What Size Space Heater Do You Need? Watts per Square Foot

A space heater needs about 10 watts per square foot of floor, the rule of thumb Ontario’s Save on Energy program says makers and retailers often suggest. That makes a 1,500 W heater right for about 150 square feet. Allow more for high ceilings, drafts or many windows, and use two heaters on separate circuits for big rooms.

How to work out heater size in three steps

Room size is the main input, and the math is short.

1. Measure the floor. Multiply the room’s length by its width in feet. A 12 by 12 foot bedroom is 144 square feet.
2. Multiply by 10. The common rule of thumb is 10 watts of heating power for every square foot. The bedroom needs about 1,440 W, so a 1,500 W heater fits.
3. Adjust for the room. The rule is a starting point for an ordinary room. Memphis Light, Gas and Water (MLGW) says space heaters work less well in very large rooms, rooms with vaulted or high ceilings and rooms with many windows, so allow extra for those.

Electric heaters turn the power they draw into heat, so the wattage on the label is a fair measure of output. One watt equals about 3.41 BTU per hour, so a 1,500 W heater puts out roughly 5,100 BTU per hour.

Space heater sizes by room

Here is the 10 watt rule applied to common rooms. Treat the right-hand column as a starting point, then check the room size printed on the heater’s box.

Most whole-room portable heaters top out at about 1,500 W. Save on Energy explains why: that is as much as you can safely plug into a residential wall outlet. Champion’s generator chart lists space heaters at 750 to 1,500 running watts, and many heaters have a lower setting, which gives one heater some range.

Why large rooms are hard to heat with one unit

A big room asks more of a heater than its floor area suggests. Warm air rises to the ceiling, and glass and outside walls pull heat out, which is why MLGW warns that a heater’s effectiveness drops in large, high-ceilinged or window-heavy rooms.

Three ways to get around the limit:

• Heat the zone, not the room. Put the heater near where people sit, still 3 feet from anything that can burn, as CPSC advises, and let the far corners stay cooler.
• Split the load. Two heaters on two different circuits share the work. The U.S. Fire Administration says not to plug anything else into the circuit a space heater is using, and CPSC says to plug each heater directly into a wall outlet, never an extension cord or power strip.
• Stop the leaks first. Save on Energy suggests weatherstripping around doors and windows, and says a home that stays drafty even with the furnace running may need insulation or window upgrades rather than more heaters.

The Department of Energy describes space heaters as supplemental heat for a specific room, not a replacement for a home’s main system. MLGW suggests turning the central thermostat down and using a portable heater in the rooms people are in.

Is an oversized space heater a problem?

Bigger is not automatically better. Save on Energy advises using the lowest wattage that meets your needs, to avoid overheating the space and paying for electricity you do not need. A heater with an adjustable thermostat lets you pick a temperature and holds it, so it is not running at full power all the time.

Sizing for a power outage

In an outage, the heater’s size decides what can power it. A 1,500 W heater on high is a heavy load for a portable generator or a battery power station. Choosing a heater with a lower setting, and heating one small room, makes the load far easier to carry. Our guide to portable generators for winter outages covers generator sizing, and space heaters in a power outage covers clearance, cords and the safety features to look for. For the running cost of a 1,500 W heater, see how much electricity an oil-filled heater uses.

Keeping one room warm also helps the plumbing. How to prevent frozen pipes lists which pipes freeze first, and door draft stoppers cover the gaps that let the heat out. More heating guides are in the heating section.