What size heat pump do I need?
Multiply the floor area by 0.13 kW per m² for a room with ceiling and underfloor insulation and a 2.4 m stud, then adjust for a higher stud, less or more insulation and a cold climate. A 25 m² insulated lounge in Auckland comes to about 3.3 kW. The calculator shows the working and the package that fits.
| 25 m² × 0.13 kW | 3.3 kW |
|---|---|
| Stud 2.4 m | — |
| Insulation: some | — |
| Auckland: mild | — |
| Sized for | 3.3 kW |
Small room package: a 2.5, 3.5 or 4.2 kW high-wall unit, standard back-to-back install and electrical certificate. We're contracting the installer for Auckland, so bookings go on the waitlist for now.
A rule of thumb, shown so you can check it. Our installer's site check confirms the size before you accept the price. How the method works.
Heat pump size chart: what size for my room?
Find your room's floor area in the left column. The middle column is a room with ceiling and underfloor insulation and a 2.4 m stud in Building Code climate zones 1–4 (Auckland, Hamilton, Rotorua, Wellington and Nelson, for example). The next column is the same room in zones 5–6 (Christchurch, Timaru and Dunedin are zone 5; Invercargill, Queenstown and Central Otago are zone 6). The last is an uninsulated room in a mild climate. Every figure comes from the same method as the calculator above.
| Room size | Insulated, zones 1–4 | Insulated, zones 5–6 | Uninsulated, zones 1–4 |
|---|---|---|---|
| 10 m² | 1.3 kW | 1.6 kW | 1.8 kW |
| 15 m² | 2.0 kW | 2.3 kW | 2.6 kW |
| 20 m² | 2.6 kW | 3.1 kW | 3.5 kW |
| 25 m² | 3.3 kW | 3.9 kW | 4.4 kW |
| 30 m² | 3.9 kW | 4.7 kW | 5.3 kW |
| 35 m² | 4.5 kW | 5.5 kW | 6.1 kW |
| 40 m² | 5.2 kW | 6.2 kW | 7.0 kW |
| 50 m² | 6.5 kW | 7.8 kW | 8.8 kW |
| 60 m² | 7.8 kW | 9.4 kW | 10.5 kW |
| 70 m² | 9.1 kW | 10.9 kW | 12.3 kW |
For a stud higher than 2.4 m, add 12% for every extra 0.3 m. If the walls are insulated and the windows double glazed, take off 15%. Climate zones are from MBIE's H1/AS1.
What size heat pump for a 25 m² lounge in my city?
| City | Climate zone | Heating capacity |
|---|---|---|
| Auckland | Zone 1 | 3.3 kW |
| Tauranga | Zone 1 | 3.3 kW |
| Hamilton | Zone 2 | 3.3 kW |
| Wellington | Zone 3 | 3.3 kW |
| Rotorua | Zone 4 | 3.3 kW |
| Christchurch | Zone 5 | 3.9 kW |
| Dunedin | Zone 5 | 3.9 kW |
| Queenstown | Zone 6 | 3.9 kW |
| Invercargill | Zone 6 | 3.9 kW |
Worked example: a 35 m² Christchurch lounge
Base: 35 m² × 0.13 kW = 4.55 kW. The 2.7 m stud is 0.3 m above 2.4 m, so add 12%. Christchurch is climate zone 5, so add 20%. The room has ceiling and underfloor insulation, so no change there. 4.55 × 1.32 = 6.0 kW, which points to a 6.0 or 7.1 kW high-wall unit: our Living area package. The same room in Auckland with a 2.4 m stud needs only 4.5 kW, a size smaller.
Is it better to oversize or undersize a heat pump?
Slightly over is better than under. An undersized unit runs flat out on the coldest mornings and still leaves the room cool. A unit one size up runs at part load most of the time, which inverter heat pumps handle efficiently. Going two or more sizes up costs more to buy and can cycle on and off in a small room. Pick the standard size at or just above your result.
Updated
From kW to a package
| Calculator result | Package | Unit fitted |
|---|---|---|
| Under 4.5 kW | Small room | 2.5, 3.5 or 4.2 kW high-wall |
| 4.5 to 7 kW | Living area | 5.0, 6.0 or 7.1 kW high-wall |
| 7 to 10 kW | Large open plan | 8.0 or 9.0 kW high-wall |
| 10 kW or more | Custom quote | Multi-split or ducted |
The adjustments
| Factor | Adjustment |
|---|---|
| Stud above 2.4 m | +12% per 0.3 m |
| No insulation | +35% |
| Ceiling and underfloor insulation | No change |
| Plus wall insulation and double glazing | −15% |
| Building Code H1 zones 5 and 6 | +20% |
What size heat pump do I need for a 40 m² room?
About 5.2 kW in Auckland, Wellington or anywhere in climate zones 1–4 with ceiling and underfloor insulation, and about 6.2 kW in zones 5–6 (Christchurch, Dunedin, Queenstown). Without insulation it rises to about 7.0 kW.
What size heat pump do I need for a 50 m² open-plan area?
About 6.5 kW in a mild climate and 7.8 kW in a cold one, insulated, 2.4 m stud. That is the top of the high-wall range, so a multi-split or ducted system is worth pricing too.
What size heat pump do I need for a 30 m² room?
About 3.9 kW in a mild or moderate climate with ceiling and underfloor insulation and a 2.4 m stud, and about 4.7 kW in Christchurch, Dunedin or further south.
How does the calculator work?
Floor area × 0.13 kW per m², then +12% for every 0.3 m of stud above 2.4 m, +35% with no insulation or −15% with walls insulated and double glazing, and +20% in Building Code climate zones 5 and 6. The full method is in our sizing guide.
What's the smallest heat pump I can buy?
About 2.0 kW cooling and 2.5–2.8 kW heating. Mitsubishi Electric's AP20 Mini is the smallest indoor unit; Fujitsu, Panasonic and Daikin each make a 2.0 kW model. If the calculator gives you under about 2.5 kW, one of these is the answer: see small heat pumps NZ.
Is 0.13 kW per m² an official standard?
No. It's a rule of thumb, shown so you can check it. The Building Code climate zones come from MBIE's H1/AS1, and Consumer NZ also says to size up for high ceilings. Our installer's site check confirms the size before you accept a price.
What if the calculator says 10 kW or more?
That's more than one high-wall unit should heat. Insulating first often brings it down. Otherwise a multi-split or ducted system suits, on a custom quote.
I'm a landlord. Can I use this for Healthy Homes?
No. Use the official Tenancy Services Heating Assessment Tool for the main living room of a rental. See our rentals guide.
Tell us the room. We'll tell you the size, the price and exactly what's included.
Size the room in two minutes. You see the kW, the package it needs and the standard install spelled out. Ducted, several rooms or a tricky wall? Ask for a custom quote.