Heat Pump COP (Coefficient of Performance)

COP (coefficient of performance) is a metric used to help measure the efficiency of heat pumps, heating systems and cooling solutions like refrigeration systems.

In simple terms, COP helps to quantify the usable energy a system can deliver against the energy input required to produce this usable energy.

COP is essential to understanding which is the best heat pump for your home, property or situation because you’ll want to chose the option that offers the highest efficiency rating versus the amount of energy it takes to produce the usable heat energy.

How is COP Defined

COP (coefficient of performance) is the amount of useful, usable heat energy, which can either be used for heating or cooling in the case of heat pumps, for each unit of energy that is used, or consumed, to operate it.

COP is a specific measurement designed to help quantify thee efficiency of systems which involve heat transfer and thermal energy, it isn’t designed to measure the efficiency of petrol engines or internal combustion engines because they operate based on different principles, burning fuel rather than transferring it and producing power rather than moving heat.

The simple mathematical expression of COP for a heat pump set to heat is;

COP (Heating) = Heat Output (Qh) / Energy Input (W)

In the heating example:

  • COP (Heating) is the Coefficient of Performance for heating.
  • Heat Output (Qh) is the amount of heat energy produced by the heat pump, in watts or BTUs (British Thermal Units) .
  • Energy Input (W) is the energy input required to operate the heat pump, in watts or BTUs.

The simple mathematical expression of COP for a heat pump set to cool is:

COP (Cooling) = Cooling Output (Qc) / Energy Input (W)

In the cooling example:

  • COP (Cooling) is the Coefficient of Performance for cooling.
  • Cooling Output (Qc) is the amount of cooling energy produced by the heat pump, in watts or BTUs (British Thermal Units).
  • Energy Input (W) is the energy input required to operate the heat pump (in watts or BTUs).

What is COP Used For?

COP, sometimes written as SCoP in reference to heat pumps allows you to quickly and easily compare the COP of different types and models of heat pump on their efficiency in producing heat for your property.

A higher COP will help you to heat your home and generate hot water more efficiently, costing you less money to do so and meaning you can enjoy a warmer, or cooler home all year round, without spending a lot of money on electricity to do so.

What Influences COP for Heat Pumps?

There are a few different factors that can influence the COP of a heat pump and these are commonly occurring changes in the natural environment and things like the design of the heat pump as well as age of components, maintenance schedules and more.

Here are the most important factors and how they can affect the COP of a heat pump.

The Temperature difference from Heat Source to Heat Sink

The bigger the difference between the heat source, in the case of heat pumps, this is the temperature of the air outside your property (in the case of an air source heat pump) or the temperature of the ground beneath your home within your borehole (in the case of a ground source heat pump) or the temperature of the water (in the case of a water source heat pump), the harder a heat pump will have to work to extract heat energy from the air to convert in to usable heat to heat your home or provide hot water.

So, if you live in a cold climate or somewhere that experiences frequent cold spells or cold winds that affect the air temperatures around your property, an air source heat pump may have a lower COP and therefore be less efficient for your home versus a ground source heat pump or water source heat pump.

Ground source heat pumps, water source heat pumps and geothermal heating systems can generally offer a more consistent COP because the temperature of the heat source they extract heat from is more consistent that the air temperatures can be, especially in cooler climates.

The Design & Quality of the Heat Pump

Heat pump design can also affect the COP of your heat pump. Older heat pumps may be less efficient than more modern heat pumps because they include older designs, simpler or more archaic technology.

Choosing a new model of heat pump, that benefits from having the latest technology and most refined heat exchanger systems included will help ensure that your heating system achieves a closer score to it’s quoted CoP than an older heat pump model.

Modern air source heat pump technology is advancing all the time and the very latest air source heat pumps from manufacturers like Vaillant can offer a SCoP (Seasonal Coefficient of Performance) rating of up to 4.88.

However, during winter this figure is likely to be significantly lower as external air temperatures drop.

Kensa heat pumps specialise in ground source heat pumps and their Evo Ground Source Heat Pump offers a SCOP of 4.72 and this is likely to be more consistent throughout the entire year, even if temperatures drop in the winter because it is based on ground temperatures of between 0°c and -3°C.

Maintenance & Age of Components

Heat pumps can naturally lose efficiency as they age. This can be because components wear and heat transfer becomes less efficient.

