Pool heat pumps: 9 kW advertised, 6.4 kW delivered — it all hangs on one line of the spec sheet

Last updated: 14 August 2026 — Calculhouse

In short. (Private residential pools, in France.) A pool heat pump advertised as "9 kW" only delivers that figure in favourable conditions (26 °C air). As soon as the air falls towards 15 °C, in spring or autumn, real power drops — by about 29 % on average across our survey of 10 references (Poolex, Astralpool, GRE, Zodiac, Hayward). For an extended April-to-October season in France, it is the power at 15 °C you should be reading, not the headline figure. And two models are only comparable if they are quoted in the same test condition.

Comparing heat pumps for your pool and going by the kilowatt figure on the box? That is the natural instinct — and it is also the surest way to get it wrong. Not because the number is false, but because it means nothing until you know the condition it was measured in. Here is how to read that number properly, once and for all.

1. The trap of the kW figure

Plenty of pool owners pick their heat pump on a single number: "5 kW", "7 kW", "9 kW". It is simple, it is reassuring, and it is incomplete. A pool heat pump does not have one power rating: it has several, and they move with the outside temperature, the water temperature and the humidity of the air.

So let us put the real question, the one that drives this whole article:

Does a heat pump advertised at 9 kW really supply 9 kW when you need it most, in spring?

The short answer: often no — and sometimes yes. It all depends on the condition hidden behind the figure. Learning to spot it is the only skill that really matters when choosing.

2. Why doesn't a heat pump have a single power rating?

Before talking about what is printed, it helps to understand why the power moves. A heat pump does not "make" heat: it takes heat from the outside air and moves it into the pool water. Its performance therefore depends directly on what that air contains:

• warm air → plenty of available heat → high delivered power;
• cool air → less heat → reduced delivered power;
• humidity counts too: the damper the air, the more so-called latent heat it carries, and the more the machine can recover.

Hence the central point to take away:

A heat pump's power is a value measured under precise conditions, not a fixed property of the machine.

French spec sheets mostly use two reference conditions:

In practice, the same machine can show 9 kW at 26 °C air and deliver only 6.4 kW at 15 °C air — same components, same unit, two different conditions. The number changes; the machine does not.

3. The two power figures an owner needs to know

The "shop window" figure (favourable condition, ~26 °C)

This is the one you most often find in large type on sales pages. It is measured in warm conditions (air ≈ 26 °C, water ≈ 26 °C, 80 % humidity). To be fair to it:

• it represents the machine's maximum potential;
• it corresponds to a real, standardised test condition, not an invention;
• and, inevitably, it produces the biggest number — so the most saleable one.

Its only flaw: it does not reflect the power actually available at the start or the end of the season, when the air is cooler.

The power at 15 °C

This is closer to real extended use at our latitudes: warming the pool back up after winter, spring mornings, the tail end of autumn.

To size an outdoor pool heat pump in France for an extended season, the power around 15 °C is the most useful piece of information.

An important nuance, worth remembering: some brands already publish their power at 15 °C. So not everyone "inflates" their figure — more on that below.

4. What this difference changes in practice

Power = heating speed

Two ideas that often get confused:

• Power (kW) = the rate at which you heat the water.
• Energy needed (kWh) = the total amount of heat you must supply to reach the temperature you want.

A rough order of magnitude, for a 50 m³ pool: it takes about 58 kWh to gain 1 °C.

Careful: those 58 kWh are a theoretical minimum, before losses. In real conditions, evaporation, wind and night-time cooling raise the requirement noticeably. If you time it at home you will always find it takes a bit longer — that is normal.

For the same requirement, a pump delivering 7 kW heats faster than one delivering 5 kW. Power does not change how much heat you have to supply; it changes how long it takes.

A pump chosen on the shop-window figure can end up undersized

Take a verified case, the Poolex Silverline On/Off 7 kW:

• advertised: 7 kW (Air 26 °C / Water 26 °C / 80 %);
• real at 15 °C: 4.7 kW, i.e. −33 %.

If you had picked that model counting on its 7 kW for spring use, you would discover: slower heating than expected, difficulty reaching the set temperature in the shoulder season, and a compressor running longer to compensate.

The role of your usage period

Here is the point that puts everything in order: your usage period is not a consequence of sizing, it is the parameter that decides which power figure to look at.

• July–August only: the air is warm, so the 26 °C condition already represents your reality. The shop-window figure is relevant, and there is no undersizing problem.
• Extended April–October use: you have to reason on the power at 15 °C, otherwise the machine struggles as soon as you step outside the height of summer.

In other words, it is not "15 °C is always the right measure". It is "15 °C is the right measure if you want to extend the season".

5. Ten real references analysed

Enough principles: here are the figures taken from actual spec sheets.

Method

• 10 representative references, drawn from a wider survey of 23 models.
• Three brands to illustrate the trend (Poolex, Astralpool, GRE) and two as counter-examples (Zodiac, Hayward).
• Two technologies (On/Off and Full Inverter), sizes from 7 to 20 kW.
• Comparison at identical conditions: Air 26 °C / Water 26 °C / 80 % against Air 15 °C / Water 26 °C / 70 %.
• The wording is deliberately neutral: "gap between advertised power and power at 15 °C", not "overstatement".

