Efficiency

Efficiency can be defined as:

Because energy cannot be created or destroyed, efficiency can never exceed 100%. However, because a heat pump transfers or “pumps” heat energy from heat source to heat sink, it is possible for more heat energy to be transferred than electrical energy supplied to the heat pump – the efficiency can therefore exceed 100% in this special case.

Since the above can cause confusion, "Coefficient of Performance" (COP) is used; this is simply the above equation without a "100" multiplication, i.e. a ratio such as 2.9.

The COP is an instantaneous measurement and reflects the sink and source temperatures at a particular moment, such as during a test. In practice, it is of limited use for evaluating or comparing performance throughout a year, where source temperatures (such as outside air) will change significantly. Water flow (sink) temperatures through the heat emitters may also change throughout the year, for example due to weather compensation controls.

In order to estimate annual performance, the European Union’s Ecodesign regulations utilise a test and calculation standard (EN14825:2016) at a wide range of temperature conditions. These are used to calculate a Seasonal Coefficient of Performance (SCOP) for a heat pump, which is used to derive an energy label class (A++ to G) for comparison purposes.

Unfortunately, the SCOP estimation of performance misses a number of important issues that may affect the performance of a heat pump when installed in homes. These include:

1. Heat loss of the actual dwelling in which the heat pump is installed is ignored
2. It uses average European climate data
3. Hot water heating operation is ignored, including its impact on space heating operation
4. Heating hours
5. For inverter (modulating) heat pumps, the minimum heat output is not defined, meaning some heat pumps may cycle on/off more than others at identical temperature conditions
6. Weather compensation is always assumed to be present

The Building Research Establishment has developed an annual efficiency calculation method that utilises the engineering standard EN15316-4-2:2017 to address these issues.

Annual efficiency (or SPF) can be defined in a number of different ways, and can include or exclude electrical energy needed for different heat pump system components. The annual efficiency estimates provided by this website include all auxiliary components necessary for operation, including back-up direct-electric heaters and circulation pump. This type of annual efficiency (or SPF) is often known as SEPEMO “SPF_H4”. The Seasonal Performance Factor (SPF) is the same as annual efficiency, but is not multiplied by 100.