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What size heat pump?

A heat pump needs to be the right size for the area it’s going to heat.

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Too small means the unit will have to work so hard that it won’t deliver its best efficiency and will be slow at heating a cold room. It’s also more likely to freeze up in very cold weather. Too large means the unit will have to cycle on and off to avoid overheating the area. It’ll also cost more to buy than a model that’s the right size.

Get the size right and the heat pump will operate somewhere in the middle of its heat-output range – and that’s where it’s most efficient.

Once you know the size, get the most efficient model you can find.

Measuring guide

Red = external wall
Blue = internal wall
A x B = area of wall
C x D = area of window
E x F = area of ceiling to unheated/heated room above
G x H = area of ceiling to un-insulated roof


Use our calculator to work out what size heat pump will best suit your needs.

The DIY approach

Here’s a quick guide if you’d prefer to work out your required heating capacity manually:

  • Allow about 44 watts per cubic metre of room volume.
  • Add another 10 percent for a large window area and another 10 to 20 percent for partial or no insulation.
  • If it's a lounge, multiply by 1.5. If it's a bedroom, make it 1.2, and for other areas multiply by 0.8.

Example: A well-insulated bedroom 3m x 4m x 2.4m high has a volume of 28.8 cubic metres. Multiply by 44 to get 1267 watts, and again by 1.2 to get 1520 watts (1.5kW).

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