How many btu needed to reach 300 degrees Fahrenheit?

Answer 1

What you are asking isn't really translatable. What do you want to reach 300F?

Air requires 75 btu per pound to reach 300F. Water needs to be enclosed in a pressure vessel and needs roughly 1180 btu/lb. Propane itself burns at 3360F under perfect conditions.

Answer 2

The answer depends somewhat on the insulation and on your region. In San Jose, California, with insulation and double pane glass, about 13,600 BTU would be appropriate. This writer uses an 8000 BTU for a room that size and it is satisfactory all but a few of the hottest days. In Houston, Texas, the A/C requirement would be 17,500 because of the fierce heat and humidity there. In Phoenix, with 114 degree summer days, the figure goes all the way to 25,000 BTU.

Answer 3

One can't simply guess at the BTU load simply by the size or location of a building. You need to know the orientation, the amount of glazing, and R-values of your insulation to find that out.