Measure the length and width of your room. It is estimated that it requires 10-watts of energy to heat 1-sq. ft. of space. So, multiply the square footage of the room by 10. For example, if your room is 10 x 12 feet, that equals 120-sq. ft. The square footage multiplied by 10 equals 1,200 BTUs required to heat the room.
Keep in mind that the square footage is calculated based on standard ceiling heights. If you have high vaulted ceilings, keeping the room comfortably warm will be difficult, since hot air rises. You may need an infrared heater with a full 1,500 watts to heat a standard living room with vaulted ceilings or a loft.