The formal term for BTU is British Thermal Units, and it refers to the amount of heat output needed to warm the air by 1 degree.    Firepit BTUs start at about 35,000 for a small portable heater and continue until about 150,000 BTUs (for residential use). Try thinking of a space heater to make it super easy to understand how much heat you are getting. One electric watt is equal to 3.41 BTUs.  That means a space heater of 1500 watts has a little over 5,000 BTUs.  Doing the math indicates that if you were inside your house, you would be warm in a room that was 15 feet by 10 feet. Outside is a different story. No ceiling or walls to keep the heat in, and the wind and humidity also will snatch up some of your heat. That's why residential gas fire pits start at 45,000 and go up to 150,000 BTUs.