ᐅ Heat Pump Efficiency at -12 Degrees?

How efficient is a heat pump really, in terms of how much energy you put in versus how much heating output you get when the temperature is around -12°C (10°F)?

I have a new air-to-air multi-split system with a SCOP of 4.6. The unit often freezes up, and after defrosting, there is a significant ice buildup underneath the device. How much of the 4.6 SCOP rating remains at temperatures between -7°C and -12°C (19°F to 10°F)?

No idea, but according to the manual for my air-to-water heat pump, the COP is still 3.3 at -7°C (19°F). The lower it gets, the less efficient it becomes. At -12°C (10°F), the COP is probably closer to 2. But that’s just a guess.

I also think it’s 2 or even less; it had to defrost constantly. For now, the gas heating is running again (-9°C (16°F)) until it reaches 0°C (32°F) again.

Well, the efficiency can be clearly understood if you have a separate submeter and a heat meter for the heat pump.
Thinking or guessing won’t get you very far here.
Especially since other data, such as the supply temperature, are missing.

For -7, the question can be answered quite easily. BAFA provides a comprehensive list of eligible heat pumps with their COP, so yours might be included. Unfortunately, it only covers up to A-7/W35 (-7 degrees Celsius (19°F) outdoor temperature, 35 degrees Celsius (95°F) supply temperature). The best heat pumps achieve a COP of around 3.0 to 3.2, the majority are around 2.6, and some air conditioning units drop to about 2.1. At -12 degrees Celsius (10°F), performance decreases significantly.

It does not matter what the performance factor is at lower temperatures. That is why it is called the seasonal performance factor (seasonal performance factor). For new buildings, it averages around 4.5 if it is not compromised by a buffer tank or underfloor heating.

It is completely normal for the heat pump to freeze once an hour and then defrost itself, and this does not indicate anything about the COP or seasonal performance factor.