Space heaters are sold by watts and rooms are sized in BTU, which is why "is a 1,500 watt heater enough for my bedroom?" is harder to answer than it should be. The conversion is simple (1 watt is 3.412 BTU per hour, so 1,500 W is about 5,118 BTU/hr), and from there the sizing follows the same logic as any other heater: work out what the room loses on a cold night, then buy something that can put that much back. This guide gives you the rule of thumb, shows where it breaks, and explains why every plug-in heater in the store tops out at exactly 1,500 W.
Watts and BTU are the same thing in different units
An electric resistance heater converts every watt it draws into heat, with no efficiency to worry about, so its output is simply wattage times 3.412. A 750 W heater gives 2,560 BTU/hr; 1,000 W gives 3,412; 1,500 W gives 5,118. This also means all 1,500 W heaters produce exactly the same amount of heat, whether they cost $25 or $250, ceramic, oil-filled, infrared, or "micathermic." The differences are in how they distribute that heat, how quietly, and how safely. A box that says one 1,500 W heater "heats up to 1,000 sq ft" while another does 300 is describing the room the copywriter imagined. Our watts to BTU converter handles other wattages.
The 10 watts per square foot rule
The classic quick rule for electric heat is 10 watts per square foot, which works out to about 34 BTU/hr per square foot. By that rule a 1,500 W heater covers 150 sq ft, a 12 × 12 bedroom with a little to spare. It assumes a room with 8 ft ceilings, decent insulation, and a moderate winter, and it is a reasonable average for a well-built house in the middle of the country.
Our calculator is a little more careful and uses 30, 40, or 55 BTU/hr per square foot depending on whether the climate is hot, moderate, or cold, then adjusts for insulation and ceiling height. In a moderate climate with good insulation the calculator's number is 34 BTU/sq ft, exactly the 10 W rule. With average insulation it is 40 (about 11.7 W/sq ft), and in a cold climate with average insulation it is 55 (about 16 W/sq ft). Here is how far a given heater goes as the only heat source in a room with average insulation:
| Heater | Output | Hot climate | Moderate | Cold climate |
|---|---|---|---|---|
| 750 W | 2,559 BTU/hr | 85 sq ft | 64 sq ft | 47 sq ft |
| 1,000 W | 3,412 BTU/hr | 114 sq ft | 85 sq ft | 62 sq ft |
| 1,500 W | 5,118 BTU/hr | 171 sq ft | 128 sq ft | 93 sq ft |
| 4,000 W | 13,649 BTU/hr | 455 sq ft | 341 sq ft | 248 sq ft |
| 5,000 W | 17,061 BTU/hr | 569 sq ft | 427 sq ft | 310 sq ft |
| 7,500 W | 25,591 BTU/hr | 853 sq ft | 640 sq ft | 465 sq ft |
The 1,500 W row is the one that matters for most people. As the sole heat source, a plug-in heater handles a small bedroom or office in a mild or moderate climate and a large very small room in Minnesota. That sounds harsh, but read the next two sections before you give up on it.
Insulation and climate move the number a lot
Take a 150 sq ft bedroom and vary only the climate and the insulation. The calculator multiplies the base load by 1.20 for poor insulation and 0.85 for good, so the spread between the best and worst case in the same climate is about 40%, and the spread between a good room in the South and a poor one in the North is nearly three to one.
| Climate | Poor insulation | Average | Good insulation |
|---|---|---|---|
| Hot (South) | 5,400 (1,583 W) | 4,500 (1,319 W) | 3,800 (1,114 W) |
| Moderate | 7,200 (2,110 W) | 6,000 (1,758 W) | 5,100 (1,495 W) |
| Cold (North) | 9,900 (2,901 W) | 8,300 (2,432 W) | 7,000 (2,051 W) |
Only three cells come in at or under 5,118 BTU: average and good insulation in a hot climate, and good insulation in a moderate one. Everywhere else a 1,500 W heater, on its own, will not hold a 150 sq ft room at 70°F on the coldest few nights of the year. On an ordinary winter evening the load is half that, and the heater is fine. Our insulation guide explains how to judge which column your room is in.
