Comparing Heater Electricity Costs: Kotatsu, Electric Blanket, Heaters and Air Conditioners

Quick-reference table of heater electricity costs

These are the electricity costs at ¥31/kWh for each device under typical use. A winter is calculated as the five months from November to March.

Per hour, there's a gap of over 20 times, from ¥1.6 for an electric blanket to ¥37.2 for an oil heater. The general trend is that devices warming your body directly are cheaper, and those warming the air or the whole room are more expensive.

Why air conditioners are cheaper than heaters

Ceramic fan heaters and oil heaters convert electricity directly into heat. 1 kWh of electricity produces only 1 kWh of heat, so efficiency tops out at 1.0.

An air conditioner doesn't make heat from electricity. It's a heat pump that collects heat from the outside air and moves it indoors. It can deliver more heat to the room than the electricity used to move it, so the same warmth takes a fraction of the power.

Heating a room for the same time costs about ¥16,000 less per winter with an air conditioner. Ceramic fan heaters suit brief use in small spaces such as changing rooms and toilets. Make one your main heat source for long hours in a whole room, and your bill swells quickly.

Oil heaters also convert electricity into heat. Their appeal is soft warmth that doesn't pollute the air or blow a draft, but they draw 1,200–1,500 W when starting up and cycle at around 500 W even after the room warms, so average power is still high. Judged on electricity cost alone, they're not suited to main heating.

Heat the whole room, or heat your body?

The gap in electricity cost comes down to the size of what's being heated. Warming just the inside of a kotatsu or your bedding takes far less heat than warming the air of a 6-tatami room.

• You spend long hours sitting alone: kotatsu (¥698 a month) + electric blanket at bedtime (¥372 a month) comes to ¥1,070 a month, less than a third of air conditioner heating.
• The family moves around the living room: the air conditioner is your mainstay. If the cold floor bothers you, using only half of a heated carpet also roughly halves its power use.
• Just changing rooms, toilets or your feet: a ceramic fan heater for short periods. 10 minutes at a time costs ¥6.2.
• When sleeping: even on high, an electric blanket costs ¥1.6 an hour. On low it's around 20 W, less than half that again.

How to choose heating that keeps costs down

1. 1

   Use an air conditioner as your main heating

   For heating a whole room, an air conditioner is the most efficient of the electric options. Just switching your long-hours mainstay from a heater to an air conditioner can cut your winter bill by more than ¥10,000.

2. 2

   Add devices that warm your body directly

   Using a kotatsu, electric blanket or heated carpet alongside makes it easier to feel warm even with the air conditioner set lower.

3. 3

   Keep heaters for short periods and small spaces

   Devices over 1,000 W cost more than ¥30 an hour. Limit them to uses like a few minutes while dressing in the morning or just the changing room.

4. 4

   Don't let heat escape

   Reducing heat loss with thick curtains, window insulation film and floor rugs lowers the power needed whichever device you use.

Using an air conditioner for heating and the cost of leaving it on are covered in detail in air conditioner electricity costs. Aquarium heaters are another appliance that stays plugged in 24 hours through winter. Choosing the wattage and guide costs are covered in what wattage aquarium heater do you need?.

Calculate with your own devices

The "Calculate by appliance" tab of the Electricity Cost Calculator includes 16 devices in heating and cooling alone. Choosing a device fills in its average power and a typical usage time, so change the time to match how you use it.

To compare two devices, start from "Air conditioner heating / ceramic fan heater" on the "Compare electricity costs" tab. By overwriting the wattage and hours on each side with your own devices, you can make other comparisons too, such as a kotatsu and electric blanket combination against an air conditioner. The appliance quick-reference table is sorted from highest monthly cost, and narrowing the category to "Heating & cooling" lets you compare just the heating devices.

FAQ

Which heating device has the lowest electricity cost?

An electric blanket. Even on high it averages around 50 W, or ¥1.6 an hour, and 8 hours a day comes to ¥372 a month. Next cheapest is a kotatsu (¥4.6 an hour on medium). Both warm your body directly and need far less power than devices that heat the whole room.

How much does a kotatsu cost per hour?

About ¥4.6 on medium. The heater itself is 500–600 W on high, but the thermostat switches it on and off, so it averages around 150 W. A guide is ¥698 for 5 hours a day over 30 days, and ¥3,488 for a winter from November to March. Setting it to high raises the average power.

Why are oil heaters so expensive to run?

They convert electricity directly to heat, so efficiency can't exceed 1.0. They draw 1,200–1,500 W when starting up and cycle at around 500 W even after the room warms. At 5 hours a day they cost ¥5,580 a month and about ¥27,900 a winter, over twice the cost of heating the same room with an air conditioner.

Which is cheaper, an air conditioner or a ceramic heater?

For heating a room, an air conditioner. Heating 6–8 tatami mats for 5 hours a day costs ¥2,325 a month with an air conditioner and ¥5,580 with a ceramic fan heater, a ¥3,255 monthly difference. An air conditioner is a heat pump that moves heat from outside, so it delivers more heat to the room than the electricity it uses.

Is there a way to lower the cost of a heated carpet?

Setting it to heat only half the surface also roughly halves its power use. A guide is ¥12.4 an hour for 2 tatami mats on medium, or ¥1,860 a month at 5 hours a day. Laying insulation underneath reduces heat lost to the floor, so it feels warm even on a lower setting.