Room Heat Pump Sizing Calculator NZ
Choosing the right size heat pump is the difference between a warm, cheap-to-run room and a unit that either struggles on the coldest days or costs more than it needed to. This New Zealand calculator estimates the heating capacity, in kilowatts, that your room actually needs, then suggests a common model size to match. Get it right and the heat pump holds your target temperature comfortably through a frosty Canterbury or Central Otago morning while sipping power; get it wrong and you either shiver or overspend. Sizing comes down to how much heat the room loses, which depends mostly on its volume, how well it is insulated, and how cold your part of the country gets. You enter the room's length, width and stud height, pick an insulation level from poor to well insulated, choose your climate zone from the warm north to the cold south, and flag whether the room has a lot of glass or exposed walls. The calculator turns that into a heating load in watts using widely used watts-per-cubic-metre rules of thumb, adjusts for your climate and exposure, and converts it to kilowatts. It then rounds up to the nearest standard heat pump capacity, the kind of figure you will see on a high-wall unit, so you can shortlist models with confidence or sense-check an installer's quote. Insulation makes a large difference, so it is worth seeing how much smaller a unit you could fit by improving ceiling and underfloor insulation first. This is a planning estimate to guide your choice, not a substitute for a professional heat-loss assessment, and you should confirm sizing with a qualified installer before buying.
A planning estimate from watts-per-cubic-metre rules of thumb, adjusted for climate and exposure, rounded up to a common model size. Heating capacity, not cooling. Confirm with a qualified installer and a heat-loss assessment before buying.
How it works
The calculator multiplies length by width by stud height to get the room volume in cubic metres. It multiplies that by a baseline heating figure in watts per cubic metre set by your insulation level, then applies a climate multiplier and an exposure multiplier for large windows or exposed walls. That gives the heating load in watts, which is divided by 1,000 for kilowatts. The suggested size rounds the load up to the nearest common heat pump capacity, so the unit comfortably meets the load on cold days.
Worked example
A 5 m by 4 m lounge with a 2.4 m stud is 48 cubic metres. At an average insulation figure of 60 watts per cubic metre, in a temperate zone with average windows, the load is about 2.88 kW. Rounded up, a 3.2 kW heat pump suits the room. Improve the insulation to well insulated and the load drops to around 2.16 kW, bringing a smaller and cheaper unit into range.
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