A good reply.
As you can see the heat loss is calculated at 16kw, this heat pump will give you about 16kw at 7 degrees, at -2 you will have an output from the heat pump at 13kw or so, during the colder spells it would be best to leave the system running 24 hours a day to keep up with the demand as it may be a little short on capacity at the lower ambient temperatures.
Check with your installer that 580 grams of refrigerant has been added as this unit is only pre charged to 10 meters so if the pipe run is 18 meters and they have not added additional gas then this could also be a sign of what you are experiencing.