The number of BTUs you'll want to heat your desired room depends on the size of that space, as well as how much insulation it has. A typical electric fireplace with a maximum output between 3500 and 9000 can comfortably heat rooms up to 400 square feet in area. If your intended use is for supplemental heating only then any amount over this range will allow you some flexibility when choosing an electric fireplace based on style rather than power output. The larger the room or more severe the climate where you live, the higher range model you'll require for adequate comfort year-round.