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The most expensive heat pump mistake in Auckland is not choosing the wrong brand. It is choosing the wrong size. Get it too small and it runs flat out on cold nights and never quite gets there. Get it too big and it short-cycles, wastes power and swings the temperature around. Size is the single decision that most affects whether you are happy with your heat pump for the next fifteen years, and the good news is you can get a solid estimate yourself in about two minutes.
Here is the simple way to work out the right size, how Auckland homes change the number, and why the final call is always worth a proper look.
Heating capacity is measured in kilowatts, and there is a simple starting calculation.
Measure your room’s length, width and height in metres, multiply them together to get the volume, then multiply that by 55 watts. Divide by 1,000 and you have the rough kilowatts you need.
Here it is with a real lounge. Say it is 5 metres long, 4 metres wide, with a 2.4 metre ceiling. That is 5 by 4 by 2.4, which is 48 cubic metres. Multiply by 55 and you get 2,640 watts, or 2.6 kilowatts. So that room wants a heat pump in the 2.5 to 3 kilowatt range. Our heat pump size calculator does this same maths for you if you would rather not reach for a calculator.
That 55 watts per cubic metre assumes a reasonably insulated house, which most Auckland homes are. If yours is not typical, adjust the figure.
For a new, well-insulated home built from about 2016 onward, use 50 watts instead, because it holds heat better and needs less input. For a cold, damp, older house with poor insulation or a lot of window glass, use 65 watts, because it loses heat faster and needs more grunt to keep up. A classic uninsulated villa sits firmly in that last group, so do not size it like a modern townhouse.
As a rough guide, here is where common Auckland rooms tend to land.
| Room | Typical size | Rough capacity |
| Bedroom or small office | Up to about 25 cubic metres | 2.0 to 2.5 kW |
| Standard lounge | Around 40 to 55 cubic metres | 2.5 to 3.5 kW |
| Large or open-plan living | 60 cubic metres and up | 4.0 to 6.0 kW or more |
| Open-plan with 3 metre ceilings | Varies, always higher | Add capacity, get it assessed |
Open-plan kitchens and lounges with high ceilings are the ones people most often underestimate. Those soaring spaces look great and demand a lot more heating power than the floor area alone suggests.
It is tempting to think bigger is safer. It is not.
An undersized unit runs at maximum output trying to reach a temperature it cannot hold, which makes it loud, thirsty and disappointing on the coldest nights when you need it most. An oversized unit has the opposite problem. It blasts the room to temperature, switches off, cools down, and switches on again, over and over. That short-cycling wastes energy, wears the unit out faster and creates the uneven, drafty temperature swings people complain about. Right sizing is what keeps a heat pump quiet, efficient and cheap to run, and it feeds directly into your heat pump running costs for the life of the system.
People spend hours agonising over which brand to buy and five minutes on the size. It should be the other way around. A perfectly good unit that is the wrong size will disappoint you, while a correctly sized unit from any of the major brands will keep you comfortable. Get the kilowatts right first, then choose between the brands. If you are weighing that up, our comparison of the best heat pump brands in NZ is a good next read. And when you do compare units, look at the Energy Rating Label, because a higher star rating means real efficiency gains at the same kilowatt size.
Not in a way you need to worry about. In Auckland, heating is the harder of the two jobs, so a heat pump sized correctly to warm your room through winter will comfortably cool that same room in summer. You size once, for heating, and the cooling looks after itself. Where people go wrong is trying to stretch a single wall unit across two rooms through a doorway. A heat pump heats and cools the space it is in, not the room next door, so if you want a warm lounge and a warm bedroom, you size and place a unit for each, or look at a multi-room system rather than hoping one oversized unit will reach everywhere. Bigger is not a shortcut to covering more rooms.
The calculation gives you a solid ballpark, but it cannot see your house. Two rooms of identical size can need different units depending on their insulation, the amount and orientation of their glazing, how exposed they are to wind, and, for many Auckland properties, salt air on the coast. That is why a number from a calculator is a starting point, not a final answer.
This is where a free on-site assessment earns its keep. Someone measures the room properly, looks at your actual insulation and windows, checks the aspect, and recommends the size that will run efficiently rather than the size that happened to be on special. Getting this right is also half of what determines your final installation cost, because the unit size and the electrical work behind it are the two biggest price drivers.
A quick calculation will get you close, and it is well worth doing before you start collecting quotes so you know what you are being sold. But the difference between close and correct is the part that shows up on your power bill every month for years.
Varcoe has sized and installed more than 20,000 systems across Auckland since 1975, and we do free on-site assessments where we measure, check your insulation and windows, and tell you the right size for your home. Not the biggest, not the one on special, just the right one. Run your numbers through our heat pump size calculator, then call us on 0800 088 888 or email info@varcoe.co.nz and we will confirm it for your specific room.
Multiply the room’s length by its width by its height in metres to get the volume, then multiply by 55 watts and divide by 1,000 for the rough kilowatts. A 48 cubic metre lounge, for example, needs about 2.6 kW, so a 2.5 to 3 kW unit.
It is easier to work in cubic metres than square metres, because ceiling height matters. Use around 55 watts per cubic metre for a typical home, 50 for a new well-insulated one, and 65 for a cold, damp or poorly insulated house.
Yes. An oversized heat pump short-cycles, switching on and off repeatedly, which wastes energy, wears the unit out faster and creates uneven temperatures. The right size runs steadily and costs less.
Open-plan areas and high ceilings need noticeably more capacity than the floor area suggests, and they are easy to underestimate. Calculate the full volume including the extra height, lean toward the higher end, and get it assessed on site.
No. Size has a bigger impact on comfort and running cost than brand. Get the kilowatts right first, then choose your brand and check the Energy Rating Label for efficiency.
We measure on site and tell you the right size for your room, not the biggest.
