Electric Floor Heating: Watts per Square Foot and Cost to Run
Part of the project planners: Planning new floors and tile and Planning heating, cooling and ventilation.
Electric floor heating is sized at about 12 watts per square foot of heated floor. A 10 × 8 ft bathroom with 75% of the floor heated (60 ft²) needs 720 watts, which is 6.0 amps on 120 volts and a 15 amp breaker, and costs about $0.69 a day ($20.74 a month) at 6 hours a day and 16¢ per kWh. The floor heating calculator does your room.
Floor heating for common bathrooms
75% of the floor heated, 12 W per ft², 120 V, 6 hours a day at 16¢ per kWh:
| Room | Heated area | Watts | Amps | Breaker | Cost per month |
|---|---|---|---|---|---|
| 5 × 8 ft | 30 ft² | 360 W | 3.0 A | 15 A | $10.37 |
| 6 × 10 ft | 45 ft² | 540 W | 4.5 A | 15 A | $15.55 |
| 8 × 10 ft | 60 ft² | 720 W | 6.0 A | 15 A | $20.74 |
| 10 × 12 ft | 90 ft² | 1,080 W | 9.0 A | 15 A | $31.10 |
| 12 × 12 ft | 108 ft² | 1,296 W | 10.8 A | 15 A | $37.32 |
| 12 × 16 ft | 144 ft² | 1,728 W | 14.4 A | 20 A | $49.77 |
Heat output
For the 10 × 8 ft bathroom:
| Output | Watts | Amps at 120 V | Cost per day | Cost per month |
|---|---|---|---|---|
| 8 W per ft² | 480 W | 4.0 A | $0.46 | $13.82 |
| 12 W per ft² | 720 W | 6.0 A | $0.69 | $20.74 |
| 15 W per ft² | 900 W | 7.5 A | $0.86 | $25.92 |
| 20 W per ft² | 1,200 W | 10.0 A | $1.15 | $34.56 |
At 240 V the current halves (3.0 A instead of 6.0 A). A cold floor or a main-heat system needs 15 W per ft² or more (900 W here).
What it does well
- Comfort: a warm floor under bare feet in a bathroom or kitchen, and it warms the room gently.
- Cost: inexpensive to run for a few hours a day if you heat only the floor area people use.
- Where: under tile, stone, vinyl, laminate or engineered wood that is rated for it.
- Limits: it rarely heats the whole room on its own unless the room is well insulated and the output is high.
Installing it
- Plan the layout so the mat or cable covers only the visible floor, and stays away from walls, fixtures and drains.
- Put insulation board or an uncoupling membrane under the heating, to send the heat up and warm the floor faster.
- Test the heating wire with a multimeter before, during and after installation, and write the readings on the box.
- Bed the mat in thinset, or in self-leveling compound, and install the floor over it. A damaged wire cannot be repaired under tile.
- Fit a thermostat with a floor sensor, on a dedicated GFCI circuit, connected by an electrician.
Cutting the cost
- Use a programmable thermostat that warms the floor shortly before you get up.
- Insulate under the floor and the room to keep the heat from leaking.
- Heat only the area that you need.
- Compare running costs with other fuels in the heating cost comparison.
Electrical work can start fires. Follow your local code and have a licensed electrician do the wiring. Estimates only.
Frequently asked questions
How many watts per square foot for floor heating?
About 12 W per ft² for a standard heated floor, 8 to take the chill off, and 15 to 20 for a cold floor or main heat. A 10 × 8 ft bathroom (60 ft² heated) needs 720 W at 12.
How much does electric floor heating cost to run?
For 720 W used 6 hours a day at 16¢ per kWh, about $0.69 a day or $20.74 a month. A thermostat with a floor sensor and a timer cuts it.
What breaker do I need for floor heating?
15 amps for 720 W on 120 V, since it is a continuous load rated at 125%. Larger floors use 240 V, which halves the current (3.0 A here).
Can floor heating heat a whole room?
Only in a well-insulated room, with 15 to 20 W per ft² and the whole floor heated. Usually it is for comfort, with another heat source for the air.
Should I heat the floor under the vanity and tub?
No. Heat only the visible floor: heating all 80 ft² of this bathroom would take 960 W, compared with 720 W for the usable 75%.
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Estimates only. Quantities depend on your materials, site and supplier. Confirm important numbers before you order, and follow local building codes for anything structural.