The 10 references

* Hayward measures here at 24 °C water (not 26 °C) — a slightly different condition, which we flag for the sake of rigour.

Each row's number matches its source, listed in the "Sources" section at the end.

Reading the results

• Across the 8 references advertised at 26 °C, the gap ranges from −21 % to −33 %, with an average of about −29 %.
• Across all 23 models in the full survey: average −27.5 %, median −26.6 %.
• A nuance by technology: On/Off models appear to lose more (≈ −33 %) than Full Inverter ones (≈ −26 %). Treat that carefully: the difference comes mostly from how each technology is specified on the sheet (a modulation range for Inverter, a fixed point for On/Off) rather than from any obvious physical superiority of one over the other.

Figure to remember (Calculhouse survey, 10 references, July 2026): between the power advertised at 26 °C and the real power at 15 °C, the average measured gap is about −29 %.

The example to keep in mind:

A heat pump advertised at 15 kW may supply only around 10 kW in conditions close to 15 °C.

6. Does COP follow the same logic? (yes)

This does not only affect power. COP (coefficient of performance = energy delivered ÷ energy consumed) is the second "too good" number on sales pages. Like power, it is measured in favourable conditions:

• at 26 °C, spec sheets show spectacular COPs — up to 12 to 16 at peak on some Full Inverter models;
• at 15 °C, the same unit falls back to between 4.4 and 8.

A verified example, again on the Poolex Silverline On/Off: COP quoted at 6.15 at 26 °C, but 4.4 to 4.6 at 15 °C.

When you read "COP up to 16", look for the condition. At 15 °C, a good COP is around 4 to 6. That is already excellent — and it is the realistic figure to plan with.

7. Do all brands inflate their advertised power?

It would be unfair — and wrong — to conclude that "every manufacturer inflates their figures". The dominant practice (Poolex, Astralpool, GRE and others) is indeed to advertise power at 26 °C. But it is not universal:

• Zodiac (PX25, Z250, Z300 ranges) publishes its power directly at Air 15 °C / Water 26 °C / 70 %;
• Hayward (EnergyLine Pro) also publishes at Air 15 °C, with NF-414 certification.

What they have in common: these brands claim NF PAC / NF-414 certification, which requires communicating on the standard 15 °C condition.

Which raises the question every reader now has:

"If 15 °C is more honest, why doesn't everyone publish it?"

The answer is simple: because 26 °C gives a bigger number, therefore a more saleable one, and nothing legally obliges a manufacturer to publish the 15 °C figure — unless it voluntarily commits to a certification scheme.

Hence the central lesson of this article:

Two "9 kW" heat pumps are only comparable if they are quoted in the same condition. A "Poolex 9 kW" (9 kW at 26 °C, so ≈ 6.4 kW at 15 °C) and a "Zodiac 9 kW" (9 kW at 15 °C) are simply not in the same class.

8. Common misconceptions worth correcting

A quick memo to keep handy:

• "A 9 kW heat pump always produces 9 kW." → No: power varies with the measurement conditions.
• "The bigger the number, the better the pump." → Not necessarily: a big figure at 26 °C can hide modest power at 15 °C.
• "Pool volume is enough to choose." → No: region, target temperature, usage period, cover and exposure matter just as much.
• "The advertised figure is a lie." → No: it is accurate within its condition. The real problem is comparing different conditions without noticing.

9. The other sizing factors

So as not to reduce everything to power, bear in mind that the real requirement also depends on:

• Pool volume: the bigger it is, the more total energy you have to supply.
• Local climate: in the north the demand is higher; in the south conditions are more favourable.
• Thermal cover: a cover or roller shutter sharply reduces losses. A good cover can have as much impact as a more powerful heat pump — it is often the most profitable investment.
• What you are aiming for: holding 28 °C in high summer is nothing like extending the season from April to October.

10. How to read a heat pump spec sheet properly

The survival checklist:

• ✅ the heating power with its condition (Air _° / Water _° / Humidity _%);
• ✅ the power at 15 °C, if your use is extended;
• ✅ the corresponding COP and its condition;
• ✅ the operating range (minimum outside temperature).

Bad reading: "9 kW heat pump". Good reading: "heat pump delivering 9 kW at Air 26 °C / Water 26 °C, and 6.4 kW at Air 15 °C / Water 26 °C".

11. What power for YOUR pool?

No magic formula, but these markers should stop you leaving empty-handed:

• for summer use, reason on the power at 26 °C;
• for extended use, reason on the power at 15 °C;
• a thermal cover changes the result substantially, in the right direction.

A good choice starts with an estimate of the pool's real heating demand, then a check that the pump actually supplies that power under realistic conditions.

That is exactly the logic of our pool heat pump power calculator, built on the 15 °C reference rather than the shop-window figure:

12. Conclusion

The right choice is not the heat pump showing the biggest kW number. It is the one that supplies enough power in the conditions you will actually use it in — and to compare two models, always check they are talking about the same condition.

Common questions