How to use it
Measure the room
Length and width, in metres. For open-plan living, measure the whole space the heat pump will warm, not just the lounge end.
Check the ceiling height
It starts at 2.4 metres, the ceiling height Consumer NZ's sizing table assumes. Change it if your ceilings are higher. Consumer NZ says to adjust estimates up for high ceilings, and the calculator does that through the room's volume.
Pick the insulation
Choose 'Older, little or no insulation' or 'Newer and well insulated'. If you're not sure, try both and take both answers to the installer.
How it works it out
It multiplies length, width and ceiling height to get the room's volume, then applies BRANZ's rough rule of thumb for heating rooms: 65 watts per cubic metre in an older uninsulated house, or 50 to 55 watts per cubic metre in a newer, well insulated one. Dividing by 1,000 turns watts into kilowatts.
The rule is per cubic metre of room, not per square metre of floor. That's why a high stud or a raked ceiling pushes the answer up.
What the answer does and doesn't tell you
Hold it up against the heat pump's nominal heating capacity: the figure on its energy label measured at 7 degrees outside. If your winter nights drop to 2 degrees or colder, EECA says the label's 2 degree heating figure should be the same or higher.
It doesn't know about your windows, which way the room faces, how draughty it is or how cold your winters get. Mitsubishi Electric NZ says areas with sub-zero winters, like most of the South Island, need a larger heat pump or an upgraded model.
Overshooting isn't a safe bet either: BRANZ says an oversized unit cycles on and off, cutting its efficiency and life expectancy. A proper heat load from your installer beats this number.
Renting the house out? A rental's legal minimum for the main living room comes from Tenancy Services' heating assessment tool, which gives its answer in kW rounded up to one decimal place. This calculator doesn't replace it.