What Size Heat Pump Do You Need for a Hot Tub or Swim Spa?
Size a heat pump by the water it heats.
A 7kW heat pump suits most hot tubs, and 11 to 12kW suits a swim spa. In 15°C air our 7kW warms a 1,000 litre hot tub by 10°C in about 2 hours, and our 12kW warms a 4,780 litre swim spa by the same in about 6.1, 2
The short answer
- 7kWfor hot tubs: it warms our 1,400 litre Sanctuary by 10°C in about 3 hours
- 12kWfor swim spas, and large hot tubs you want to heat faster
- -14°Cthe coldest air our 12kW keeps working in1
The size in kW is how much heat the heat pump gives. Our 7kW gives 7.11kW in 26°C air and 5.36kW in 15°C air, and our 12kW gives 12.5kW and 9.10kW.1 The more water you have, the more heat it takes to warm it.
How long a heat-up takes
Each litre of water needs 4.18 kJ to rise by one degree.3 Here is the heat our hot tubs and swim spas need to warm by 10°C, and the time each heat pump takes to supply it in 15°C air.1, 2
| Hot tub or swim spa | Water | Heat to warm 10°C | With the 7kW | With the 12kW |
|---|---|---|---|---|
| Harmony hot tub | 480 litres | 5.6 kWh | about 1 hour | about 0.6 hours |
| Comfort hot tub | 645 litres | 7.5 kWh | about 1.4 hours | about 0.8 hours |
| Lagoon hot tub | 1,000 litres | 11.6 kWh | about 2.2 hours | about 1.3 hours |
| Sanctuary hot tub | 1,400 litres | 16.3 kWh | about 3 hours | about 1.8 hours |
| Active swim spa (13ft) | 4,780 litres | 55.5 kWh | about 10.4 hours | about 6.1 hours |
| Active Pro swim spa (15ft) | 5,853 litres | 68.0 kWh | about 12.7 hours | about 7.5 hours |
| Active Pro Plus swim spa (19ft) | 7,162 litres | 83.2 kWh | about 15.5 hours | about 9.1 hours |
Times are for the heat going into the water, with the cover on. A first fill from a cold tap usually needs more than 10°C, so take the time for 10°C and multiply it up.
Winter heat-ups take longer
Less heat from colder air
At -7°C our 7kW gives 2.77kW of heat and our 12kW gives 5.07kW, about half what they give at 15°C.1 So a winter heat-up takes about twice as long: around 4 hours for a 1,000 litre hot tub on the 7kW, and around 11 hours for the Active swim spa on the 12kW.
Keeping it hot is the easy part
Once the water is up to temperature, the heat pump only tops up what the water loses, so it runs for much shorter spells. The 12kW carries on working down to -14°C, and the 7kW down to -7°C.1
Does a bigger heat pump cost more to run?
Not for the same heat. Your water needs the same kWh whichever heat pump you choose, so the running cost comes down to the COP, the heat out for each unit of electricity in. In 15°C air our 7kW has a COP of 5.41 and our 12kW 4.82.1
That makes the 7kW the efficient choice for a hot tub, and the 12kW the right one when you want a swim spa up to temperature in hours rather than a day. Our guide is a heat pump worth it for a hot tub? works through what one heat-up costs.
Our heat pumps
See all our air source heat pumps, or start with these two.
Outdoor Living 7kW air source heat pump
£1,699
The right size for most hot tubs, with the higher COP of the two: 5.41 in 15°C air. Quiet at 54 dB(A).1
Outdoor Living 12kW air source heat pump
£2,199
For swim spas and large hot tubs: 12.5kW of heat in 26°C air and 9.10kW in 15°C air, working down to -14°C.1
Heat pump size questions
Open a question to read the answer. The full figures are in the installation manual for our 7kW and 12kW heat pumps.
What size heat pump do I need for a hot tub?
A 7kW heat pump suits most hot tubs. Our 7kW gives 5.36kW of heat in 15°C air, enough to warm a 1,000 litre hot tub by 10°C in about two hours. Choose the 12kW for a large hot tub you want to heat faster, or for a swim spa.
Is a 7kW heat pump big enough for a swim spa?
A 7kW will heat a swim spa, but slowly: warming our 4,780 litre Active swim spa by 10°C takes about 10 hours in 15°C air. Our 12kW does the same in about 6 hours, which is why we suggest 11 to 12kW for swim spas.
How long does a heat pump take to heat a hot tub?
Divide the heat the water needs by the heat pump's output. A 1,000 litre hot tub needs about 11.6 kWh to rise by 10°C, so our 7kW takes about 2 hours in 15°C air and about 4 hours at -7°C. Heat lost through the cover adds a little more time.
Does a bigger heat pump cost more to run?
Not for the same heat. The water needs the same kWh of heat whichever size you choose, so running cost depends on the COP. In 15°C air our 7kW has a COP of 5.41 and our 12kW 4.82, so the 7kW is the more efficient choice for a hot tub and the 12kW the faster one for a swim spa.
What power supply does a hot tub heat pump need?
A single phase 220 to 240V supply with a 30mA RCD. Our 7kW draws up to 8A on a 16A D-curve breaker, and our 12kW up to 13A on a 25A D-curve breaker. A qualified electrician should connect it.
Sources
- Outdoor Living, Hot tub and swim spa air source heat pump user and installation manual, OLHP07 and OLHP13, technical data, electrical installation and bypass assembly, version V20221221.
- Outdoor Living, product pages for the Harmony hot tub, Comfort hot tub, Lagoon hot tub, Sanctuary hot tub, Active swim spa, Active Pro swim spa, Active Pro Plus swim spa, water capacity, accessed October 2026.
- Engineering ToolBox, Specific heat capacity of water, accessed October 2026.
Paul Margerison
Managing Director & Founder, Outdoor Living
Match the heat pump to the water and it will keep your hot tub or swim spa ready all year without working any harder than it needs to.