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.