COP 5 in the catalogue: why it does not mean a bill five times lower

A heat pump can have a high rated COP and operate in a correctly sized system. But a single rated figure cannot be converted into an annual bill by simple division.

I would first define exactly what we are comparing: an individual unit at a specified test point, a standard season, or the whole property including pumps, supplementary heating and domestic hot water.

Three metrics for different questions

COP gives the ratio of heat output to electrical input under specified operating conditions. SCOP describes a standard seasonal scenario. Certification materials address heating capacity, COP, design load and seasonal efficiency separately. (Eurovent — Liquid Chilling Packages and Heat Pumps)

For real-world operation, it is useful to define your own measurement boundary and calculate the seasonal ratio:

SPF₍system₎=(Q₍useful heat over the period₎) / (E₍all included consumers₎)

An SPF value is also incomplete without an explanation of its boundaries. Is supplementary electric heating included? External circuit pumps? Storage losses? Where exactly is heat measured? These points must be established before making a comparison.

Fraunhofer ISE field measurements show a spread in actual seasonal efficiency across properties. This is not a ready-made forecast for Ukraine, but evidence that a real system deserves its own assessment. (Fraunhofer ISE — findings from research into heat pumps in existing buildings)

How a small share of supplementary heating changes the result

Consider a hypothetical plant room that supplies 18 000 kWh of heat to the system over a season. All the values below are assumptions for this worked example.

The heat pump supplies 16 200 kWh at an assumed average COP of 3,6. Its electricity consumption is:

E₍HP₎=16 200/3,6=4500 kWh

The supplementary electric heater produces a further 1800 kWh of heat at an assumed conversion factor of 1. Separately accounted for, the circulation pumps and controls consume 300 kWh.

Total:

E₍\Sigma₎=4500+1800+300=6600 kWh;

SPF=18 000/6600≈2,73

Supplementary heating supplied only 10% of the heat but used approximately 27% of the total electricity consumption accounted for. This does not necessarily mean that the heat pump itself is faulty.

In this example, heat is measured at the plant room outlet; downstream distribution losses are not assessed. The auxiliary 300 kWh is not included in the assumed COP, to avoid double counting.

Keep space heating and domestic hot water separate

Underfloor heating and heating water in a cylinder require different operating conditions. They should therefore be assessed separately before compiling an energy balance. SEAI also distinguishes between assessment methods for space heating and water heating. (SEAI — Heat pumps: system assessment and input data)

A ‘before and after’ comparison must cover the same services. If the old bill excluded domestic hot water but the new one includes it, the difference does not describe space heating efficiency.

What financial savings mean

Provided that the same 18 000 kWh of heat was previously produced entirely by electric heating with a conversion factor of 1, our example gives a reduction in electricity consumption of approximately 63%.

But a comparison with gas requires calculating the cost of a useful kilowatt-hour of heat from both sources. COP alone does not determine the price of gas or electricity.

A high rated COP is a good equipment characteristic. A clear seasonal energy balance is a characteristic of an engineered solution.