Is a buffer tank useful in a heat pump?

Ground source heat pump 3-12 kW modulating
Optimal heat pump size according to calculation: 8 kW

Why do you want a 12 kW "monster"? Don't oversize the heat pump! Even the calculated 8 kW feels too large to me. Is there a room-by-room heating load calculation?

Why do you want a 12 kW "monster"? Don't oversize the heat pump! Even the calculated 8 kW feels too big to me. Is there a room-by-room heat load calculation?

I already have a modulating heat pump with 3-12 kW.

The consideration is to retrofit a buffer tank.

How do you come to think you might need one? Does the heat pump cycle too frequently for you? Is the consumption too high?
The heat pump is probably significantly oversized.
First of all, you should check if it is really a problem and if so, whether it can be controlled with settings.

I already have a modulating heat pump with 3-12 kW.

The idea is to retrofit a buffer tank.

That means the house and heating system are already built or installed and you want to change something, or have you only ordered/bought the heat pump so far? Maybe you would like to describe the initial situation in more detail so that you get meaningful answers…

Just to clarify: our architect and the heating planner were completely in agreement and roughly recommended the following to us: heat pump, stratified storage tank and fresh water station, no buffer, no ERR.

Just to explain: our architect and also the heating planner were completely in agreement and roughly recommended the following to us: heat pump, stratified storage tank and fresh water station, no buffer, stratified storage tank and fresh water station, no buffer, no ERR.

1) House & heat pump are 7 years old. The heat pump made 24 starts per day, now the compressor is broken, I have already discussed everything. One solution option is a buffer tank. Since I don't want the discussion to get out of hand and I specifically want to learn something about the topic of buffer tanks, I have formulated the inquiry this way.

2) "Stratified storage tank and fresh water station, no buffer"

Huh? A stratified storage tank with a fresh water station is a buffer tank after all.