This is exactly the kind of everyday decision where "just crank the heater" quietly drains your bank account for three months straight. Mio doesn't care about cozy vibes — here's the math.
Older Japanese apartments (apāto and many mansion units) often have single-pane windows, thin walls and little insulation. Whatever you heat, a lot of it escapes. That flips the usual instinct: instead of buying a bigger heater to warm a leaky room, the cheaper strategy is to only heat the small space around your body and let the room stay cold.
Aircon (heat pump): the wall unit runs in reverse to heat. Efficient per unit of heat and good for a whole room, but a tired old unit in a leaky room can run for hours to hold temperature — that's where the bill climbs. Kerosene (tōyu) heater: strong, cheap heat, but you buy and haul kerosene, must ventilate regularly, and accept an open-flame fire risk. Kotatsu / electric blanket / panel heater: these heat the person, not the room — the lowest running cost of all.
| Method | Heats… | Relative monthly cost |
|---|---|---|
| Aircon (old unit, whole room, long hours) | The air | Highest |
| Kerosene heater (whole room) | The air | High–medium (plus fuel runs & risk) |
| Panel / halogen heater (spot) | Nearby air/body | Medium |
| Electric blanket | The person | Low |
| Kotatsu + blanket + layers | The person | Lowest |
Exact yen depends on your unit, tariff and how many hours you run it — check your own plan. The ordering is the point: anything that heats a whole leaky room sits at the top; anything that heats just you sits at the bottom. A kotatsu draws roughly the power of a couple of lightbulbs, not a room-sized appliance.
Aircon: the sticker running cost assumes a sealed room — in a leaky one it runs far longer than you expect. Kerosene: the fuel looks cheap, but add the trips to buy tōyu, the humidity/condensation it creates, mandatory ventilation (which vents heat), and fire risk — many rental contracts even restrict open-flame heaters. Kotatsu: almost no hidden cost, which is exactly why it wins.
Smart middle path: keep one aircon or heater to take the morning chill off a room for 15–20 minutes, then switch to kotatsu + layers for the hours you're actually sitting still. You get warm fast without paying to heat empty air all evening.
Heat the body, not the air. In a poorly insulated Japanese apartment, a kotatsu (plus an electric blanket and a warm layer) beats blasting the aircon on running cost, every time. Use aircon or a heater as a short burst to warm a room, not as your all-evening default. The "right" answer isn't about comfort tech — it's about refusing to pay to heat air that immediately leaks outside.