If you plan to use electric heater, switch to AC - regular heater or whatever have efficiency of mere 100%, while AC is a heat pump and can transfer heat at 200-500+% efficiency
This is quite good advice. The trick that that the portable AC units most people have in EU require an open window for the vent tube and create a positive or negative pressure which can push all your savings right out the window. If you have a split wall unit with a heat exchanger outside then you are in good shape.
I have a free-standing air-conditioner that I got because temperatures hit records this year, are you saying I should put the cold side of it outside and pipe the tube into my home?
Split A/C units (like ceiling mounted ones) are uncommon in most of Northern Europe residential buildings..
Unfortunately those free standing units create a pressure differential and you will see all your hot air pushed outside. The split units use a fluid line to the outside to exchange temperature, so you would need one of those.