run it continuously for the two days it takes. Variable speed pool pumps make that cheap, since running long and slow costs far less than running short and hard.
Wind and surface area
Wind across an open pool strips heat fast, and a long shallow pool loses more than a compact deep one holding the same litres. Surface area is where the loss happens, so a 12 metre lap pool at 1.2 metres deep is harder to heat than a small pool of identical volume. Windy coastal sites are the classic case where people wish they had gone one size up.
Holding temperature is the easy part
Holding a pool at temperature costs a fraction of the first heat up, typically two to five run hours a day. A covered pool only drops one to two degrees overnight, and putting one or two degrees back is a short job for a correctly sized unit.
This is also where an inverter earns its money. A fixed speed heat pump switches hard on, gets there, and switches off. An inverter drops to a low speed and trickles heat in, which uses less power and holds the water steadier. The guide to pool heat pump COP covers why that low speed running is so much more efficient.
Buying for speed, not just for size
Going one size up is the only reliable way to heat a pool faster. There is no setting, no additive and no trick plumbing that beats extra kilowatts. If waiting three days in September annoys you, buy the bigger unit up front, because capacity cannot be added later without replacing the whole thing.
For a standard 40,000 litre family pool the sensible options are the Midea M Series 20kW with WiFi at $3,799, our best seller and the one covered on the Midea 20kW page, or the Guardian Eco Pro 20kW at $4,940. Step up for faster shoulder season heating and quieter running with the Guardian Lux-V 17kW full DC inverter at $6,990. Pools past 60,000 litres want the Guardian Eco Pro 35kW at $9,451.
Prices current as of August 2026, AUD including GST.
The full range from 9kW to 35kW sits in Heat Pumps for Pools & Spas.
Frequently asked questions
How long does a heat pump take to heat a pool from cold?
A correctly sized heat pump takes 24 to 36 run hours to take a typical 40,000 litre Australian pool from about 18 degrees to 28 degrees, which is two to three days on a normal ten to twelve hour filter cycle. Cold air or no pool cover can double that.
How long does it take to heat a pool by 1 degree in Australia?
About 2 to 3 hours of run time for a correctly sized heat pump on a covered pool in mild conditions. The figure comes from the 1.16 kWh of heat needed per 1,000 litres per degree, divided by the heat pump output in kW, plus about 25 percent for losses while heating.
Can a pool heat pump heat a pool overnight?
No, not from cold. Overnight running gives you maybe eight hours, which is three degrees at best, and night air is the coldest air of the day so output is at its lowest. Heat pumps do their best work in the afternoon when the air is warm, which is why most owners run them through the middle of the day.
Does a pool cover make a heat pump heat faster?
Yes, a cover roughly halves heat loss and cuts heat up time close to half on an exposed pool. The cover does not add heat, it stops the pool giving back what the heat pump just put in, especially overnight through evaporation. It is the cheapest performance upgrade available.
Will a bigger heat pump heat my pool faster?
Yes, and it is the only real way to speed it up. Doubling the kW output roughly halves the run hours, and a larger unit also copes better in cold air where a smaller one struggles to reach temperature at all. Oversizing slightly costs a little more to buy and nothing extra to run, because the bigger unit reaches temperature then idles.
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Send through your pool volume, whether it is covered, your postcode and the temperature you want to swim at. We will tell you the model that fits and roughly how long the first heat up will take. It beats guessing off a table.
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This guide is general information for Australian pool owners and is not a substitute for advice on your specific pool and site. Heating times vary with climate, pool shape, wind exposure and equipment condition. Pool heat pumps are electrical appliances and must be connected by a licensed electrician. Confirm sizing with us or your pool professional before you buy.