A BTU or British Thermal Unit is a standard unit of energy that is used in the United States and sometimes in the United Kingdom. It represents the amount of thermal energy necessary to raise the temperature of one pound of pure liquid water by one degree Fahrenheit at the temperature at which water has its greatest density (39 degrees Fahrenheit).
Generally, in a room with adequate insulation and typical ceilings, each 10,000 BTU's of heat output from an efficient gas appliance will heat approximately 350 sq. ft. If the area to be heated has pitched or vaulted ceilings, poor insulation, or drastic air movement then the heating ability will be reduced.
Though the amount of warmth generated varies among manufacturers and models, normally around 4,500 to 5,000 BTUs of heat is generated from an average electric fireplace and 3,000 BTUs from a gel fireplace.