Published on · By the Faster-EV team
A rule of thumb: 100 W per m²
To cool a room with a 2.5 m ceiling in a reasonably insulated home, a cooling capacity of about 100 W per m² is often used. A 12 m² bedroom therefore needs around 1.2 kW, and a 30 m² living room around 3 kW.
Indoor units come in capacity steps (for example 2.5 or 3.5 kW): choose the step equal to or just above your estimated need.
What changes the need
Several factors increase the required capacity: a higher ceiling, full south or west exposure, large windows, poor insulation, a room under the roof or many heat sources (kitchen, electronics, occupants). Conversely, a recent, well-insulated home may need less.
In these cases, the 100 W per m² estimate is only a starting point: a room-by-room calculation remains the reliable method.
For heating, think in heat losses
If you also plan to heat with a reversible air conditioner, capacity is determined by the home’s heat losses in cold weather, which depend on your region and insulation. A unit’s heating capacity drops as the outdoor temperature falls: check the capacity the manufacturer states at low temperatures, not just the nominal value.
Neither too much nor too little
An oversized unit reaches the set temperature quickly, then stops and restarts frequently: comfort is less stable, the air is less well dehumidified and wear increases. An undersized unit runs flat out without reaching the desired temperature on very hot days. Inverter models modulate their output, but only within a certain range: correct sizing is still necessary.
Equipping several rooms
With a multi-split, estimate each room’s need to choose its indoor unit, then check that the outdoor unit accepts the chosen combination: each manufacturer publishes combination tables. Your installer, who must hold a refrigerant handling certificate to connect the equipment anyway, can carry out a heat load calculation and confirm the choice before you order.