Heat Pump Size Calculator: Get the Right kW for Your Room
Sizing your heat pump correctly matters more than which brand you buy. Undersized? It’ll be loud, inefficient, and never quite warm enough. Oversized? It cycles on and off constantly, wastes energy, and creates temperature swings.
There’s a simple calculation.
Here it is.
The Formula
Room volume (m³) × 55 watts = kW you need
That’s for a standard well-insulated home. Done.
Example
You’ve got a lounge: 5m long, 4m wide, 2.4m high ceiling.
5 × 4 × 2.4 = 48 cubic meters 48 × 55 = 2,640 watts = 2.6kW
So you’d want a 2.5-3kW heat pump for that room.
But That's the Baseline
The 55 watts per cubic meter assumes a reasonably insulated house. If yours is different, you adjust:
For a new, well-insulated home (built 2016 or later):
Use 50 watts instead. Saves you a bit of kW.
For an older house with average insulation:
Stick with 55 watts. That’s the middle ground most Auckland homes are.
For a cold, damp house with poor insulation or lots of windows:
Use 65 watts. The unit needs to work harder.
For the South Island or high-altitude areas:
Add 10–15% to your calculation because it gets colder.
Adjusted Example (Cold South Island House)
Same 48m³ room but in Christchurch with older insulation:
48 × 65 = 3,120 watts = 3.1kW
Plus 10–15% for climate = 3.5kW would be safer
Heat Pump Sizes You'll Actually See
- 2.5kW: Small bedroom, home office
- 3.5kW: Medium bedroom, small lounge
- 5kW: Large bedroom, medium lounge, small open-plan
- 6kW: Large open-plan living room, moderate whole-home use
- 7–8kW+: Large open-plan, whole-home systems, commercial
Most single-room installations in Auckland homes are 3.5kW or 5kW. That’s the sweet spot.
Variables You Need to Factor In
- Window size and type: Large windows or single glazing? Add 10%. Double glazing helps a lot.
- How new is your house: Older homes leak heat. Newer homes (especially post-2016) are more airtight.
- Sunlight exposure: A north-facing room gets passive solar heat in winter. Reduce your calculation by 5–10% if it’s sunny.
- Ceiling height: Standard is 2.4m. Higher ceilings? Multiply by 1.2–1.5 for the extra volume.
- North vs. South Island: South Island’s colder, so you need extra capacity.
What You Shouldn't Do
Don’t just pick the biggest unit available hoping it’ll be “more powerful.” Oversizing causes problems:
- Frequent cycling (on/off/on/off) wastes energy
- Temperature swings—too hot then too cold
- Higher upfront cost with no benefit
- Reduced equipment lifespan from stress
Professional installers won’t recommend oversizing because it’s inefficient.
The Professional Assessment
This calculator gives you a starting point. But the best approach is having someone visit your home and actually look at:
- Your insulation level
- Window placement and condition
- How airtight the space actually is
- Your local climate zone
- Which room you’re heating and whether heat needs to carry elsewhere
Varcoe does free on-site assessments. We measure, we look at your actual insulation, we check your windows, and we tell you exactly what size you need. Not a sales pitch, just the right size.
Quick Sizing Guide for Auckland
Bedrooms (typical 3m × 4m × 2.4m high):
- New/insulated home: 2.5kW
- Average home: 3.5kW
- Older/poor insulation: 3.5-5kW
Living rooms (typical 5m × 6m × 2.4m high):
- New/insulated home: 4kW
- Average home: 5-6kW
- Older/poor insulation: 6-7kW
Open-plan areas (7m × 7m or larger):
- New/insulated home: 6-7kW
- Average home: 7-8kW
- Older/poor insulation: 8-10kW
South Island equivalents:
- Add 1kW to these figures.
After You've Sized It, Installation Matters
Right sizing is half the battle. Installation is the other half.
Your installer needs to:
Mount the indoor unit high on an exterior wall (not corners)
Run pipes with proper insulation
Size the electrical circuit correctly
Test and balance the system
Show you how to use it efficiently
A perfectly sized unit installed badly performs worse than a slightly oversized unit installed right.
One More Thing
Check the Energy Rating Label when comparing units. More stars = more efficient. A 3-star unit generates 34% fewer emissions than a 1-star unit of the same size. Efficiency varies between brands, so look at the actual ratings, not just the kW number.
Get Your Size Right
Use the formula above as a starting point. Measure your room, calculate it, then ring us on 0800 088 888 and we’ll confirm whether your calculation makes sense for your specific Auckland home. Free, no obligation. We’d rather size you correctly than have you buy the wrong unit.