What capacity must the heat pump have?

Hello everyone,
we are currently building a two-family log house from Fullwood and are in the process of sizing a heat pump. Since we don’t have much knowledge on the topic and received different statements from specialized companies, we are a bit uncertain.
The heated area is 200m² and the heat pump only needs to supply the underfloor heating. We had a heating load calculation made according to DIN EN 12831, which resulted in a standard building heating load of 5.7kW.
We have also received an offer for a Panasonic Monoblock Aquarea system with either 7kW or 9kW, each with a 300-liter buffer tank.
The system should not be designed too small or too large. What capacity would you recommend, or are there additional parameters that need to be considered?

Thanks in advance for your reply.

Regards, Dieter

At 5,x kW I would definitely risk the 5kW monoblock, if it is not enough then add a second one. If it regularly runs at full load, then pay attention to the acoustics towards the neighbor.

For our house 4kW was calculated without hot water, in reality 3kW including hot water is enough.

Just get an inverter-controlled one... it only draws what is needed and then doesn’t make such a noise

Panasonic is one such example. At our 3kW heating load, it remains very quiet; when running at 5kW, it becomes noticeably louder. Hence the note that with tight sizing, acoustics become more important because it will then run at high power more often.

By the way, the 7kW heat pump is definitely not sensible, as it loses power with increasing cold, unlike the popular 5kW model, and no longer delivers 7kW. At higher outdoor temperatures, the 7kW power is not needed.

And please without buffer storage, but this is surely also mentioned in the [HTD Forum].

Thank you very much for your answers. You really gave us some good additional information. The topic of heat pumps is somewhat complex, and during a consultation we were already recommended a much larger unit (16kW) with a buffer tank at an unacceptable price. We will forgo the buffer tank. The system is located 4.5m from the neighboring house, so the acoustics are not unimportant. Is it sensible to use one heat pump as we currently do with our two-family house (two-story), or would it be better to have two smaller ones, each serving its own condominium?

The matter with the two smaller heat pumps for each household is already settled, as the smaller units start at 5KW and that would not be effective in our case. It was just a thought anyway.