Not to brag but this winter I didn't turn on radiator once. The temperature never dropped below 20°C inside (I live in a flat that few years ago had 10cm of isolation added to it). Yes I know, some of the heat was probably from neighbors but I can't do anything about that. I also shortened hot showers to minimum and wore warm underwear and sweater. The winter was quite warm [1], coldest was around -5°C
[1] https://www.shmu.sk/sk/?page=1&id=klimat_operativneudaje1&id...