I'm currently living like that in Japan. In fact I'm writing this sitting in a Kotatsu. Last week it once got 6C in the apartment. We use a DeLonghi oven, a small heater and a Kotatsu whenever we need a place to get warm. With the Kotatsu we were able to get the bill from 120$/m down to 80$/m. I have to say, one can get used to it - the constant change in temperature keeps up the blood flow and I feel a lot stronger against the cold compared to when I stayed in Switzerland, which gets a bit colder in winter outside - but always has cozy 21C inside. Only one thing: A Kotatsu is such a trap to get lazy. It's also dubbed 'bad people maker' in Japan - I can totally see why.