Moving parts in air source heat pumps can become dirty and draw more power to turn to effectively and draw in the external air for heat to be extracted. If the fan is weighed down with dust or debris or gets damaged and becomes slow and heavy it’ll affect efficiency.

Compressors are the other crucial part of a heat pump which can dictate and impact efficiency. Much like a fridge, the compressor unit is responsible for the heat transfer of a heat pump.

By transferring refrigerant from low pressures and low temperatures to high pressure and high temperatures it is responsible for helping to effectively heat your home.

The compressor, therefore, is a critical part in helping determine the COP of your heat pump. You’ll want to make sure you know what type of compressor is used in your heat pump, how it is manufactured and ensure that you’re satisfied with the quality of the parts used to make it.

As compressors age, they can become less efficient and noisier, which could mean your heat pump gets more intrusive as it produces heat for your property.

Seasonal Variations and Load Conditions

Depending on where you are in the world and the temperature variations you experience in the different seasons you may find that the CoP of your heat pump varies throughout the year.

For example, air source heat pumps claiming a CoP of 4.88 may have achieved this score through testing, in a highly controlled environment which simulates a Spanish summer (there’s no information on the Vaillant website detailing how they calculate their CoP scores).

SCoP – Seasonal Coefficient of Performance is calculated to try to factor in seasonal temperature variations, however without context it is difficult to assume that manufacturer SCoPs are accurate.

For example, if they’re based on a mild winter, better SCoP is expected, if you experience cold winters, where air temperatures frequently drop and remain low for long periods of time, you can expect you CoP to be worse during the winter and better during warmer, summer months.

Even Vaillant reference this on their website:

The SCOP follows the same principle. However, it considers the entire year with all existing temperature variations. Therefore, it can only be calculated retrospectively. An installed heat pump will show a different SCOP for every year – depending on the usage pattern and the weather conditions. A mild winter, for example, will result in a significantly better SCOP than a long and hard winter with many cold days and temperatures below 0°C.

Vaillant website, Aug 2023.

SCoP is less of an issue for ground source heat pumps because the temperature of the ground hundreds of metres below the surface isn’t impacted by seasonal temperature changes.

The ambient temperature of the ground at around 100m beneath the surface tends to remain at a constant from around 9 – 9.5°C to 14.5 – 15°C*

With 1m temperatures around 12.7°C in southern England and 8.8°C in northern Scotland (winter temperatures being 10.3°C in southern England and 7.9°C in northern Scotland)**.

So, when comparing ground source heat pumps to air source heat pumps, you can expect the CoP figures to be more consistent throughout the year as temperature variations are less severe underground than they are for air temperatures when comparing average British Summer temperatures to average temperatures of British winters.

Heat Pump CoP vs Temperature Graph

As air temperatures decrease, it is common to see air source heat pumps CoP decline.

The data here shows a clear decline in the CoP of air source heat pumps as temperatures decline below freezing.

Whereas a ground source heat pump might decline in CoP as temperatures increase and it is increasingly used to provide cooling for the home.

heat pump cop vs temperature graph

The graph above is for illustrative purposes only and is not based on real world data.

Limitations and Considerations

CoP offers a useful guide to helping you understand the efficiency and potential efficiency you can expect to experience with your new heat pump.

However, there are clear limitations because of the lack of clarity over how scores are calculated, how different manufacturers test and score their heat pumps and what their scoring would translate to when their heat pumps are used in real world conditions.

Consider your options when choosing a heat pump and don’t just chose the option that claims to offer the best CoP.

You’ll also need to consider things like, how much space you have for a heat pump at the property where you’re installing it, how much noise you’re willing to put up with and whether or not you have any geothermal heat sources you can draw from and what type of heat pump you’d prefer to have.

If you have enough space, there are underground heat sources keeping your ground temperatures stable or elevated throughout the year, such as the granite rock layer found in Cornwall then you might want to consider a ground source heat pump because you might achieve more consistent heat generation throughout the year and get closer to the manufacturer quoted CoP.

If you’re occupying a smaller premises with little outside space, in an area with minimal seasonal temperature variations and want a heat pump that’s cheaper to buy and install, then an air source heat pump may be the better option for you.

Sources and References

*, ** Nora UK Shallow Ground Temperatures