No products in the cart.
What size heat pump do I need? The answer is a number in kilowatts (kW), and it depends on more than floor area. EECA, the Energy Efficiency and Conservation Authority, says a heater that is too small will struggle and one that is too big costs you more. This guide covers what changes the kW for a New Zealand room, why your local winter low matters, and how to get a proper figure.
What heat pump size actually means
A heat pump is rated by the heat or cooling it delivers, not by the power it draws. Model names often follow the cooling figure, and the heating figure is usually higher. Daikin New Zealand lists its Cora wall-mounted range at 2.0 to 7.1 kW cooling but 2.8 to 8.1 kW heating. Panasonic New Zealand lists one model as 3.5 kW cooling and 4.0 kW heating.
Consumer NZ calls heating capacity the most important specification when you buy, so size on heating, not cooling. The Energy Rating Label on every heat pump shows heating capacity at two outdoor temperatures, 7°C and 2°C, and cooling capacity at 35°C. If your winters are cold, compare the 2°C heating figure.
Looking for a heat pump? High wall sets from $859.99 incl. GST, delivered NZ wide. We arrange installation in Auckland, Hamilton, Tauranga, Rotorua, Wellington, Christchurch and Invercargill.
What decides the size you need
Floor area and ceiling height
A heat pump heats the air in the room, so volume matters, not just floor area. Consumer NZ’s sizing estimates assume a standard 2.4 m ceiling and say to size up for higher ceilings. Count any space that is always open to the room, such as an open-plan kitchen, hallway or stairwell. The government’s heating assessment tool treats those spaces as part of the room.
Insulation
Panasonic’s buyers guide notes that a room without wall or ceiling insulation needs more capacity than an insulated one. EECA’s advice is to insulate first: ceiling and underfloor insulation where practicable, weather stripping on doors and windows, and no draughts.
Windows, glazing and sun
Windows are one of the surfaces heat escapes through, so their size and whether they are single or double glazed change the load. Direction matters too. A room with direct afternoon sun from the west behaves differently from one that only gets morning sun, and curtains or blinds reduce heat loss through the glass.
Your local climate
This is the factor most people miss. Heating output falls as the outdoor temperature falls, and EECA notes that some models keep performing down to -20°C while others struggle below freezing. Tenancy Services puts it plainly: you need more heating capacity if you live somewhere cold.
Installers size against an external design temperature, the cold winter figure set for each council area in Schedule 2 of the Residential Tenancies (Healthy Homes Standards) Regulations 2019. EECA’s heat pump installation guide lists them. A selection:
| Council area | External design temperature (°C) |
|---|---|
| Whangarei | 2 |
| Auckland | 1 |
| Hamilton City | -3 |
| Tauranga City | 0 |
| Rotorua Lakes | -3 |
| Palmerston North City | -3 |
| Wellington City | 2 |
| Christchurch City | -4 |
| Dunedin City | -4 |
| Queenstown Lakes | -6 |
| Central Otago | -8 |
| Invercargill City | -4 |
| Mackenzie | -10 |
EECA adds that a sunless or damp valley can be one or two degrees colder than the figure for its area.
Why the rated kW is not the whole story
EECA’s guide works through an example. A room needs 5.6 kW of heat and the design temperature is +1°C. On a sample performance chart, the heat pump rated at 6 kW only delivers 5.1 kW at 0°C, so it falls short. The 8 kW rated model delivers 6.2 kW at 0°C and does the job.
So check heating output at your winter low, not the headline rating. EECA recommends products that publish this performance data, particularly in colder climates.
Too small or too big
- Undersized: defrosts too often, so at times it is not delivering heat or blows cold air. It runs at full capacity too often, which raises running costs, lowers efficiency and adds wear.
- Significantly oversized: uses more power starting up, short cycles because output exceeds demand, and runs at low load too often. That can mean draughts even on low fan, more noise and lower efficiency.
According to EECA, the extra running costs of an undersized heat pump generally outweigh the extra cost of installing the next size up. A slightly oversized unit gives a safety margin. Big oversizing is what to avoid.
For a large or open-plan area, EECA notes that two smaller units or a multi-split system can spread heat better than one large unit. See our multi-split heat pump systems, or for central heating, ducted heat pump systems.
