What Size Heat Pump Do I Need? kW by Room (NZ Guide)

Quick answerMost bedrooms need a heat pump of about 2 to 3 kW, a typical lounge 3.5 to 5 kW and a large open-plan living area 6 kW or more. The exact size depends on floor area, ceiling height, insulation, glazing and which way the room faces. Size for the heating output the unit delivers on a cold morning, not the headline figure on the brochure.

Heat pump assessor measuring a living room wall in an Auckland home

Size is the most common heat pump mistake. Too small and the unit runs flat out on cold nights without keeping up. Too big and it cycles on and off, wastes power and blows draughts. Getting it right is mostly a matter of measuring the room and being honest about how well it holds heat.

This guide gives you rule-of-thumb sizes by room, explains what pushes the number up or down, and shows how the official healthy homes tool works it out for rentals. For a quick estimate on your own rooms, try our free heat pump size calculator, which uses the same method.

How many kW heat pump do I need per square metre?

A rough starting point is about 0.1 to 0.2 kW of heating per square metre of floor with a standard 2.4 metre ceiling, with the low end for a well insulated, double glazed room and the high end for a cold, poorly insulated one. It is better to think in cubic metres, because a high ceiling adds air to heat.

Our calculator multiplies floor area by ceiling height, then by a heat load of about 45 watts per cubic metre for a well insulated room, 60 for some insulation and 75 for little or none, and adjusts for sun. The table shows what that gives for common rooms with a standard ceiling. It is a rule of thumb, not a measured heat loss calculation. Consumer NZ notes sizing tables usually assume 2.4 metre ceilings, so rooms with higher ceilings need more capacity.

Room (2.4 m ceiling)	Well insulated	Some insulation	Little or none
Bedroom, about 12 m2	1.3 kW	1.7 kW	2.2 kW
Small lounge, about 20 m2	2.2 kW	2.9 kW	3.6 kW
Large lounge, about 35 m2	3.8 kW	5.0 kW	6.3 kW
Open-plan living, about 50 m2	5.4 kW	7.2 kW	9.0 kW

What size heat pump for an open-plan living area?

A typical open-plan kitchen, dining and living area needs 6 kW or more, and large or poorly insulated spaces can call for 8 kW or more. Include every space that is always open to the room, such as the kitchen, hallway or stairwell, because the heat flows into them.

One very large unit is not always the best answer. EECA's installer guide notes that where a space is large, two smaller units or a multi-split can distribute heat better than one big unit at one end of the room. That is often the case in long, L-shaped living areas or rooms that wrap around a corner, where a single high wall unit leaves the far end cool.

What size heat pump for a bedroom?

Most bedrooms need about 2 to 3 kW. The heat load of a small, well insulated bedroom can be lower than the smallest units on the market, so the choice usually comes down to the smallest quiet model rather than an exact calculation.

For bedrooms, noise matters as much as capacity. Look for a low indoor noise figure on the lowest fan speed and a quiet or night mode, and make sure the outdoor unit is not under the bedroom window. A cold south-facing bedroom with single glazing may need the next size up. Our high wall heat pumps page covers the quiet bedroom models we fit most often.

Want the exact size for your rooms?

Cheeky's Heat Pumps measures each room and works out the capacity you need at a free in-home assessment, then quotes the right unit.

0800 432 878 Book Online Get a quote

What happens if a heat pump is too small?

An undersized heat pump runs at full output for long periods, struggles to reach the set temperature on cold nights and can spend more time in defrost. Consumer NZ says undersized units struggle to heat the room, and EECA's installer guide notes repeated defrost cycles within an hour or two can be a sign a unit is too small for the space.

Oversizing is not a safe hedge either. EECA's guide says a slightly larger unit gives a safety margin, but a significantly oversized one cycles on and off, wastes power, creates draughts and is noisier, and Consumer NZ adds that oversized units cost more upfront. The aim is a unit with a modest margin over the calculated load.

Does insulation change the size I need?

Yes, a lot. In the rule of thumb above, the same 35 square metre lounge needs about 3.8 kW when well insulated and about 6.3 kW with little or no insulation. Ceiling height, glazing, orientation and draughts change it too.

Ceiling height is the one people miss. In Ponsonby, many homes are Victorian and Edwardian timber villas with tall ceilings, so the air volume is much bigger than the floor area suggests. Location matters as well. EECA's installer guide tells installers to size for the capacity a unit delivers at the local design temperature, which it lists as 1 degree for the Auckland Council area, and inland spots such as Pukekohe can be colder on still winter nights than coastal suburbs. Shade adds load too: in Epsom, mature trees keep many rooms shaded through winter, especially south-facing living areas.

How does the healthy homes heating calculator work out size?

The Tenancy Services heating assessment tool calculates the minimum heating capacity a rental's main living room needs, in kW, rounded up to one decimal place. It is a more detailed heat loss calculation than a rule of thumb.

It asks for your council area, the property type and consent date to estimate insulation, the room size, any spaces always open to the living room, which walls, floors and ceilings face outside, and the size of external windows and glazed doors. The heating standard then requires fixed heating that meets that figure, with a thermostat, and each heater at least 1.5 kW. If you own a rental, our healthy homes heating assessments run this calculation on site. Homeowners can use the same tool as a free cross-check on their own living room.

How do you get an exact heat pump size?

Measure the room properly and work out its heat loss, or have an installer do it. A calculator gives a good starting range, but it cannot see a draughty sash window, a cold concrete floor or a wall of glass facing south.

Use our heat pump size calculator to get a first estimate, then compare it with what installers recommend. If two quotes suggest very different sizes for the same room, ask each one how they worked it out. At Cheeky's Heat Pumps we measure every room at a free in-home assessment and write the sizing basis on the quote. You can book online, and Cheeky's Heat Pumps will bring the tape measure.

Frequently Asked Questions

Is a 5 kW heat pump big enough for a living room?

Often, for a living room of around 20 to 35 square metres with standard ceilings and reasonable insulation. A bigger, open-plan or poorly insulated room may need 6 kW or more. Check the unit's heating output at 2 degrees on the energy label, because that is closer to what you get on a cold Auckland morning.

Can one heat pump heat a whole house?

Rarely well. A single unit heats the room it is in and some of the spaces open to it, but closed bedrooms and long hallways stay cooler. For whole-house heating, look at a multi-split with units in several rooms, or a ducted system with an outlet in each room.

Does a heat pump's kW rating mean how much power it uses?

No. The kW figure is heating or cooling output. Heat pumps move heat rather than make it, so a good unit delivers about three or more units of heat for each unit of electricity, which Consumer NZ calls a coefficient of performance of 3 or more. Power use is shown as annual kWh on the energy label.

What size heat pump do I need for a garage or sleepout?

Use the same method, but expect a higher load per square metre, because garages and sleepouts are often poorly insulated, with roller doors, concrete floors and thin walls. Improving insulation and sealing gaps first usually lets you buy a smaller, cheaper unit that keeps up.

Do I need a bigger heat pump for cooling?

Usually not in Auckland. Heat pumps sized for winter heating normally have enough cooling capacity for the same room. Rooms with large west-facing windows or skylights can be the exception, so mention afternoon sun when you get a quote.