Sizing starts with heat loss
A heat pump should be sized to meet the home's design heat loss during cold conditions. Floor area can give a rough starting point, but it cannot replace a room-by-room assessment.
Coastal exposure, older construction, attic insulation, windows, draughts, and emitter sizing all affect the result.
Example rough range
A 120 m2 home with average insulation might roughly fall around 6 to 9 kW depending on age, exposure, ventilation, and radiators. A better insulated home of the same size may need less.
Use the calculator for a broad check only. Final equipment selection should come from a qualified survey and design.