Pool Heat Pump Sizing: How to Choose the Right kW

Choosing a pool heat pump usually starts with a number: how many kilowatts. It is the right question, but the answer depends on more than the size of the pool. A unit that is too small runs for hours, never quite reaches temperature on cold days and disappoints. A unit that is too large costs more to buy than you needed to spend. This guide explains the physics behind sizing in plain terms, so you can make sense of a specification sheet and understand what we ask when we size a heat pump for you.

We do not quote generic savings or monthly running costs here, for the same reason we do not on our pool heat pump service page: the honest figure depends on your pool, your tariff and how you use it. What we can do is show you how the numbers work.

Step one: know how much water you have

Everything starts with the volume. For a rectangular pool, multiply the length by the width by the average depth, all in metres, then multiply by 1,000 to get litres. For the average depth, add the shallow-end depth to the deep-end depth and halve it. Freeform and kidney-shaped pools are harder; an approximation is fine for a first conversation, and we measure properly before recommending a unit.

The surface area matters too, separately from the volume. Most heat is lost from the surface, so a large, shallow pool can need more heating than a smaller, deeper one holding the same number of litres.

Step two: the energy to warm the water

Heating water is simple physics. One kilowatt-hour of heat raises about 860 litres of water by 1 °C. From that you can work out how much heat it takes to warm your pool.

A worked example: a 50,000-litre pool that you want to raise by 5 °C.

  • 50,000 litres multiplied by 5 °C gives 250,000 litre-degrees.
  • Divide by 860 and you get roughly 291 kWh of heat.
  • That is the heat the water must receive, before counting any heat lost along the way.

Step three: heating output, not electrical input

Heat pump specification sheets list two different power figures, and mixing them up is the most common sizing mistake.

  • Heating output (kW): the heat the unit delivers into the water. This is the figure that sizes the unit.
  • Power input (kW): the electricity the unit draws from the supply. It is much smaller than the heating output, because a heat pump moves heat rather than making it.

Divide the heat needed by the heating output and you get a rough number of hours. In the example above, a unit delivering 15 kW of heat would take around 19 to 20 hours of running to supply 291 kWh, and a unit delivering 25 kW would take around 12 hours. Those figures are before any losses, and on a real pool the losses change the picture completely.

Why heat loss decides the size

A pool is not a sealed tank. It loses heat all the time, and most of that loss is from the surface:

  • Evaporation: the biggest loss. Every litre that evaporates takes a large amount of heat with it, and the dry Highveld air evaporates water readily.
  • Night-time cooling: the air on a clear Gauteng night can be far colder than the water, and the surface radiates heat into the sky.
  • Wind: air moving across the surface strips heat away and speeds up evaporation.
  • Shade: a pool in shade for most of the day gains little from the sun to offset its losses.

The first heat-up happens once. After that, the heat pump spends the rest of the season replacing what the pool loses each day and night. On an uncovered pool in a windy spot, that daily loss can be large enough that a small unit simply cannot keep up, however many hours it runs. This is why real sizing has to account for heat loss, and why simple rules of thumb that give a fixed number of kilowatts per thousand litres can be badly wrong for a particular pool.

COP: what it is and why it drops in winter

The coefficient of performance, or COP, is the heat delivered divided by the electricity used. A COP of 5 means five units of heat into the water for every unit of electricity. It is the reason heat pumps are far more efficient than a plain electric element.

COP is not a fixed property of the machine. It is highest when the air is warm and humid, and falls as the air gets colder, because there is less heat in the air to move. Heating output usually falls with it. A unit that performs well on a March afternoon has to work much harder, and delivers less heat, on a frosty July morning.

That is why the conditions behind a specification matter. Manufacturers quote heating output and COP at stated test conditions: a particular air temperature, humidity and water temperature. Read those conditions. A figure measured in warm air says little about how the unit will perform when you actually want heat, and two units can only be compared fairly at the same conditions.

Working out the electricity for yourself

You can estimate the electricity for any amount of heat with one division. Take the heat needed in kWh and divide it by the COP at the conditions you expect. In the worked example, 291 kWh of heat at a COP of 5 would need roughly 58 kWh of electricity, before losses. To turn that into money, multiply by the rate per kWh on your own Eskom or municipal bill, including any time-of-use rates. We deliberately do not put a rand figure on it, because tariffs differ and change.

A cover changes the answer

Because most heat is lost from the surface at night, covering the pool is the single biggest thing you control. A cover reduces evaporation and night-time loss, which reduces both the size of unit you need and the hours it has to run. If you will not use a cover, a heat pump can still work, but it needs to be larger and it will cost more to run. Our comparison of pool covers: solar, safety, automated and net explains the options, and we measure and fit covers through our pool covers service.

Inverter units

An inverter heat pump varies its compressor and fan speed to match demand instead of switching fully on and off. It can run hard during the first heat-up and then settle at a lower output to hold the temperature, which is quieter and more efficient once the pool is warm. When comparing inverter units, look at the heating output range and at the conditions each figure was measured at.

What to decide before sizing

  • Which months you want to swim. Stretching the season into spring and autumn is a much smaller job than swimming through a Highveld winter.
  • Target temperature. Every extra degree adds to the daily heat loss the unit must replace.
  • Whether you will cover the pool every night.
  • Where the unit can stand, on a level base with clear airflow and away from bedroom windows.
  • The electrical supply. A heat pump needs its own correctly rated supply, installed by a qualified electrician who issues a certificate of compliance.

For a wider comparison with solar and other heating methods, read our overview of pool heating options in South Africa.

What to send us

If you would like us to recommend a size, WhatsApp us on 081 577 5051 with:

  • The pool's length, width, and shallow and deep end depths, or its volume if you know it
  • How much sun the pool gets, and whether it is exposed to wind
  • Whether you have, or will use, a cover
  • The months you want to swim and roughly how warm you want the water
  • A photo of the equipment area where the heat pump would stand

We will recommend a unit for your pool and your goal and quote the installation in writing, and we will tell you plainly if the goal is unrealistic without a cover. You will find the heat pumps in our online shop in the Heating department, and we install what we sell at a fixed fitting price.

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