Skandi Rogers wrote:What is the heating at the moment? I would think the quickest way to guess how many kW you need is to see how many you are using at the moment. For example we have a 30kW output pellet boiler that runs around 30% most of the time so our house takes 10kW to heat with the outside at 0 and the inside at 20 using a mix of radiators and underfloor heating. However to heat the house downstairs (175m) from cold takes all 30kW for 4-5 hours. we have poor insulation and a lot of heat loss from badly insulated pipes between the boiler and house, but also have south facing windows which help a lot, IF the sun shines Your heating needs to be sized for the worst case so after a week of overcast weather at -6. my fear with a 5kw system is that from cold which it will be every time you let the fire go out for more than a day it's going to take you 6-7 hours to reheat the air and even longer to reheat the mass.
The official way to calculate is very complicated you need to know the R value of all walls roofs and windows which in practice is impossible to find on an old house especially if it's been retrofitted with insulation. I can find estimates online especially if you look at air to air heat pumps they estimate that a new/well insulated house requires 55w per m3 so guessing your house is 2.5m tall inside you would need 13kW to heat it.