BTU (British Thermal Unit) measures heat output and applies it to the power generated by your grill smoker combo. For example, you'll need one BTU of heat if you want to raise a substance's temperature by one Fahrenheit. The faster and more efficient your grill smoker's BTU is, the better it will be able to cook the meat. The ideal BTU for a smoker grill should range between 80 and 100 BTUs per square inch of the cooking surface.