Heat pumps are most efficient when running steadily at 60–80% of their max output. Oversized units short-cycle: they hit the set temperature, switch off, then start again — and starts/stops are where compressors wear out and where efficiency tanks.
In a typical Kiwi lounge of 25 m² with average insulation, the right-sized heat pump is usually between 4 kW and 5 kW. Yet it's common to see 7 kW units installed because the homeowner saw a "bigger is safer" sticker.
What goes into the maths
- NZ region — a Christchurch lounge needs ~60% more capacity than the same room in Auckland
- Insulation level — pre-1978 homes lose heat 30–50% faster
- Glazing area — big sliders and bifolds add a lot of load
- Ceiling height — open voids and cathedral ceilings need a bigger unit
Before you accept any installer's quote, run the numbers yourself with our free Heat Pump Sizer. Then ask them to justify their recommendation.