Recommended heater power
What size space heater do I need?
For supplemental heating, the rule of thumb is about 10 watts of heater power per square foot of room, adjusted for how well the space holds heat. Measure your room’s length and width, pick how well-insulated it is, and this calculator gives you the wattage to look for (and the equivalent in BTUs). Most portable electric heaters top out at 1,500 watts, which comfortably warms a room up to roughly 150 square feet.
A space heater is for taking the chill off one room, not heating a whole house. Plug it straight into a wall outlet (never a power strip), keep 3 feet clear of anything flammable, and choose a model with tip-over and overheat protection.
Frequently Asked Questions
How many watts to heat a 12×12 room?
A 12×12 room (144 sq ft) with average insulation needs about 1,440 watts — so a standard 1,500-watt heater is ideal. Draughty or poorly insulated rooms need more; well-sealed rooms need less.
How do watts relate to BTUs?
One watt equals about 3.41 BTU per hour, so a 1,500-watt heater puts out roughly 5,100 BTU. The calculator shows both so you can compare electric and gas heaters.
Can one space heater warm my whole house?
No — portable heaters are designed for a single room. For whole-home heating you need a central system sized by a professional (a very different calculation).
Why does insulation matter so much?
A drafty or uninsulated room (like a garage or older sunroom) loses heat quickly, so it needs more power to stay warm. A well-sealed modern room holds heat and needs less.