A quick rule of thumb, and its limits
Hitachi New Zealand gives a rough indication of 0.12 kW per square metre. On that rule a 25 m² room works out at about 3 kW and a 40 m² lounge at about 4.8 kW. Use it as a starting point only. It ignores ceiling height, insulation, glass and your design temperature, which can all push the number up.
Get a proper number with the heating assessment tool
The Tenancy Services heating assessment tool works out the minimum heating capacity for a main living room under the healthy homes standards. It asks for your region, the property type and when it got building consent, the space you need to heat, which surfaces heat can escape through and how well they are insulated. The result is in kW, rounded up to one decimal place.
It was built for rentals, but EECA’s installation guide says it suits any room in rental and owner-occupied homes. Where it has to estimate, it picks safe values. For sloped or irregular ceilings it assumes a 45 degree slope and the highest wall height, so it will usually overestimate. Check each model you compare can deliver the tool’s figure at your design temperature.
When to get a site assessment
EECA says a good supplier should be able to assess the size of the space, the number of windows and your climate before recommending a size. It is worth doing when:
- the room is open-plan or joins a hallway or stairwell
- ceilings are high, sloped or raked
- your design temperature is below 0°C
- the home is older or you are unsure what insulation it has
- you want to heat several rooms from one system
EECA recommends the indoor unit high on an exterior wall, away from corners, with the outdoor unit ideally within about 15 metres and clear space around it for airflow and maintenance.
NZ DEPOT offers installation within 15 km of Auckland, Wellington, Christchurch, Hamilton, Tauranga, Rotorua and Invercargill. Elsewhere you can use your own licensed installer. Delivery takes 3 to 9 working days depending on location. Request a heat pump installation quote with your room size, ceiling height and town. If you already know your size, our high wall heat pumps start from $859.99 for a set.
Frequently asked questions
What size heat pump do I need for a 20 m² bedroom?
Hitachi New Zealand’s rough guide of 0.12 kW per square metre gives about 2.4 kW. Treat that as a rough start. A cold location, high ceiling, poor insulation or lots of glass push it up, so run the heating assessment tool and compare the 2°C heating figure on each label.
Should I size on heating kW or cooling kW?
Heating, for most New Zealand homes. The two figures differ and the model name often follows cooling, so read the heating capacity on the Energy Rating Label.
Is a bigger heat pump always better?
No. EECA says slight oversizing is fine, but a significantly oversized unit short cycles, can feel draughty, is noisier and less efficient. Undersizing is usually the costlier mistake.
Do high ceilings need a bigger heat pump?
Yes. Consumer NZ’s estimates assume a 2.4 m ceiling, so higher ceilings mean more air to heat. Consumer NZ also notes floor-mounted units may heat more effectively in high-ceiling rooms.
Can one heat pump heat my whole house?
A single split system heats one room. EECA says multi-split systems run several indoor units from one outdoor unit, and ducted systems heat multiple rooms through vents. Consumer NZ points out a multi-split cannot cool one room while heating another.
Can I use the healthy homes heating tool if I own my home?
Yes. EECA’s installation guide says the tool suits any room in both rental and owner-occupied homes in New Zealand.
More heat pump guides
Sources
- EECA, Good practice guide: heat pump installation, March 2022 (sizing, design temperatures, performance chart example)
- EECA, Buy an efficient heater (right size, site assessment)
- EECA, Heat pumps (system types, installation position)
- EECA, Zoned Energy Rating Label for heat pumps
- Tenancy Services, Heating assessment tool guide (last updated 21/09/2026)
- Consumer NZ, How to buy the best heat pump (updated May 2026)
- Daikin New Zealand, Cora wall mounted heat pump (heating and cooling capacity range)
- Panasonic New Zealand, Single room air conditioners (heating and cooling capacities)
- Panasonic New Zealand, Air conditioning buyers guide (insulation, windows and aspect)
- Hitachi New Zealand, What size heat pump do I need?
Ready to choose a heat pump? High wall sets from $859.99 incl. GST, delivered NZ wide. We arrange installation in Auckland, Hamilton, Tauranga, Rotorua, Wellington, Christchurch and Invercargill.
