Understanding your result
The headline is the heater size to buy in watts, from the sizes sold (500, 750, 1,000 and 1,500 W plug-in; 2,000 to 5,000 W hardwired), with the heat the room actually needs in the sub-line. The stats give the need in watts and BTU per hour and the current the chosen heater draws; the table adds the basis of the estimate, the factors applied, whether the heater is a plug-in or a hardwired unit, and the cost per hour at the US average electricity price.
The split at 1,500 W is the important one. Up to that size the heater plugs into a wall outlet, and the notes remind you that a 1,500 W unit draws 12.5 A and wants an outlet of its own. Above it, the room needs a hardwired 240 V heater on its own circuit, installed by a licensed electrician, and the calculator says so rather than pretending a 1,500 W heater will do.
For a permanent installation the baseboard heater calculator sizes electric or hot-water baseboard to the same room load, and the heating cost by fuel calculator shows what electric resistance heat costs for a season against gas or a heat pump. If the aim is to heat one room and let the rest of the house cool, the HVAC running cost calculator gives the other side of that comparison.
How we calculate this
The 10 watts per square foot rule is the US Department of Energy’s guidance for supplemental heating of a room with average insulation while the central system holds the rest of the house; the insulation and ceiling factors scale it the same way the sizing calculators do. For primary heat the calculator uses the climate-zone heating loads from the BTU per square foot table, which is the load the room loses on the design night with nothing else heating it.
Electric resistance heat converts every watt to 3.412 BTU per hour, with nothing lost, so the heater’s rating is also its output; the unit conversions page lists that and the other factors.
The assumptions behind the numbers
| Assumption | Default | Where it comes from |
|---|---|---|
| Supplemental heat | 10 W per sq ft | US Department of Energy, Energy Saver, portable heaters |
| Primary heat | Climate-zone heating load per sq ft | Rule-of-thumb climate-zone charts; see the BTU per square foot table |
| Insulation and ceiling factors | 1.2 / 1.0 / 0.85; +10% per ft above 8 ft | Common published adjustments |
| Plug-in limit | 1,500 W at 120 V | 80% of a 15 A circuit for a continuous load (National Electrical Code practice) |
| Sizes sold | 500, 750, 1,000, 1,500 W plug-in; 2,000, 2,500, 3,000, 4,000, 5,000 W hardwired | US retail space heater and wall heater sizes |
| Electricity price for the cost row | $0.17 per kWh | US residential average, EIA, 2025 |
Assumptions last reviewed October 7, 2026.
The calculator does not model the heater’s thermostat cycling, which decides the real running cost, the comfort difference between radiant and convective heat, or the fuel-burning space heaters (kerosene, propane) that need ventilation and carbon monoxide precautions and are outside its scope. The guide to comparing fuels per million BTU puts the cost of resistance heat in context, and the guide to thermostat setbacks covers the “heat one room, set back the house” strategy and when it saves money.
Two worked examples
Topping up a home office
A 150 sq ft office with average insulation and 8 ft ceilings, central heating on, supplemental heater.
- Watts: 150 × 10 × 1.0 × 1.0 = 1,500 W
- BTU: 1,500 × 3.412 = 5,118 BTU/hr
- Heater: 1,500 W plug-in, 12.5 A at 120 V
- Cost: $0.26 per hour running flat out at 17 cents per kWh
The room sits exactly at the plug-in limit, and the note says to give the heater its own outlet and never an extension cord. With a thermostat the heater will cycle; at half duty over an 8-hour working day it costs about $1 a day.
The only heat in a poorly insulated room in a cool climate
A 200 sq ft room in zone 5 (Chicago), poor insulation, 8 ft ceiling, no other heat source.
- Load: 200 × 45 × 1.2 = 10,800 BTU/hr
- Watts: 10,800 ÷ 3.412 = 3,165 W
- Heater: 4,000 W hardwired 240 V, 16.7 A
- Cost: $0.68 per hour running flat out
No plug-in heater comes close; the room needs a 4,000 W wall or baseboard heater on a dedicated 240 V circuit (a 20 A breaker at 125% covers 3,840 W, so this one needs 25 or 30 A). At $0.68 an hour and perhaps 10 hours a day in January, the room costs over $200 a month to heat with resistance, which is the case for insulating the room first and considering a small heat pump.
