At ITS, we supply both standard and inverter type pool heat pumps. We have over a decade’s worth of experience in the design, installation and maintenance of heat pumps and decided that it was time for us to add and inverter range due to the many advantages that it holds when it comes to pool heating.

What is it about inverter pool heat pumps that makes it the heat pump of choice? In this article we will explain in detail the differences between inverter and standard (also called on/off or fixed output) pool heat pumps.

What is the difference between the two technologies?

The energy needed to maintain a certain pool temperature varies greatly with a change in ambient temperature and with air movement (wind) over the pool surface.

A fixed output heat pump can only be on or off. When turned on, the fixed output heat pump works at 100% capacity to meet the heating demand of the pool. It will continue to do this until the desired pool temperature is met and will then cycle between off and on to try and maintain this temperature.

An inverter heat pump, however, uses a variable speed compressor which modulates its output, increasing or decreasing its output power to match the exact heat demand requirements of the pool.

When the demand is low the heat pump will reduce its output, limiting the electricity usage and the exertion placed on the heat pump’s components and limiting the start-up cycles.

Start-up Current

Inside a standard heat pump is an AC electric motor driven compressor. Any AC motor has a start-up current that is usually up to seven times higher than the normal operating current. This means that it requires much more power to start turning than when it is at operating speed.

An inverter heat pump uses a DC electrical motor and is able to ramp up slowly and avoid a high start-up current. There are a number of advantages to this:

  1. It uses less power to get started- saving you money.
  2. It reduces the electrical cable thickness required for the installation. This can result in a significant saving on installation costs.
  3. It eliminates nuisance tripping that is sometimes a problem with bigger single phase powered units.
  4. It reduces start-up “stress” on the machine.
  5. It enables the possibility to run the heat pump from correctly sized off-grid solar electric systems.


Inverter heat pumps are able to run much quieter than standard heat pumps. In 100% output power mode an ITS inverter pool heat pump is already quieter than an equivalent size standard heat pump and when the inverter unit is operated in “quiet” mode it would be equivalent to putting earplugs in when using a standard heat pump.


When compared to traditional fossil fuel-burning heating systems, both fixed output and inverter heat pumps offer far greater levels of energy efficiency. A well-designed standard pool heat pump system will provide an average coefficient of performance (CoP) of around 5. This means that for every 1kW of electrical energy used to power the heat pump it will return 5kW of heat energy. With an inverter pool heat pump it is possible to get CoP values of more than 12! Some marketing material shows values much higher than this but it is important to know that the CoP value is dependent on the ambient and water temperature and testing it in unrealistic conditions to show a higher COP value is misleading.

It is also important to know that inverter heat pumps will give you more or less the same CoP value than a standard heat pump when operated at 100% output capacity. It is only when an inverter heat pump “backs off” it’s output power that the efficiency starts to increase.

This is shown in the below table

COP vs Output Power Level Diagram

Let’s assume a client would like their pool to always be 28oC – even during a worst-case winter ambient temperature of 0oC. Let’s also assume that for the specific pool size a 16kW inverter heat pump will be able to fulfil this requirement when operating at 100% output capacity. When the ambient temperature is 0oC (running at 100% output capacity) the inverter heat pump will not provide a greater electricity saving than a standard heat pump. However, when the ambient temperature increase and the heat pump do not need to run at 100% output capacity, the inverter heat pump will become more efficient and can provide a significant electricity saving advantage. At ambient temperatures of 18oC the inverter heat pump could already have reduced its output power to 30% and use less than half the electricity of a standard heat pump. From this it should be clear that a correctly sized inverter pool heat pump can provide a significant saving in electricity consumption when compares to a standard pool heat pump.