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The short answer: budget 10 watts per square foot for an insulated room with an 8 foot ceiling. That is 750 watts for 75 sq ft, 1,000 watts for 100 sq ft and 1,500 watts for 150 sq ft. The 1,500 watt figure is a hard ceiling on any plug-in heater, because a 15 amp 120V circuit allows only 1,440 watts of continuous load under the National Electrical Code 80 percent rule. Poorly insulated rooms, high ceilings or lots of glass need 12 to 15 watts per square foot.
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How many watts for my room size?
Measure length by width, multiply by 10, and adjust for the room. The 10 watts per square foot figure comes from typical residential heat loss and assumes the heater is topping up a house that already has central heating, not carrying the whole load. If the room is a converted garage, a three-season porch or a basement with an uninsulated slab, use the higher column. A portable heater is a supplemental appliance by design and it will disappoint in any room fighting genuine building heat loss.
| Room size | Example | Insulated room | Drafty / high ceiling | Practical choice |
|---|---|---|---|---|
| Up to 75 sq ft | Small office, bathroom | 750 W | 900 W | 750 W with thermostat |
| 100 sq ft | 10 x 10 bedroom | 1,000 W | 1,300 W | 1,500 W on low setting |
| 150 sq ft | 12 x 12 bedroom | 1,500 W | Not enough on 120V | 1,500 W ceramic or oil-filled |
| 200 sq ft | Living room | 2,000 W needed | 2,400 W+ | 240V unit or mini split |
| 300 sq ft | Open family room | 3,000 W needed | 3,600 W+ | Hardwired or ductless system |
| Garage / workshop | Uninsulated 400 sq ft | 4,000 W+ | 5,000 W+ | 240V garage heater |
Will the circuit handle it?
A 1,500 watt heater draws 12.5 amps at 120V. On a 15 amp branch circuit the code-permitted continuous load is 12 amps, so that heater is already over the limit before anything else is plugged in — which is why a heater plus a hair dryer or a vacuum on the same circuit trips the breaker so reliably. Give the heater a circuit with nothing significant on it, plug it straight into a wall receptacle, and never use an extension cord or power strip. If the plug or outlet feels warm, stop and have the receptacle inspected.
If yours trips regularly, work through why does my space heater trip the breaker. If it stops on its own instead, see why does my space heater keep shutting off.
Does the type matter as much as the wattage?
It changes how the heat feels, not how much you get. Every electric resistance heater turns essentially all of its input into heat, so 1,500 watts is 1,500 watts of output whether it is ceramic, oil-filled or infrared. What differs is delivery: ceramic with a fan warms a room fastest, oil-filled warms slowest but holds heat and runs cooler to the touch, and infrared or quartz heats people and objects in its line of sight rather than the air, which suits drafty spaces and garages where heating the air is hopeless.
- Fast warm-up, small room: ceramic with a thermostat.
- Bedroom or long evening use: oil-filled — see oil-filled vs ceramic.
- Drafty garage or workshop: infrared — see ceramic vs infrared.
- Large room: start at best space heater for a large room.
Running cost by wattage is in how many watts does a space heater use, and safety rules for unattended use in are space heaters safe to leave on overnight. More in the heating and cooling hub.