Standard space heaters with 5,100 Btus of output are able to heat a standard-size room of 144 square feet. Larger space heaters heat larger rooms. To calculate the power needs of a living space, a safe estimate is that each square foot of space requires an additional 10 watts.
Rooms may need more than 10 watts of power per square foot if the room has a lot of windows, is poorly insulated, or has a tile or cement floor, high ceilings or a staircase. Rooms with few windows, those that have low ceilings, are well insulated or that are used several people at a time may require fewer Btus than other rooms.