What size heat pump is needed for a 180 square meter house?

You need to first work out your heat load for the house which depends on lots of factors. (Insulation, exposure, window area etc.). Since most heat pumps give an efficiency ratio of no more than 4 to 1. This means for every kilowatt of electricity you burn driving the pump you will get 4 kilowatts of heat. So if you think you can heat your house with 12 kilowatts of electric fires then you heat it with a 4 or 5 kilowatt heat pump. The trick with heat pumps is to find a good source of low grade heat that is still there in the winter. Ground coils, bore holes , streams, even pools, are all better than air to air systems because when you need most heat the air outside is coldest.