Where to find your inputs
Floor area and ceiling. Length × width of the room; ceilings above 8 ft add to the load.
Insulation. Judge the room, not the house: an uninsulated sunroom, a converted garage or a room over a garage is “poor” even in a well-built house.
Supplemental or only heat. If the thermostat for the rest of the house is set to 68 °F and this room is just cold, it is supplemental. If the room has no register, radiator or baseboard, or the central system is off, it is the only heat.
Heater wattage. Printed on the heater’s label; most plug-in units are 1,500 W on high and 750 or 900 W on low.
Common mistakes
- Believing the box. “Heats up to 1,000 sq ft” assumes the furnace is doing the work; as supplemental heat a 1,500 W unit covers about 150 sq ft.
- Running two heaters on one circuit. Two 1,500 W heaters draw 25 A; a 15 A breaker trips, or worse, the wiring overheats.
- Using an extension cord or power strip. Most are not rated for 12.5 A continuous; plug the heater straight into the wall.
- Choosing by “efficiency”. All electric heaters are 100% efficient; pick by how the heat is delivered and how quiet it is.
- Heating a cold room instead of fixing it. A drafty window or an uninsulated wall costs more every winter than it costs to fix.
- Leaving a heater running unattended. Use the thermostat and tip-over switch, keep 3 ft clear of anything that burns, and switch it off when leaving the room.
Questions people ask
- What size space heater do I need for a 150 square foot room?
- As supplemental heat in a room with average insulation and 8 ft ceilings, about 1,500 watts, which is the largest plug-in size and gives 5,100 BTU per hour. As the room's only heat, it depends on the climate. 150 sq ft needs about 1,700 watts in a mixed climate and 2,000 in a cool one, which means a hardwired 240 V heater; in a warm climate (zone 2) about 1,100 watts and a plug-in unit will do.
- How many square feet does a 1,500 watt heater heat?
- About 150 sq ft as supplemental heat at the Department of Energy's 10 watts per square foot, more in a well-insulated room (175 sq ft) and less in a drafty one (125 sq ft). As the only heat, 1,500 W is 5,118 BTU per hour, which covers roughly 135 sq ft in a mixed climate, 115 in a cool one and 85 in a very cold one, all with average insulation. Marketing claims of "up to 1,000 sq ft" assume the central heating is doing the real work.
- Why do all space heaters max out at 1,500 watts?
- Because a standard 120 V household circuit is rated 15 A, and a continuous load should not exceed 80% of that, which is 12 A, or 1,440 to 1,500 W. A heater that drew more would trip breakers in most homes. Anything above 1,500 W is sold as a hardwired 240 V unit (wall, ceiling or baseboard heaters of 2,000 to 5,000 W) or as a 240 V plug-in garage heater on a dedicated circuit.
- Are infrared, ceramic and oil-filled heaters different sizes for the same room?
- No. A 1,500 W heater of any kind puts 1,500 W of heat into the room, and all of them are 100% efficient at turning electricity into heat. They differ in how the heat is delivered. Fan-forced ceramic units warm the air quickly; oil-filled radiators warm slowly and keep radiating after they switch off; infrared quartz heaters warm people and surfaces in their line of sight before the air. Pick by comfort and noise, not by claimed efficiency.
- How much does it cost to run a space heater?
- Watts ÷ 1,000 × hours × price per kWh. A 1,500 W heater running flat out for 8 hours at the US average of 17 cents per kWh costs $2.04; one with a thermostat that cycles half the time costs about $1. Over a 150-day heating season at 8 hours a night that is $150 to $300 per heater. Electric resistance heat costs about three times as much per BTU as a heat pump and about three times natural gas; the heating cost by fuel calculator compares them.
- Can a space heater heat a whole room in winter?
- As the only heat, a 1,500 W plug-in heater covers a small room in a mild climate and falls short elsewhere; the calculator's "only heat" mode shows the room size it can carry in your zone. For a larger room or a cold climate, a hardwired 240 V heater or baseboard sized to the room's load is needed, and the heating cost of electric resistance makes a heat pump worth considering for anything used all winter.