COP stands for coefficient of performance. It is the thermal energy the heat pump sends to the heating system, divided by the electrical energy it uses to do so. If the machine brings 4 kW of heat into the house and the meter shows 1, the COP is 4: for every kWh paid on the bill, four reach the radiators or the floor. The other three it takes from the outdoor air for free.

The calculation starts from three measurements. On the water circuit you read the flow rate, the flow temperature and the return temperature. The heat delivered is the flow rate times the temperature difference times the specific heat of water: 1.72 m³/h with a 5 °C difference make 10 kW. On the electrical side a wattmeter measures what the compressor, fan, electronics and internal pumps draw. Dividing the first number by the second gives the COP.

It is the figure that says how good a machine is, and it is indeed the first one compared. The problem is where it is measured.

What makes the difference is the quality of the refrigerant circuit, piece by piece.

• Coil and fan. With fins spaced 2.5 mm apart and a large exchange surface, frost takes longer to close the passages, and defrosts are less frequent and shorter. We explained this in the tip on fin spacing and in the one on defrosting.
• Pipe diameter. Narrow pipes cost less but hold the refrigerant back: the compressor has to push harder to move the same amount of gas, and that extra effort is paid on the bill.
• The 4-way valve. It is the valve that reverses the cycle for defrosting and for cooling. If it is small for the machine, the gas struggles to get through, and efficiency is lost in every hour of operation.
• The water exchanger, in air-to-water units. More plates mean more surface between refrigerant and water: the compressor can work at a lower pressure to give the system the same temperature, and the COP rises.
• The distributor inside the plates. When present, it splits the refrigerant equally among all the plates. Without it, some plates work hard and others hardly at all, and the exchanger performs as if it were smaller.
• Compressor and inverter. The compressor must be chosen for heating in winter, not adapted from cooling (tip on compressors), and the inverter must let it work at half capacity for most of the season, where efficiency is highest (tip on sizing). The inverter uses energy too: part of the current entering it becomes heat in its components. On our residential machines the inverter is silicon carbide (SiC): it switches with fewer losses than traditional silicon, runs cooler and leaves more energy for the compressor. It is efficiency gained without touching the circuit: we explain the difference between SiC and IGBT in the tip on the inverter.

These are components you cannot see in a data sheet. In the laboratory COP they weigh little; in January fog they make all the difference.

The COP is a snapshot at a single temperature. The SCOP, seasonal coefficient of performance, is the film of the whole winter: the EN 14825 standard takes the hours of a typical winter, splits them by outdoor temperature, measures the COP at -7, +2, +7 and +12 °C and takes the hour-weighted average. The result says how much heat you get for every electrical kWh over the whole season, and from there comes the energy class on the label.

For the same machine as in the table, the certified SCOP is 5.75 with water at 35 °C and SCOP 4.20 at 55 °C. It is the number to compare between two heat pumps, at the same water temperature and climate: the Keymark sheet gives it for the average climate and, when available, for the cold one. The SCOP too, however, is born in the laboratory from the same tests we discussed above: it says who is more efficient, not how much you will consume in your house.

In Italy “etichetta energetica pompa di calore” is searched 210 times a month, in Germany “Energieeffizienz Wärmepumpe” 260. The label does not show the SCOP but the seasonal efficiency ηs: the SCOP divided by 2.5, the factor Europe uses to convert electricity into primary energy, minus 3 points for the controls.

The class depends on that number and on the water temperature. With water at 35 °C, as for underfloor heating, A+++ means at least 175%; with water at 55 °C, as for radiators, at least 150% (EU Regulation 811/2013; today’s label has been in use since 26 September 2019). Our Kita X has 221.5% at 35 °C and 172.7% at 55 °C: A+++ in both.

Two A+++ units can still be very different: the class is a step and does not say how far above the threshold a unit is. To compare them, look at the SCOP, or the ηs, at the same water temperature and in the same climate.

• At what conditions the COP is given: air, water, full or part load. A COP without its conditions says nothing.
• Who measured it: an HP Keymark certification or an independent laboratory is worth more than a catalogue number.
• The SCOP at 35 and at 55 °C, not only at 35.
• The capacity delivered at -7 °C: a machine can have a good COP and not enough capacity when it is needed.
• The weight, which tells how much coil and how much exchanger are inside (tip on air conditioners and heat pumps).
• The fin spacing and how it handles defrosting.
• What water temperature it reaches without electric heaters.