Why every plug-in heater stops at 1,500 watts
A standard North American outlet is on a 120 V, 15 A circuit, which can carry 1,800 W. For anything that runs for hours, electrical code limits continuous loads to 80% of the circuit rating, which is 1,440 W, and the safety standard for portable heaters (UL 1278) caps them at 1,500 W. That is why the "high" setting on every plug-in heater in North America is 1,500 W and the "low" is usually 750 or 900. You cannot buy a more powerful plug-in heater, and you should not try to run two on the same circuit: 3,000 W on a 15 A breaker trips it, and on an old fuse or a worn outlet it can start a fire before it does. Putting two heaters on different circuits is fine, but most bedrooms only have one.
A 20 A circuit can carry more, but heaters are still built to 1,500 W because the maker cannot know which outlet you will use. Anything above that is a 240 V unit with a different connector.
Supplemental heat is a different calculation
Most space heaters are not the only heat source. They top up a room the furnace does not quite reach, or let you keep the thermostat at 62°F and be comfortable in the one room you are actually using. In that case the heater only has to cover the difference between what the central system is providing and what you want.
A 300 sq ft living room in a moderate climate has a full heating load of 12,000BTU at design conditions, far beyond a plug-in heater. But if the furnace is holding the house at 62°F and you want 70°F in that room, the extra 8°F is roughly a quarter of the temperature difference the full load was calculated for (70°F inside vs. about 35°F outside on a moderate-climate design night), so the heater needs about a quarter of the load: around 3,000 BTU, well within 1,500 W. This is why a single space heater makes a real difference in most rooms even though the table above says it "cannot" heat them. The table is for a room with no other heat.
When you need 240 volts, or gas
Garages, workshops, basements, and additions with no ductwork are where 1,500 W runs out. A 20 × 20 garage in a cold climate with typical uninsulated walls and a big steel door has a heating load of about 26,400 BTU/hr, which is 7,737 W, or five plug-in heaters. The realistic options are a 240 V electric unit heater (4,000 W = 13,600 BTU, 5,000 W = 17,000 BTU, 7,500 W = 25,600 BTU; the 7,500 W is the common garage size) on a dedicated 30–40 A circuit, or a gas unit heater or vented wall furnace, which start around 30,000 BTU and cost far less to run per BTU than electric resistance heat. The garage heater page works through the common sizes.
A finished 600 sq ft basement in a moderate climate is around 24,000 BTU. That is 240 V territory too (a 7,500 W unit at 25,591 BTU fits; a 5,000 W unit at 17,061does not quite), or a ductless heat pump, which delivers two to three units of heat per unit of electricity. Below-grade walls lose less than exposed ones, so "average" insulation is usually fair for a basement even in an older house. See basement heater BTU for a range of sizes. To translate a heater rated in kilowatts, the kW to BTU converter is the quickest route.
Choosing a type once you know the wattage
Since output is fixed by wattage, the type is about delivery. Ceramic fan heaters warm a room fastest and are the most compact, at the cost of fan noise. Oil-filled radiators are silent, take 20 minutes to get going, and hold heat after they cycle off, which makes them the best choice for a bedroom overnight. Infrared heaters warm people and objects in their line of sight rather than the air, so they are right for a large, leaky space where heating all the air is hopeless (a garage workbench, a drafty porch) and wrong for evenly warming a closed room. Whatever you buy, get a tip-over switch, an overheat cutoff, and a thermostat; the thermostat is what turns a 1,500 W heater into a 750 W average draw once the room is up to temperature.
What to do next
Measure the room, decide whether the heater is the only heat or a top-up, and run the free BTU calculator in heating mode with your climate and insulation. If the result is under about 5,118 BTU, one 1,500 W plug-in heater covers it as the sole heat source; if the result is higher but you have central heat, a plug-in heater will still make a noticeable difference; and if the result is higher and there is no other heat, you are shopping for a 240 V unit, a gas heater, or a mini-split. For a bedroom in the 6,000 BTU range, choose an oil-filled radiator with a thermostat and stop worrying about it. Convert anything rated in BTU back to watts with the BTU to watts converter if the box does not say.