How Long Does a Pool Take to Heat Up? The 58 kWh Rule

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How long does a pool take to heat up? A 50,000-litre pool needs about 58 kWh of heat per 1°C. A 21 kW heat pump takes about 2.8 hours a degree, so 22 hours for an 8°C rise; a 44 kW gas heater about 11 hours. Those are floors: the pool keeps losing heat while you heat it.

The short answer

The 58 kWh rule: 50,000 kg of water × 4.186 kJ/kg·K = 209,300 kJ, or 58.1 kWh, for every 1°C. That is physics, not a maker’s figure.
Heat pump: one maker’s manual rates its units at 16 kW and 21 kW with 19°C air, and the larger at 26.1 kW with 27°C air. An 8°C rise takes 17.8–29.1 hours of running before losses.
Gas: one residential range is published at 24–94 kW output, so 4.9–19.4 hours for 8°C before losses. Gas is the only type indifferent to the weather.
Solar has no kW rating. Even capturing every joule of Sydney’s October sun on a 32 m² collector, an 8°C rise takes at least 2.6 days.
Losses decide the real answer. energy.gov.au says a bubble cover cuts heat losses from a heated pool by as much as 75%; outside NSW the building code requires a cover and a time switch with gas or heat pump heating.

Time to heat a 50,000 litre pool, by published heater output (58.1 kWh per °C; outputs from makers’ manuals, read 4 October 2026; the 30% loss column is our assumption, not a measurement)
Heater and published output	Per 1°C, no losses	8°C rise, no losses	8°C rise if 30% of output is lost (assumption)	Where the output figure comes from
Heat pump, 16 kW (air 19°C, water 26°C)	3.6 hours	29.1 hours	41.5 hours	Heat pump maker’s manual, smaller model
Heat pump, 21 kW (air 19°C, water 26°C)	2.8 hours	22.1 hours	31.6 hours	Same manual, larger model
The same 21 kW heat pump on a warm day: 26.1 kW (air 27°C)	2.2 hours	17.8 hours	25.5 hours	Same manual: output rises with air temperature
Gas heater, 24 kW output	2.4 hours	19.4 hours	27.7 hours	Gas heater maker’s operating instructions, smallest model
Gas heater, 44 kW output	1.3 hours	10.6 hours	15.1 hours	Same range, natural gas
Gas heater, 94 kW output	0.6 hours	4.9 hours	7.1 hours	Same range, largest model
Solar, 32 m² collector (80% of a 40 m² pool) in Sydney’s October sun, every joule captured	0.33 days	2.6 days (a floor)	Not computed: no independent collector efficiency figure	BoM mean daily solar exposure 20.0 MJ/m², Sydney Airport, October; SPASA 80% collector area

How long does a pool take to heat up? The worked calculation

Every heater question comes down to one sum, and you can do it with your own numbers:

Water mass. One litre of water is about 1 kg, so a 50,000 litre pool is about 50,000 kg. For a rectangle, volume in litres is length × width × average depth in metres × 1,000; a 10m × 4m pool averaging 1.25m holds 50,000 litres.
Energy per degree. Water’s specific heat is 4.186 kJ per kg per °C. 50,000 × 4.186 = 209,300 kJ. Divide by 3,600 to convert to kilowatt-hours: 58.1 kWh per °C.
The rise you want. Lifting the pool from 20°C to 28°C is 8°C, or 8 × 58.1 = 465 kWh.
Divide by heater output in kW, not input. 465 ÷ 21 kW = 22.1 hours of running.
Allow for losses. The pool loses heat to air, wind and the night sky while it heats. We have used 30% as an illustration; it is an assumption, not a measured figure, and an uncovered pool on a windy night can lose more.

The quotable version: a 50,000 litre pool needs about 58 kWh of heat for every 1°C, so hours to heat = 58 × degrees ÷ heater kW, before losses.

If you do not know your volume, the turnover table in how long a pool pump should run each day uses the same pool sizes.

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

Slowly, by design. One heat pump maker’s Australian manual publishes two ratings for each model: 19.3 kW and 26.1 kW with air at 27°C, falling to 16 kW and 21 kW with air at 19°C, both with water at 26°C. The colder the air, the less heat there is to move, so the same unit heats more slowly in spring than in summer.

On the 58 kWh rule, the 21 kW model needs 2.8 hours per degree with 19°C air: 22.1 hours for an 8°C rise, or 31.6 hours if 30% of its output is lost on the way. A second heat pump maker’s manual warns owners that ‘the initial temperature rise for swimming pool heating may seem slow but the unit is probably working’, because heat pumps have a much lower heat output than gas ‘and will need to operate for more time to deliver the same heat transfer’.

Those are running hours, not elapsed hours. Outside NSW, clause 13.7.8 of the National Construction Code’s housing provisions requires a time switch on a gas or heat pump pool heater (the clause does not apply in NSW), and if yours runs 8 hours a day, 22 hours of heating is about three days. That is why a heat pump suits holding a temperature all season rather than raising it for one weekend; solar vs gas vs heat pump pool heating sets out the running costs, and whether you need a timer on your pool heater covers the rule.

