Calculator
Rough heat pump size for your room
Room of about 30 m², estimated heating need
4.3 to 5.2 kW
That usually means a 5.0 kW high-wall heat pump.
Get it confirmed by a local installerWhat the kW number means
The kW figure on a heat pump is how much heat it can put into the room, not how much power it uses. A 6 kW heat pump typically draws a fraction of that from the wall, because it moves heat from outside rather than making it.
Most units list a rated capacity and a maximum. The rated figure is the one to size on. The maximum is what the unit can push for short bursts, often at lower efficiency.
Typical sizes by room
These ranges are for a home with average insulation and standard ceilings. Colder towns, high ceilings and draughty rooms push you towards the top of the range or beyond.
| Room | Typical size | Usual type |
|---|---|---|
| Small bedroom or study | 2.0 to 3.0 kW | High-wall |
| Large bedroom | 3.0 to 3.5 kW | High-wall |
| Small lounge | 3.5 to 5.0 kW | High-wall |
| Average living area | 5.0 to 7.0 kW | High-wall or floor console |
| Large open-plan | 7.0 to 9.0 kW | Large high-wall or ducted |
| Whole house | 10 kW and up | Ducted or multi-split |
What changes the size
Floor area is only the starting point. Installers look at the volume of air, how fast the room loses heat, and how cold it gets outside.
- Ceiling height. A villa with three-metre studs has around a quarter more air to heat than a modern room of the same floor area.
- Insulation and glazing. Ceiling and underfloor insulation and double glazing can reduce the size you need noticeably.
- Climate. A Dunedin or Invercargill lounge needs more heating capacity than the same room in Auckland.
- Open stairwells and hallways. Heat escapes upstairs or down a hall, so the unit is effectively heating more than one room.
- Large windows and sun. A west-facing glass wall loses heat on winter nights and gains it on summer afternoons.
Why too big or too small both cost you
An undersized heat pump runs flat out on cold mornings and still does not get the room warm. It uses more power per unit of heat because it never gets to cruise.
An oversized heat pump reaches temperature too quickly and switches on and off. That short-cycling wastes energy, wears the compressor and does a poorer job of drying the air.
The right size runs most of the time at a moderate output, which is where heat pumps are most efficient.
How an installer sizes it on site
A good installer measures the room, checks insulation in the ceiling and under the floor, looks at windows and doors, and asks how you use the space. They also check where the outdoor unit can go, because that sets the pipe run.
If a quote arrives with a kW figure and no one has been to look at the room, ask how it was worked out.
The calculator on this page is a rough guide only. Your installer confirms the size during a site visit.