The quotable version: a 21 kW heat pump needs about 22 running hours to lift a 50,000 litre pool by 8°C, and longer in cold air.

How long does a gas pool heater take?

Gas is the fast option. One gas heater maker’s Australian operating instructions publish outputs of 24, 36, 44, 62, 76 and 94 kW on natural gas across its residential range, slightly less on propane. Gas heaters are sold by their input in megajoules an hour; the output in kW is the figure that heats water, so use it in the sum.

For the same 50,000 litre pool and 8°C rise: 19.4 hours on the 24 kW model, 10.6 hours on 44 kW and 4.9 hours on 94 kW, before losses. Gas output does not depend on air temperature, which is why it is the only type that can reliably warm a pool by the weekend.

Speed is paid for in running cost. The live comparison on this site puts a six-month gas season at $600–$1,500, against $300–$1,020 for a heat pump; see pool running costs per year.

The quotable version: a 94 kW gas heater can lift a 50,000 litre pool 8°C in about 5 hours; a 24 kW one takes about 19.

How long does a pool take to heat up naturally, or with solar?

Solar heaters have no kW rating, because their output is the sunshine on the day. So work out the ceiling instead. The Bureau of Meteorology’s mean daily solar exposure for October is 20.0 MJ/m² at Sydney Airport and 21.5 MJ/m² in Brisbane; for January, 22.8 in Sydney and 24.3 in Melbourne. SPASA recommends a collector area of at least 80% of the pool’s surface for most installations, so a 10m × 4m pool gets 32 m².

32 m² × 20.0 MJ = 640 MJ a day, or 178 kWh, if every joule landing on the collector reached the water. An 8°C rise needs 465 kWh, so even that impossible best case takes 2.6 days. Real collectors capture only part of the sun and the pool loses heat overnight, so it is longer; we found no independent collector efficiency figure, so we have not estimated how much longer. SPASA says solar can extend pool use by up to four months, which is the honest way to think of it: a season, not a weekend.

‘Naturally’, the cover does most of the work. energy.gov.au says translucent bubble covers ‘trap heat from the sun and can warm your pool by as much as 7 degrees’.

The quotable version: solar pool heating cannot lift a 50,000 litre pool 8°C in under about 2.6 days in Sydney in October, even capturing every joule of sun.

What makes my pool take longer to heat?

Heat lost while you heat. The table shows a 30% loss column as an assumption; the real share depends on wind, shade, the season and above all whether the surface is covered. energy.gov.au says bubble covers can cut heat losses from a heated pool by as much as 75%. Outside NSW, the National Construction Code’s housing provisions already require a pool heated by gas or heat pump to have a cover with a minimum R-Value of 0.05 and a time switch on the heater; the clause does not apply in NSW.

No cover overnight: the pool gives back much of the day’s heat; what a pool cover costs and whether it pays back works the numbers.
Cold air for a heat pump: the same unit’s output fell from 26.1 kW to 21 kW between 27°C and 19°C air in its maker’s table.
A heater too small for the pool: divide 58 kWh per degree by its output and see if the hours fit your day.

When you get heating quotes, ask each for the output in kW at 19°C air for a heat pump, or output (not input) for gas, and do the sum yourself. You can compare pool quotes with heating included, from businesses in Sydney, Melbourne or Brisbane.

The quotable version: the cover, not the heater, decides how much of the heat you pay for stays in the pool.

Common questions

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

Hours to heat = 58 × degrees ÷ heater output in kW for a 50,000 litre pool, before losses. A 21 kW heat pump needs about 22 hours for 8°C; a 44 kW gas heater about 11. Outputs are from makers’ manuals such as this gas heater’s operating instructions.

How long does a pool take to heat up naturally?

Days, and only in sunny weather. A translucent bubble cover can warm a pool by as much as 7 degrees, according to energy.gov.au, and cuts heat losses from a heated pool by up to 75%.

How long does a spa pool take to heat up?

Use the same sum with the spa’s volume: litres × 4.186 × degrees ÷ 3,600 ÷ heater kW. For example, a 1,500 litre spa raised 20°C by a 3 kW heater (an example input; read your spa’s plate) needs about 11.6 hours before losses.

Why is my heat pump heating my pool so slowly?

Usually because it is working as designed. Heat pump output falls as air cools: one maker’s manual lists 26.1 kW at 27°C air but 21 kW at 19°C for the same unit, and a time switch limits its hours.

How much solar collector do I need to heat my pool?

SPASA says AS3634 suggests a minimum of 60% of the pool’s surface area in ideal conditions, and SPASA recommends at least 80% for most installations.

Sources

General information only, current at 2026-10-04. Pool rules differ by state and by council, and penalty amounts are updated regularly. Confirm your own obligations with your council or a registered inspector before acting.